Showing papers in "European Journal of Soil Science in 2014"

Biochar-root interactions are mediated by biochar nutrient content and impacts on soil nutrient availability

Abstract: Summary Roots are the first point of contact between biochar particles and growing plants, yet detailed studies of biochar‐root interactions are few. Biochar may affect root growth, and therefore plant performance, through two mechanisms: (i) as a direct nutrient source and (ii) through impacts on nutrient availability. To test the hypothesis that biochar‐root interactions occur and are determined by biochar nutrient supply and impacts on soil nutrients, spring barley (Hordeum vulgare L.) was grown with and without biochar addition in rhizobox mesocosms. Biochar from unaltered and artificially weathered Miscanthus or willow (Salix sp) biochar types was used and was manipulated to alter its structure and nutrient content. After 28days of plant growth, biochar nutrient content, soil nutrient content and the amount of biochar were measured in the bulk soil, the rhizosphere and the rhizosheath. Plants in biochar-amended soils had larger rhizosphere zones than the control treatment. The rhizosphere contained more biochar particles than the bulk soil, an indication that roots preferred soil containing biochar particles. Biochar particles retained soil nitrogen (N) in the form of nitrate, and also supplied phosphorus (P) to the soil and plant. Miscanthus biochar had a larger extractable P content than the Salix biochar, with different effects on plant growth and root responses. Although artificial physical weathering had no effect on overall plant growth, weathering effects on N retention and P content were dependent on biochar type. Our results indicate that roots are attracted towards biochar, resulting in its partitioning between bulk and rhizosphere soil. Biochar thus controls plant root nutrient acquisition directly as a nutrient source and indirectly by altering soil nutrient content.

Biochar addition rate influences soil microbial abundance and activity in temperate soils

Abstract: Summary Biochar (BC) amendment to soils is a proposed strategy to improve soil fertility and mitigate climate change. However, before this can become a recommended management practice, a better understanding of the impacts of BC on the soil biota is needed. We determined the effect of addition rates (0, 1, 5, 10 and 20% by mass) of a fast-pyrolysis wood-derived BC on the extraction efficiency (EE), abundance and temporal dynamics of phospholipid fatty acids (PLFAs, microbial community biomarkers) in four temperate soils during a 1-year incubation. Additionally, the effects on microbial mineralization/incorporation of BC-C were determined by measuring CO2 efflux and the BC contribution to CO2 and PLFA-C using the natural 13C abundance difference between BC and soils. Biochar addition proportionally increased microbial abundance in all soils and altered the community composition, particularly at the greatest addition rate, towards a more gram-negative bacteria-dominated (relative to fungi and gram-positive) community. Though chemically recalcitrant, the BC served as a substrate for microbial activity, more so at large addition rates and in soil with little organic matter. Microbial utilization of BC-C for growth could only partially explain the observed increase in microbial biomass, suggesting that other, potentially abiotic, mechanisms were involved. The strong decrease in PLFA EE (−77%) in all soils with biochar addition emphasizes the need to measure and correct for EE when using PLFA biomarkers to estimate soil microbial responses to BC additions. Overall, our study provides support for BC use as a soil amendment that potentially stimulates microbial activity and growth.

Short-term effects of biochar on soil heavy metal mobility are controlled by intra-particle diffusion and soil pH increase

Abstract: Summary Biochar, the solid product of biomass pyrolysis, can be used as a soil amendment to stabilize metals in contaminated soils. The effects of biochar on the mobility of metals in soils are, however, poorly understood. To identify the predominant processes, we focused on (i) a possible kinetic limitation by transport in biochar particles, (ii) the evolution of biochar mineral phases and (iii) the effect of biochar on soil pH. Batch experiments were conducted to measure the sorption kinetics of copper (Cu), cadmium (Cd) and nickel (Ni) and the sorption-desorption isotherms for lead (Pb), Cu, Cd, zinc (Zn) and Ni in a wood-derived biochar. Sorption data were then compared with extraction test results using biochar with one acidic and one basic soil contaminated by Zn, Cd and Pb. Kinetic results showed that biochar particle sizes controlled metal sorption rate despite a similar specific surface area, which indicated a limitation by intra-particle diffusion. Isotherms showed a partially reversible sorption to biochar following the order Pb > Cu > Cd ≥ Zn > Ni, which we explained primarily by the (co)precipitation of metals or their adsorption on specific biochar mineral phases. Effective metal immobilization was observed with biochar in both contaminated soils but could not be predicted from the sorption isotherms. This immobilization appeared to be governed by the soil pH increase, which induced a greater retention of metals on soil particles. Short-term effects of biochar on contaminated soils may therefore be controlled by diffusion in biochar particles and by soil alkalinization processes.

Biochar carbon stability in four contrasting soils

Abstract: Summary There is a limited understanding of the effects of soil properties on biochar carbon (C) stability. This knowledge is essential to evaluate the capacity of biochar for long-term soil C sequestration fully. In this study two biochars, produced by slow pyrolysis at 450 or 550 ◦ C from a δ 13 C-depleted (−36.4‰) Eucalyptus saligna Sm. woody material, were incubated in four soils (Inceptisol, Entisol, Oxisol and Vertisol) of contrasting chemical and mineralogical properties. The total biochar-C mineralized over 12 months was 0.30–1.14 and 0.97–2.71% from the soil-biochar mixtures incubated at 20 and 40 ◦ C, respectively. The total biochar-C mineralized (mg CO2-C per unit of native soil organic C (SOC) basis) from soils incubated with the 450 ◦ C biochar was approximately twice the corresponding amount mineralized from the 550 ◦ C biochar systems. The influence of soil properties on biochar-C mineralization was greater for the 450 ◦ C biochar than the 550 ◦ Cb iochar. The smallest proportion of C mineralized from the 450 ◦ C biochar occurred in the Inceptisol incubated at 20 ◦ Ca nd in the Oxisol at 40 ◦ C. However, when expressed on a per unit of native SOC basis, the C mineralization of the 450 and 550 ◦ C biochars was least in the Oxisol and greatest in the Inceptisol at both incubation temperatures. Mean residence times (MRTs) of the biochars estimated using the two-pool exponential model varied between 44 and 610 years. The estimated MRT of the biochars may vary under field conditions depending upon the environmental conditions and addition of labile C from plants. Our results indicate that biochar-C was stabilized by variable charge minerals in the Oxisol and that the stabilization occurred rapidly at high temperatures.

Rhizospheric organic compounds in the soil–microorganism–plant system: their role in iron availability

Abstract: Summary Poor iron (Fe) availability in soil represents one of the most important limiting factors of agricultural production and is closely linked to physical, chemical and biological processes within the rhizosphere as a result of soil–microorganism–plant interactions. Iron shortage induces several mechanisms in soil organisms, resulting in an enhanced release of inorganic (such as protons) and organic (organic acids, carbohydrates, amino acids, phytosiderophores, siderophores, phenolics and enzymes) compounds to increase the solubility of poorly available Fe pools. However, rhizospheric organic compounds (ROCs) have short half-lives because of the large microbial activity at the soil–root interface, which might limit their effects on Fe mobility and acquisition. In addition, ROCs also have a selective effect on the microbial community present in the rhizosphere. This review aims therefore to unravel these complex dynamics with the objective of providing an overview of the rhizosphere processes involved in Fe acquisition by soil organisms (plants and microorganisms). In particular, the review provides information on (i) Fe availability in soils, including mineral weathering and Fe mobilization from soil minerals, ligand and element competition and plant-microbe competition; (ii) microbe–plant interactions, focusing on beneficial microbial communities and their association with plants, which in turn influences plant mineral nutrition; (iii) plant–soil interactions involving the metabolic changes triggered by Fe deficiency and the processes involved in exudate release from roots; and (iv) the influence of agrochemicals commonly used in agricultural production systems on rhizosphere processes related to Fe availability and acquisition by crops.

Impact of biochar addition on water retention, nitrification and carbon dioxide evolution from two sandy loam soils

Abstract: Summary Some intensive agricultural practices result in soil degradation through loss of soil organic matter. Organic farming may mitigate this problem, if managed properly, but may result in a yield penalty compared with conventional systems. Biochar addition to soil could influence both agricultural systems, but previous studies are not definitive about its impact on soil processes. Sandy soils are more susceptible to the effects of reduced soil organic matter on soil hydrology and nutrient dynamics. Nitrogen (N) is important for crop growth and soil water content can influence its transformation and cycling. This study explored the effect of biochar amendment on soil water retention and nitrification processes in soils under organic and conventional management. Carbon dioxide evolution was used as an indicator of related microbial activity. A water release curve study and a 60-day incubation experiment were set up to consider the effect of biochar application on organically and conventionally managed sandy loam soils. The results showed that addition of biochar increased water retention for both soils and this is attributed to its porous structure. On incubation of an organically managed soil, with green-waste compost, initial ammonium level was small, reflecting microbial demand for N. The large cation exchange capacity of the organically managed soil retained ammonium, reducing availability for nitrification. Carbon dioxide evolution increased with continuing small contents of ammonium and nitrate when biochar was added to the organically managed soil. Biochar enhanced nitrification without increased respiration during incubation of a conventionally managed soil with added mineral N; a possible explanation for this enhancement is the increase in pH resulting from the biochar addition.

Optimizing chamber methods for measuring nitrous oxide emissions from plot-based agricultural experiments

Abstract: Nitrous oxide emissions (N2O) from agricultural land are spatially and temporally variable. Most emission measurements are made with small (MUCH LESS-THAN 1 m(2) area) static chambers. We used N2O chamber data collected from multiple field experiments across different geo-climatic zones in the UK and from a range of nitrogen treatments to quantify uncertainties associated with flux measurements. Data were analysed to assess the spatial variability of fluxes, the degree of linearity of headspace N2O accumulation and the robustness of using ambient air N2O concentrations as a surrogate for sampling immediately after closure (T-0). Data showed differences of up to more than 50-fold between the maximum and minimum N2O flux from five chambers within one plot on a single sampling occasion, and that reliability of flux measurements increased with greater numbers of chambers. In more than 90% of the 1970 cases where linearity of headspace N2O accumulation was measured (with four or more sampling points), linear accumulation was observed; however, where non-linear accumulation was seen this could result in a 26% under-estimate of the flux. Statistical analysis demonstrated that the use of ambient air as a surrogate for T-0 headspace samples did not result in any consistent bias in calculated fluxes. Spatial variability has the potential to result in erroneous flux estimates if not taken into account, and generally introduces a far larger uncertainty into the calculated flux (commonly orders of magnitude more) than any uncertainties introduced through reduced headspace sampling or assumption of linearity of headspace accumulation. Hence, when deploying finite resources, maximizing chamber numbers should be given priority over maximizing the number of headspace samplings per enclosure period.

Soil microbial communities responded to biochar application in temperate soils and slowly metabolized 13C-labelled biochar as revealed by 13C PLFA analyses: results from a short-term incubation and pot experiment

Abstract: Summary This study investigates (i) the effect of biochar amendments on soil microbial communities in temperate agricultural soils, (ii) the involvement of microorganisms (MOs) in degradation of biochar and (iii) techniques to quantify degradation of biochar in short-term experiments. The study involved an incubation experiment and a pot experiment with two arable soils (a sandy acidic Planosol and a calcareous loamy Chernozem) amended with 13C-depleted biochar from wheat husk and willow plants. Phospholipid fatty acids (PLFAs), 13C-PLFA, CO2, 13C-CO2, soil organic carbon (Corg) and 13C-Corg were monitored for 100 days. Effects of biochar application on the soil microorganisms (MOs) were generally minor. In the incubation experiment, microbial biomass was elevated by wheat husk biochar, especially in the Planosol. The increase in PLFAs was attributed to Gram-negative bacteria and actinomycetes. Fungi and Gram-positive bacteria were less affected. In the pot experiment, MOs did not respond to the addition of willow biochar. The effects of biochar were mainly attributed to an increase in the pH of the Planosol. Additionally, MOs were probably less responsive to inorganic fertilizer in biochar-amended soil. In the incubation, only the actinomycetal PLFA 10Me18:0 incorporated biochar C, while in the pot experiment, Gram-negative bacterial PLFAs (16:1ω7c, 16:1ω5c, 18:1ω7c) and Me16:0 & i17:1ω8 and i17:0 indicated degradation of biochar after 5 weeks. Uptake of around 20% biochar C in these PLFAs was monitored, which accounts for 2% biochar C in the total microbial biomass. From the PLFA data the mean residence time of biochar carbon was estimated in time-scales of centuries to millennia. The CO2 concentration decreased after biochar addition until its production was masked by root respiration. The use of 13C-CO2 labelling to estimate degradation was complicated by the interference with an initial negative priming effect and the dissolution/precipitation of carbonate. In conclusion, soil MOs were not particularly affected by addition of biochar, and the effects recorded were mainly attributed to changing environmental conditions after biochar addition. Nonetheless, uptake of 13C label into microbial PLFAs was successfully used to estimate microbial degradation of biochar in short-term experiments.

Improving the prediction performance of a large tropical vis-NIR spectroscopic soil library from Brazil by clustering into smaller subsets or use of data mining calibration techniques

Abstract: Summary Effective agricultural planning requires basic soil information. In recent decades visible near-infrared diffuse reflectance spectroscopy (vis-NIR) has been shown to be a viable alternative for rapidly analysing soil properties. We studied 7172 samples of seven different soil types collected from several regions of Brazil and varying in organic matter (OM) (0.2–10.3%) and clay content (0.2–99.0%). The aim was to explore the possibility of enhancing the performance of vis-NIR data in predicting organic matter and clay content in this library by dividing it into smaller sub-libraries on the basis of their vis-NIR spectra. We used partial least square regression (PLSR) models on the sub-libraries and compared the results with PLSR and two non-linear calibration techniques, boosted regression trees (BT) and support vector machines (SVM) applied to the whole library. The whole library calibrations for clay performed well (ME (modelling efficiency) > 0.82; RMSE (root mean squared error) 0.60; RMSE < 0.55%). The best results were, however, found when dividing the large library into smaller subsets by using variation in the mean-normalized or first derivative spectra. This divided the global data set into clusters that were more uniform in mineralogy, regardless of geographical origin, and improved predictive performance. The best clustering method improved the RMSE in the validation to 8.6% clay and 0.47% OM, which corresponds to a 21% and 15% reduction, respectively, as compared with whole library PLSR. For the whole library, SVM performed almost equally well, reducing RMSE to 8.9% clay and 0.48% OM.

Reducing capacity of water extracts of biochars and their solubilization of soil Mn and Fe

Abstract: Summary Biochar, being produced in an oxygen-restricted environment, is chemically more reduced than the original feedstock. Consequently, it was hypothesized that reduced biochar components could participate in redox-mediated reactions in the soil. This hypothesis was tested by measuring the reducing capacities of aqueous extracts of biochars and the reduction and solubilization of soil Mn and Fe oxides by the extracts. The reduction capacity of extracts from biochars produced from three feedstocks (eucalyptus wood, EUC; olive pomace, OP; and greenhouse waste, GHW) at different highest pyrolysis treatment temperatures (HTT; 350, 450, 600 and 800°C) was less for the EUC feedstock than the others, and was greater for biochars produced at lower HTTs. The organic fraction of the extracts apparently was responsible for the major part of the reducing capacity. Extracts of smaller-HTT biochars, having greater dissolved organic carbon (DOC) contents, had greater reducing capacities than extracts of larger-HTT biochars from the same feedstock. Extracts of two GHW biochars (GHW-450 and GHW-600) solubilized Mn and Fe from soils at pH values below 8. The extract with the greater reducing capacity (GHW-450) solubilized both metals to a significantly greater extent. Smaller-HTT biochars produced from agricultural wastes, having a greater variety and concentration of soluble reducing agents, are expected to have more impact on soil redox reactions than larger-HTT biochars. By participating in chemical and biological redox-mediated reactions in the soil, biochar could influence microbial electron shuttling, nutrient cycling, pollutant degradation, contaminant mobilization and abiotic formation of humic structures.

Effect of pine wood biochar on ammonium nitrate leaching and availability in a South African sandy soil

Abstract: Summary Biochar has shown potential in reducing inorganic nitrogen (N) leaching losses from inorganic and organic fertilizer sources in coarse-textured soils. Little information, however, is available on the effect of biochar on the availability of the retained inorganic N in biochar-amended sandy soil. The objective of our study was to determine the potential of pine wood biochar to reduce the leaching of ammonium nitrate fertilizer (100 kg N ha−1) from sandy soil (W. Cape, South Africa) and to quantify the exchangeable inorganic N (2 m KCl) remaining after intensive leaching. Laboratory columns containing sandy soil and biochar (0, 0.5, 2.5 and 10.0% w/w) were leached weekly over a period of six weeks simulating heavy winter rainfall. Biochar (0.5, 2.5 and 10.0% w/w) significantly reduced the cumulative amount of ammonium (12, 50 and 86%, respectively) and nitrate (26, 42 and 96%, respectively) leached relative to the control soil. Despite the observed strong reduction in inorganic N leaching, the leached biochar-amended soils contained only small amounts of exchangeable ammonium (0–7.3 mg kg−1) and nitrate (5.8–8.0 mg kg−1). The results show that pine wood biochar can strongly reduce not only the amount of ammonium and nitrate leached from sandy soils, but also the amount of recoverable exchangeable ammonium and nitrate after leaching. Furthermore, the 2.5 and 10.0% biochar application rates led to over-liming. This raises some concerns as to the practical use of biochar in improving N fertilizer-use efficiency of plants.

Carbon dioxide emissions from biochar in soil: role of clay, microorganisms and carbonates

Abstract: Summary The stability of biochar in soil is of importance if it is to be used for carbon sequestration and long-term improvement of soil properties. It is well known that a significant fraction of biochar is highly stable in soil, but carbon dioxide (CO2) is also released immediately after application. This study investigated the nature of the early release of CO2 and the degree to which stabilizing mechanisms protect biochar from microbial attack. Incubations of 14C-labelled biochar produced at different temperatures were performed in soils with different clay contents and in sterilized and non-sterilized soils. It emerged that carbonate may be concentrated or form during or after biochar production, resulting in significant carbonate contents. If CO2 released from carbonates in short-term experiments is misinterpreted as mineralization of biochar, the impact of this process may be significantly over-estimated. In addition to the CO2 released from carbonates, there appears to be a labile fraction of biochar that is oxidized quickly during the first days of incubation, probably by both abiotic and biotic processes. Later in the incubation, biotic mineralization appears to be the primary cause of CO2 evolution. Finally, we found that both production temperature and clay content affect biochar mineralization. As protective mechanisms hypothesized to prevent degradation of organic matter in soil usually implicate clay, we conclude that biochar is likely to be protected from mineralization during the early stages of incubation by its own recalcitrant chemical and physical nature as well as by physical protective mechanisms.

An incubation study on the stability and biological effects of pyrogenic and hydrothermal biochar in two soils

Abstract: Summary The success and feasibility of CO2-sequestration through incorporation of biochar into soils depends strongly on the long-term biochar stability and on the improvements of physical and microbial soil properties. In this study, the stability of two maize-derived biochars (from pyrolysis and hydrothermal carbonization) and of a compost-biochar mixture and their effects on microbial biomass and enzyme activity were determined in two soils during a 57-day incubation. Soil samples amended with biochar increased soil organic carbon (SOC) content by 20 or 40%. Samples amended with hydrothermal biochar showed the largest respiration rates and the largest increase in microbial and enzymatic activity compared with the untreated controls. Carbon and 13C mass balances showed that between 13 and 16% of the added hydrochar was mineralized within 8 weeks. In the arable soil, hydrochar additions greatly stimulated the degradation of SOC, thus inducing positive priming effects. The mineralization of pyrogenic biochar (pyrochar and a pyrochar-compost mixture) was significantly less (1.4–3%) and comparable to the SOC mineralization in the control soils.

Assessment of soil organic carbon at local scale with spiked NIR calibrations: effects of selection and extra-weighting on the spiking subset

Abstract: Summary Spiking is a useful approach to improve the accuracy of regional or national calibrations when they are used to predict at local scales. To do this, a small subset of local samples (spiking subset) is added to recalibrate the initial calibration. If the spiking subset is small in comparison with the size of the initial calibration set, then it could have little noticeable effect and a small improvement can be expected. For these reasons, we hypothesized that the accuracy of the spiked calibrations can be improved when the spiking subset is extra-weighted. We also hypothesized that the spiking subset selection and the initial calibration size could affect the accuracy of the recalibrated models. To test these hypotheses, we evaluated different strategies to select the best spiking subset, with and without extra-weighting, to spike three different-sized initial calibrations. These calibrations were used to predict the soil organic carbon (SOC) content in samples from four target sites. Our results confirmed that spiking improved the prediction accuracy of the initial calibrations, with any differences depending on the spiking subset used. The best results were obtained when the spiking subset contained local samples evenly distributed in the spectral space, regardless of the initial calibration's characteristics. The accuracy was improved significantly when the spiking subset was extra-weighted. For medium- and large-sized initial calibrations, the improvement from extra-weighting was larger than that caused by the increase in spiking subset size. Similar accuracies were obtained using small- and large-sized calibrations, suggesting that incipient spectral libraries could be useful if the spiking subset is properly selected and extra-weighted. When small-sized spiking subsets were used, the predictions were more accurate than those obtained with ‘geographically-local’ models. Overall, our results indicate that we can minimize the efforts needed to use near-infrared (NIR) spectroscopy effectively for SOC assessment at local scales.

Batjes, N. H. 1996. Total carbon and nitrogen in the soils of the world. European Journal of Soil Science, 47, 151–163. Reflections by N.H. Batjes

Abstract: I was pleasantly surprised, and honoured, by the Editor-in-Chief’s request to provide some reflections on my paper published in the European Journal of Soil Science in 1996. During my early career as a land evaluation expert, I largely considered sound soil organic matter management as being essential to safeguarding the fertility and physical properties of soils. However, it was when I joined ISRIC World Soil Information, then known as the International Soil Reference and Information Centre, in 1990 that I became more aware of, and interested in, the role of soils (and the soil organic matter they hold) in regulating ecosystem functions and processes. Under the stimulating Directorship of the late Dr Wim Sombroek, ISRIC had just organized an international conference that led to the widely cited book on ‘Soils and the Greenhouse Effect’ (edited by Dr Lex Bouwman, 1990). Interestingly, one of the outcomes of the conference was the identification of research gaps with respect to the geographical distribution and characterization of the world soils and land cover types as required for improved, quantitative model-based studies of land use and climate change-induced changes in biogenic greenhouse gas (CO2, N2O and CH4) fluxes, and the associated changes in carbon stocks held in vegetation and soil at various spatial and temporal scales. Such information is needed to inform land-use policies and to propose interventions that promote a reduction in biogenic greenhouse gas emissions at a global scale while ensuring food security, biodiversity and human well-being and livelihood at the local scale. One direct spin-off of the conference on ‘Soils and the Greenhouse Effect’ was the methodological development and subsequent compilation of a harmonized, quantified soil profile database, now known by its acronym ISRIC-WISE, which has been linked to the soil mapping units shown on the broad-scale digital FAO-UNESCO Soil Map of the World. Key site and soil attributes to be considered in the global database were identified from an extensive review of soil factors and processes that control fluxes of heat, moisture and greenhouse gases (with my esteemed former colleague Dr Mike Bridges). By its nature, the database drew heavily on the essential and voluntary contributions from a wide range of soil institutes and experts contacted across the various continents; each contributed a selection of soil profiles considered best to represent soil conditions (defined in terms of the unifying Soil Map of the World Legend: FAO-UNESCO, 1974) in their respective countries. These materials were supplemented with site, soil morphological and soil analytical data derived from the vast collection of survey reports and paper maps safeguarded in ISRIC’s soil reference library. Because of this concerted effort, WISE soon provided a larger consistent profile dataset for the world than previously available, albeit still with recognized soil geographical and soil taxonomic gaps, that is freely accessible through the ICSU World Data Centre for Soils (http://www.isric.org). The WISE soil profiles and suite of spatial GIS databases derived from them have been used for a diverse range of studies by numerous research groups and organizations throughout the world. They have been cited, for example, in research on pedotransfer function development, global agro-ecological zoning, assessments of crop production potential and yield gap analyses, soil vulnerability to pollution, soil gaseous emission potentials, and mathematical modelling of soil organic carbon stock changes at national scale. One of my first studies using the initial WISE dataset was to present estimates for global stocks of soil carbon, organic as well as inorganic, held in the various soil types of the world to a depth of 2 m; earlier global assessments only considered the top 1 m of soil and were based on datasets that are more limited. The ‘1996 estimates’ implied that the soil organic carbon pool to 1-m depth is about twice the size of the atmospheric carbon pool and about three times the amount of carbon in vegetation; hence, the overall size of the soil carbon reservoir is large when compared with the gross annual fluxes of carbon. My paper thereby pointed to the importance of soils in sequestering atmospheric CO2 and in emitting biogenic trace gases that are radiatively active and enhance the ‘greenhouse’ effect. Changes in land use and management as well as predicted global warming and climate change, through their effects on net primary productivity, the plant community and soil conditions, have important effects on the size and composition of the organic matter pool in the soil and directly affect the atmospheric concentration of these trace gases. In retrospect, the still widely cited paper was arguably among the first to draw attention to world-scale carbon in soil in relation to the importance of soils in sequestering atmospheric CO2 and in emitting biogenic trace gases. It is encouraging for soil scientists and other stakeholders to see that managing soils for multiple economic, societal and environmental benefits now figures prominently on the international agenda (UNEP, 2012). A

Changes in carbon stocks of Danish agricultural mineral soils between 1986 and 2009

Abstract: Summary To establish a national inventory of soil organic carbon (SOC) stocks and their change over time, soil was sampled in 1986, 1997 and 2009 in a Danish nation-wide 7-km grid and analysed for SOC content. The average SOC stock in 0–100-cm depth soil was 142 t C ha−1, with 63, 41 and 38 t C ha−1 in the 0–25, 25–50 and 50–100 cm depths, respectively. Changes at 0–25 cm were small. During 1986–97, SOC in the 25–50-cm layer increased in sandy soils while SOC decreased in loam soils. In the subsequent period (1997–2009), most soils showed significant losses of SOC. From 1986 to 2009, SOC at 0–100 cm decreased in loam soils and tended to increase in sandy soils. This trend is ascribed to dairy farms with grass leys being abundant on sandy soils while cereal cropping dominates on loamy soils. A statistical model including soil type, land use and management was applied separately to 0–25, 25–50 and 50–100 cm depths to pinpoint drivers for SOC change. In the 0–25 cm layer, grass leys added 0.95 t C ha−1 year−1 and autumn-sown crops with straw incorporation added 0.40 t C ha−1 year−1. Cattle manure added 0.21 t C ha−1 year−1. Most interestingly, grass leys contributed 0.58 t C ha−1 year−1 at 25–50 cm, confirming that inventories based only on top-soils are incomplete. We found no significant effects in 50–100 cm. Our study indicates a small annual loss of 0.2 t C ha−1 from the 0–100 cm soil layer between 1986 and 2009.

Suitability of biochars (pyro- and hydrochars) for metal immobilization on former sewage-field soils

Abstract: Summary Mineralization of organic material and decreasing pH are expected to increase bioavailability and leaching of metals in soils formerly contaminated by irrigation with wastewater. Biochar has recently been proposed as a sorbent for metals, although the effects described in the literature are still inconsistent. Therefore, we conducted pot experiments to study the effects of the application of two different biochars to a sewage-field soil on growth and nutrition of oat (Avena sativa L.) as well as on the mobility of zinc (Zn), copper (Cu), cadmium (Cd) and lead (Pb) in the soil solution. Poplar-derived hydrochar (obtained by hydrothermal carbonization) and maize-derived pyrochar (obtained by pyrolysis) were mixed with the soil at concentrations of 0, 1, 2.5 and 5% (g 100 g−1). Soils were leached after 2, 3 and 5 weeks with deionized water. Hydrochar reduced biomass production and had almost no effect on metal concentrations in plants and leachates. The negative effect on biomass yield may be attributable to decreased nitrogen (N) availability. Pyrochar increased biomass production and reduced plant Zn and Cd concentrations. In contrast, metal concentrations in soil leachates generally increased. Comparison of ultra-centrifuged and filtered samples, microprobe analysis of centrifugation residues and Visual Minteq calculations suggest colloidal transport of Zn precipitates, whereas Cu is transported in the dissolved fraction. We conclude that the tested hydrochar is not suitable for metal immobilization. In the case of pyrochar, further research on colloidal transport of metal precipitates is needed to assess its suitability for soil remediation, despite positive effects on plant growth and the reduced uptake of Zn and Cd.

Effect of biochar addition on soil respiration partitioning and root dynamics in an apple orchard

Abstract: Summary Biochar addition to soil has been suggested as a promising strategy to increase soil carbon storage with important side-effects on soil fertility and crop productivity. Understanding the effect of biochar on soil respiration partitioning into rhizosphere-derived (Fr) and soil organic carbon-derived (Fsoc) components and on plant root dynamics and microbial activity is a crucial issue in the prediction of the impact of biochar on soil organic carbon and nutrient cycles. Within this framework, an experiment was carried out in an apple (Malus domestica Bork) orchard located in the experimental farm of the Bologna University (Italy). In spring 2009, 10 t of biochar per hectare were incorporated into the surface 20-cm soil layer by soil ploughing. The trenching method was used in order to partition total soil respiration (Fs) into Fr and Fsoc components in both biochar-treated and control soil. Soil respiration measurements were performed from June 2009 to March 2011. To study root dynamics, polycarbonate boxes were built and buried into the soil. Soil profile pictures were collected fortnightly with a CCD sensor scanner inserted in the boxes and analysed with the WinRHIZO Tron MF software. Biochar addition increased Fsoc and reduced Fr, even if the root length intensity (La) increased in biochar-treated soils relative to that in the control. A decrease in root metabolic activity was postulated to explain these contrasting results.

Sorption and desorption characteristics of organic phosphates of different structures on aluminium (oxyhydr)oxides

Abstract: Summary Sorption and desorption characteristics of four organic phosphates (OPs) with different molecular sizes and structures (glycerophosphate, GP; glucose-6-phosphate, G6P; adenosine triphosphate, ATP; myo-inositol hexakisphosphate, IHP) and inorganic phosphate (Pi) on three aluminium (Al) (oxyhydr)oxides (amorphous Al(OH)3, boehmite and α-Al2O3) were investigated. The maximum sorption amounts of OPs and Pi increased with decreasing crystallinity of the minerals on a per mass basis: α-Al2O3 < boehmite < amorphous Al(OH)3. With an exception of IHP sorption on amorphous Al(OH)3, the maximum surface area-based sorption densities increased with decreasing molecular weight (MW) of OPs and Pi: IHP < ATP < G6P < GP < Pi. Despite having the largest MW, IHP had greater sorption amounts on amorphous Al(OH)3 than the other OPs because of the transformation of surface complexes to surface precipitates. Sorption kinetics of OPs was first a rapid sorption followed by a long and slow sorption process. Of the three Al (oxyhydr)oxides, amorphous Al(OH)3 had the greatest first rapid sorption density and initial sorption rate of OPs within 5 minutes, both factors decreasing with increasing MW of OPs. The initial desorption percentages of OPs by KCl generally increased with decreasing MW of OPs, whereas the maximum desorption percentages of OPs by citrate were four to five times those achieved with KCl. Overall, strong specific sorption of OPs occurs on the surface of Al (oxyhydr)oxides, and molecular structure and size of OPs, as well as crystallinity and crystal structure of the minerals, are the key factors affecting the interfacial reactions and environmental behaviour of OPs.

Nitrous oxide emission reduction with greenwaste biochar: comparison of laboratory and field experiments

Abstract: Summary We compared (i) nitrous oxide (N2O) fluxes from a 9-month field experiment established on a temperate, newly sown, intensively managed meadow using automated chambers with fine time resolution, and (ii) fluxes measured at several occasions throughout the experiment under controlled laboratory conditions. Twenty tonnes dry matter ha−1 greenwaste biochar were added to three plots and compared with three control plots. Cumulated N2O field measurements revealed a reduction of 21.5% in the plots with biochar. The reductions for samples where the biochar was added at the beginning of the experiment in the field and samples which were collected each month and measured in the laboratory were in the same range (11.4–39.2%). Emission reductions from laboratory incubations when biochar was freshly mixed with soil in the laboratory were about twice as large (46.5–58.0%). Our results indicate provisionally that, at our site, biochar controls N2O emission through its capacity for reducing NO3− availability to denitrifiers, with the efficiency being related to the effectiveness of mixing of biochar in soil.

Long‐term irrigation effects on soil organic matter under temperate grazed pasture

Abstract: Summary Irrigation of grazed pasture significantly increases plant and animal production, which may in turn increase soil organic carbon (SOC), depending on the balance between primary production and below-ground allocation of C on the one hand, and the decomposition and export of C from the soil on the other. To evaluate the effect of irrigation on SOC we sampled a grazed pasture field experiment maintained under different irrigation treatments for 62 years. The dry-land treatment in this experiment only received rainfall at an average of 740 mm year−1. The 10 and 20% irrigation treatments involved application of 100 mm of irrigation when the soil reached 10 and 20% gravimetric moisture content, respectively. The 10 and 20% irrigation treatments received average total annual irrigation inputs of 260 and 770 mm year−1, respectively. The 10 and 20% irrigation treatments increased pasture production by 44 and 74%, respectively, compared with that from the dry-land. Analysis of soils taken to 1-m depth revealed that amounts of SOC were not significantly different between the dry-land (125.5 Mg ha−1) and 10% irrigation (117.8 Mg ha−1) treatments, but these were significantly greater than the 20% irrigation treatment (93.0 Mg ha−1). At 50–100 cm, SOC was also less (34%) for the 20% irrigation treatment than for the 10% irrigation treatment. The relative quantities of carbon (C) and nitrogen (N) in the light fraction (LF) at all soil depths decreased successively from dry-land to the 20% irrigation treatment, suggesting that wetter soil conditions accelerated decomposition of the LF fraction, a comparatively labile SOC fraction. The C-to-N ratio of the bulk soil was also less for the 20% irrigation treatment, indicating more decomposed SOM in the irrigated than in the dry-land treatment. There were no significant differences in the microbial biomass between the three different irrigation treatments, but the respiration rate (CO2 production) of soil organisms in the 20% irrigation treatment was consistently greater than in the other two treatments. It was concluded that large increases in plant productivity as a result of irrigation had either no effect or significantly reduced SOC stocks under grazed pasture. The reduced SOC content observed in the 20% irrigation treatment was attributed to a combination of increased C losses in animal products and drainage associated with greater stocking, together with accelerated decomposition of organic C resulting from elevated soil moisture maintained throughout the growing season.

A comparison of extraction procedures for water-extractable organic matter in soils

Abstract: The characteristics of dissolved organic matter (DOM) in soils are often determined through laboratory experiments. Many different protocols can be used to extract organic matter from soil. In this study, we used five air-dried soils to compare three extraction methods for water-extractable organic matter (WEOM) as follows: (i) pressurised hot-water-extractable organic carbon (PH-WEOC), a percolation at high pressure and temperature; (ii) water-extractable organic carbon (WEOC), a 1-hour end-over shaking; and (iii) leaching-extractable organic carbon (LEOC), a leaching of soil columns at ambient conditions. We quantified the extraction yield of organic carbon; the quality of WEOM was characterized by UV absorbance, potential biodegradability (48-day incubation) and parallel factor analysis (PARAFAC) modelling of fluorescence excitation emission matrices (FEEMs). Biodegradation of dissolved organic carbon (DOC) was described by two pools of organic C. The proportions of labile and stable DOC pools differed only slightly between the WEOC and LEOC methods, while PH-WEOC contains more stable DOC. The mineralization rate constants of both labile and stable DOC pools were similar for the three methods. The FEEMs were decomposed into three components: two humic-like fluorophores and a tryptophan-like fluorophore. The effect of extraction method was poorly discriminant and the most similar procedures were PH-WEOC and LEOC while WEOC extracts were depleted in humic-like fluorophores. This study demonstrates that WEOM quality is primarily determined by soil characteristics and that the extraction method has a smaller, but still significant, impact on WEOM quality. Furthermore, we observed considerable interaction between extraction procedure and soil type, showing that method-induced differences in WEOM quality vary with soil characteristics.

Biochars in soils: new insights and emerging research needs

Abstract: The quantity, quality and breadth of research connected to biochar have grown rapidly since 2009/2010 when a number of books and reviews were published, some with policy support (Lehmann & Joseph, 2009; Shepherd, 2009; Sohi et al., 2009; Verheijen et al., 2010), and a European Commission biochar event was held at the COP15 meeting in Copenhagen in 2009. Bi-annual conferences organised by the IBI (International Biochar Institute: http://www.biochar-international.org/) have been superseded by sessions and symposia at many disciplinary conferences. Research networks have emerged, notably a European COST action as well as a number of journal special issues (Pesquisa Agropecuária Brasileira , 2012, 47(5); Global Change Biology Bioenergy , 2013, 5(2); Agronomy , 2013, vol. 3(2); Carbon Management, 2014; and a virtual special issue of Soil Biology & Biochemistry . This special issue of the European Journal of Soil Science is the fruit of biochar sessions at the EUROSOIL 2012 conference (held at Bari, Italy). At the time of writing, 1038 articles which included the word ‘biochar’ or ‘bio-char’ in the topic had been indexed in the ISI Web of Science from 2005 to 2012, of which 698 included ‘soil’ in the topic (Figure 1). Approximately one third of all biochar publications do not include ‘soil’ as a topic (black bars in Figure 1), despite soils being their suggested direct or indirect objective. Analysis of key words of these non-soil related biochar publications reveals that the main subject areas are chemical engineering and energy. This possibly reflects a search for better understanding of biochar as a material and the co-production of energy and biochar.. From 2010, the proportion of soil-related biochar publications increased (green bars in Figure 1). To put these numbers into context, we compared the number of biochar publications against a well-established and related topic, that of crop residue return and impact on soil. The red line in Figure 1 (the secondary y-axis) shows publications with ‘biochar & soil’ in the topic as a proportion of publications with ‘crop residue & soil’ in the topic, increased from 5% in 2005 to 37% in 2012. The increasing interest in biochar in soil science stems predominantly from its potential for increasing crop productivity (Atkinson et al., 2010; Jeffery et al., 2011) at the same time as efficiently sequestering carbon in soils (Woolf et al., 2010). Nevertheless, there is a range of additional soil functions and land uses, as well as reports on direct and indirect interactions between soils, biochar and biota. These include, among others, contaminated soil remediation (Beesley et al., 2011; Ennis et al., 2012), restoration of grasslands (Ohsowski et al., 2012); forest management (Zwart & Kim, 2012; Stavi, 2013), promotion of mycorrhizal activity (Warnock et al., 2007), seed germination (Solaiman et al., 2012), plant disease suppression (Elad et al., 2010; Meller Harel et al., 2012), interactions with soil fauna (reviewed by Lehmann et al., 2011; Ameloot et al., 2013) and impacts on pesticides (Kookana, 2010; Graber et al., 2011). It is now firmly understood that as well as influencing soil fertility by improving nutrient retention and exchange, addition of biochar to soils can affect numerous other soil properties and processes, including abiotic and biotic interactions. With biochar production and application to soil increasing in most parts of the world, for scientific research purposes as well as a burgeoning interest for commercial purposes, the need to understand how biochar additions affect soil properties and processes in order to inform regulation has become urgent. Given its wideranging effects and longevity and reactivity in the soil, acquiring such an understanding requires contributions from all relevant soil science disciplines and as well as environmental science and plant science disciplines. As a step towards providing a platform for interdisciplinary exchange, a session titled ‘Effects of Biochar on Soil Properties, Processes and Functions’ was convened at the EUROSOIL 2012. A total of 85 oral and poster presentations were made, constituting the third largest session at the conference after the classical sessions on soil erosion and soil organic matter. Twenty-four manuscripts based on session presentations were submitted to this EJSS peer-reviewed special issue. The 16 accepted manuscripts submitted from Africa, Asia, Australia & New Zealand, Europe, North and Latin America and the Middle East, describe the latest findings on how biochar affects autoand heterotrophic soil respiration, nutrient dynamics, sorption of soil contaminants, water dynamics, redox reactions, and rhizosphere interactions. The methodologies employed span multiple spatial scales including field plots, root-box experiments, lysimeters and greenhouse and laboratory experiments, and also studies that compare effects between different scales. Temporal scales of experiments ranged from 1 day to 21 months (the median was 100 days), while one study compared charcoals added to the soil over 770 years.

Comparison of silicate minerals as sources of potassium for plant nutrition in sandy soil

Abstract: Summary Given the cost of conventional fertilizers and increasing demand as a result of increasing population growth, new sources of potassium (K) for plant nutrition need to be considered. Readily soluble nutrients are rapidly lost from well-drained soils, and so it is appropriate to consider silicate minerals that release K slowly during weathering. In this paper, we compare the availability to plants grown in sandy soils of K from microcline (feldspar), biotite (mica) and nepheline syenite (nepheline + microcline) using leek (Allium ampeloprasum var. porrum L.) as a model plant. Pot experiments were carried out under controlled environmental conditions using natural and artificial soil. The performance of the minerals was compared with treatment with KCl and a negative control (no K added). Plant shoot diameter was measured weekly to assess growth rates. After 10 weeks, plant dry mass and soil and plant contents of soluble K were measured to determine offtake; mineralogical changes in biotite-treated soils were assessed. Results for artificial and natural soil differed, reflecting differences in their mineralogy. With no added K, plant growth ceased after 2 weeks. Growth rates were greatest for KCl, followed by biotite; linear growth continued for 5 weeks in the natural soil and for the entire 10 weeks in the artificial soil. Growth rates with nepheline syenite (natural soil) and microcline (both soils) did not differ significantly from the negative control, but for nepheline syenite, leek shoot K content was significantly greater, demonstrating availability of K from this source. X-ray diffraction analysis showed that biotite reacted to form vermiculite.

Early transformation and transfer processes in a Technosol developing on iron industry deposits

Abstract: Summary Large surface areas covered with man-made materials are subject to pedogenetic evolution However, pedogenetic processes in the resulting Technosols are seldom fully assessed This work was conducted to identify and characterize the processes occurring on deposits of industrial technogenic materials A former settling pond of the iron and steel industry where a forest has established since termination of the industrial activity approximately 50 years ago was chosen A 2-m deep pit was opened, and a series of layers of the soil profile were sampled The macro- and micro-structure were studied, and soil samples were analysed for structural, chemical and mineralogical assessment (chemical analyses, X-ray diffraction, infrared and Mossbauer spectroscopies, scanning and transmission electron microscopies coupled with energy dispersion spectrometry) Results showed that the profile was composed of a succession of sub-horizontal layers arranged in two groups according to their structure and composition, linked to the composition of the industrial effluent Group 1 was composed of iron-, carbonate- and aluminosilicate-rich layers exhibiting a compact structure Group 2 contained manganese-rich layers with a friable structure Pedogenetic processes of various intensities were detected at different depths Besides an accumulation of organic matter at the surface, transformations of minerals were recorded all along the soil profile, with weathering, leaching and precipitation of new phases Phenomena occurred primarily in specific zones, such as cracks and interfaces between two layers In conclusion, the soil maintained characteristics of the original industrial material and displayed several stages of pedogenesis, which were controlled chronologically by climatic and biological factors

Pedogenic and biogenic alkaline‐extracted silicon distributions along a temperate land‐use gradient

Abstract: The primary source of dissolved silicon (Si: DSi) is the weathering of silicate minerals. In recent years, it has been shown that Si cycling through vegetation creates a more soluble Si pool in the soil, as amorphous Si (ASi) deposits in plants (phytoliths) are returned to the soil through litter. Amorphous Si accumulation in soils depends on a number of factors, including land use. In addition to the biogenic ASi fraction, soils contain other non-biogenic amorphous and sorbed Si fractions that could contribute significantly to DSi export to rivers, but hitherto these Si fractions have been difficult to separate from each other with traditionally applied extraction methods. The objective of this paper is to understand better how land use affects the distribution of the different extractable Si fractions. We re-analysed samples from the land-use gradient studied previously by Clymans et al. () with a continuous Si and aluminium (Al) extraction technique. Different extractable Si fractions of biogenic or pedogenic origin were successfully separated on the basis of their dissolution in alkaline solutions (Na2CO3 and NaOH) and Si:Al ratios. We show that forests store almost all alkaline extractable Si (AlkExSi) in the pedogenic fraction while the importance of phytoliths increases with human disturbance to become the dominant fraction in the AlkExSi pool at the arable site. The pedogenic AlkExSi pool is also more reactive than the phytolith-bound Si. Conversely, pastures and croplands tend to preserve phytoliths in the soil, which are less reactive, decreasing the potential of DSi export relative to forested ecosystems. (Less)

Aggregate stability of a crusted soil: differences between crust and sub-crust material, and consequences for interrill erodibility assessment. An example from the Loess Plateau of China

Abstract: Summary Soil interrill erodibility is a key component of soil erosion models. However, when using aggregate stability to assess soil erodibility, samples are usually collected from the plough layer, while soil erosion occurs at the soil surface. Hence, the potential changes in erodibility caused by crusting are ignored. Moreover, soil interrill erodibility is difficult to predict accurately. This lack of predictability means that current erosion models use a constant erodibility value for a given soil, and thus do not consider potential heterogeneity of erodibility. This study was conducted to (i) assess the heterogeneity of aggregate stability for a crusted soil and (ii) relate this heterogeneity to the aggregate stability of the underlying material (sub-crust) and to standard soil properties. A field study was conducted in a small area of the Loess Plateau in China in which the crust and the sub-crust soils were sampled. Standard soil properties (organic matter content, sand content, silt content, clay content, cation exchange capacity (CEC), pH in water, and water content at the time of sampling) were measured as potential explanatory factors of aggregate stability. The results showed a large heterogeneity in aggregate stability among the sites, even though the sites had the same soil type. The mean weight diameter (MWD) of the crust varied between 0.33 and 2.04 mm while the MWD of the sub-crust varied between 0.23 and 1.42 mm. Soil texture and pH were uniform among the sampling sites, whereas water content, organic matter content and CEC varied more. Even though some correlations existed (for example r = 0.57 between MWD for the slow wetting test and organic matter content), none of the standard soil properties was able to predict aggregate stability accurately. The aggregate stability of the crust was significantly greater than that of the sub-crust. The large differences in aggregate stability imply large differences in soil interrill erodibility. Because a single soil type was investigated, this finding proves that erodibility can vary greatly in space even for a given soil type. Soil interrill erodibility should be estimated from the exact material exposed to erosive forces, the soil surface material. Using the sub-crust would have led to greatly over-estimated erodibility and thus to a marked bias in erosion model predictions. Resume La stabilite structurale d'un sol encroute : differences entre la croute et le materiau sous-jacent, et consequences pour l'estimation de l'erodabilite inter-rigole. Un exemple dans le Plateau de Loess (Chine) L'erodabilite inter-rigole est un parametre clef des modeles d'erosion du sol. Cependant, lorsque des tests de stabilite structurale sont utilises pour evaluer l'erodabilite, les mesures sont habituellement realisees sur des echantillons preleves dans l'horizon laboure alors que l'erosion a lieu a la surface du sol. Ainsi, les changements potentiels d'erodabilite causes par la formation de croute sont ignores. De plus, l'erodabilite inter-rigole reste encore difficile a predire avec precision. Ces difficultes conduisent les modeles d'erosion a utiliser une erodabilite constante pour un type de sol donne, et donc a ne pas considerer l'heterogeneite potentielle de l'erodabilite. Cette etude a ete conduite pour (i) evaluer l'heterogeneite de la stabilite structurale pour un sol encroute et (ii) relier cette heterogeneite a la stabilite structurale du materiau sous-jacent (sous-croute) et aux proprietes standards du sol. Une etude de terrain a ete realisee sur un secteur de surface limitee du Plateau de Lœss (Chine). Des echantillons provenant de la croute et de la sous-croute ont ete collectes. Les proprietes standards (teneur en carbone organique, teneurs en sable, limon et argile, CEC, pH, et teneur en eau au prelevement), ont ete mesurees en tant que facteurs explicatifs potentiels de la stabilite structurale. Les resultats ont montre une grande heterogeneite de la stabilite structurale entre les differents sites alors que ces derniers presentaient le meme type de sol. Le MWD de la croute variait entre 0.33 et 2.04 mm tandis que le MWD de la sous-croute variait entre 0.23 et 1.42 mm. La texture du sol et le pH etaient tres homogenes entre les sites etudies, tandis que la teneur en eau, la teneur en matiere organique et la CEC variaient plus fortement. Bien que certaines correlations aient ete identifiees (par exemple r = 0.57 entre le MWD du test a l'humectation lente et la teneur en carbone organique), aucune de ces proprietes n'a permis de predire precisement la stabilite structurale. La stabilite structurale de la croute etait significativement superieure a celle de la sous-croute. Les grandes differences de stabilite structurale mesurees impliquent des erodabilites tres contrastees. Comme un seul type de sol a ete etudie, ce resultat prouve que l'erodabilite peut etre tres variable spatialement pour un type de sol donne. L'erodabilite inter-rigole du sol devrait etre mesuree sur le materiau exact qui subit l'erosion, c'est-a-dire le materiau de surface. L'utilisation du materiau sous-jacent aurait engendre une forte surestimation de l'erodabilite et donc un biais important dans les predictions d'un modele d'erosion.

Ranking factors affecting emissions of GHG from incubated agricultural soils.

Abstract: Agriculture significantly contributes to global greenhouse gas (GHG) emissions and there is a need to develop effective mitigation strategies. The efficacy of methods to reduce GHG fluxes from agricultural soils can be affected by a range of interacting management and environmental factors. Uniquely, we used the Taguchi experimental design methodology to rank the relative importance of six factors known to affect the emission of GHG from soil: nitrate (NO3-) addition, carbon quality (labile and non-labile C), soil temperature, water-filled pore space (WFPS) and extent of soil compaction. Grassland soil was incubated in jars where selected factors, considered at two or three amounts within the experimental range, were combined in an orthogonal array to determine the importance and interactions between factors with a L16 design, comprising 16 experimental units. Within this L16 design, 216 combinations of the full factorial experimental design were represented. Headspace nitrous oxide (N2O), methane (CH4) and carbon dioxide (CO2) concentrations were measured and used to calculate fluxes. Results found for the relative influence of factors (WFPS and NO3- addition were the main factors affecting N2O fluxes, whilst glucose, NO3- and soil temperature were the main factors affecting CO2 and CH4 fluxes) were consistent with those already well documented. Interactions between factors were also studied and results showed that factors with little individual influence became more influential in combination. The proposed methodology offers new possibilities for GHG researchers to study interactions between influential factors and address the optimized sets of conditions to reduce GHG emissions in agro-ecosystems, while reducing the number of experimental units required compared with conventional experimental procedures that adjust one variable at a time.

Phosphorus fractions in bulk subsoil and its biopore systems

Abstract: Summary Improving phosphorus (P) accessibility in subsoils could be a key factor for sustainable crop management. This study aims to explain the quantity of different P fractions in subsoil and its biopore systems, and to test the hypothesis that crops with either fibrous (fescue) or tap-root systems (lucerne and chicory) leave behind a characteristic P pattern in bulk subsoil, biopore linings and the rhizosphere. The crops were cultivated for up to 3 years in a randomized field experiment on a Haplic Luvisol developed from loess. Aqua regia-extractable P (referred to as total P) and calcium acetate lactate-extractable P (PCAL) were assessed at 0–30 (Ap horizon), 30–45 (E/B horizon), 45–75 and 75–105 cm subsoil depths. In addition, sequential P fractionation was performed on different soil compartments between 45 and 75 cm depths. The results showed that total P stocks below the Ap horizon (30–105 cm) amounted to 5.6 t ha−1, which was twice as large as in the Ap, although the Ap contained larger portions of PCAL. Both PCAL and sequential P extractions showed that biopore linings and the rhizosphere at the 45–75 cm depth were enriched, rather than depleted, in P. The content of inorganic P (81–90% of total P) increased in the following order: bulk soil = biopores 2 mm. Biopores >2 mm and rhizosphere soil were clearly enriched in resin- and NaHCO3-extractable Pi and Po fractions. However, we failed to attribute these P distribution patterns to different crops, suggesting that major properties of biopore P originated from relict biopores, rather than being influenced by recent root systems. The stocks of the sum of these P fractions in the bulk subsoil (182 kg ha−1 at 45–75 cm depth) far exceeded those in the biopores (3.7 kg ha−1 in biopores >2 mm and 0.2 kg ha−1 in biopores <2 mm). Hence, these biopores may form attractive locations for root growth into the subsoil but are unlikely to sustain overall plant nutrition.

Nitrogen deposition impacts on the amount and stability of soil organic matter in an alpine meadow ecosystem depend on the form and rate of applied nitrogen

Abstract: The effects of atmospheric nitrogen (N) deposition on carbon (C) sequestration in terrestrial ecosystems are controversial. Therefore, it is important to evaluate accurately the effects of applied N levels and forms on the amount and stability of soil organic carbon (SOC) in terrestrial ecosystems. In this study, a multi-form, small-input N addition experiment was conducted at the Haibei Alpine Meadow Ecosystem Research Station from 2007 to 2011. Three N fertilizers, NH4Cl, (NH4)(2)SO4 and KNO3, were applied at four rates: 0, 10, 20 and 40 kg N ha(-1) year(-1). One hundred and eight soil samples were collected at 10-cm intervals to a depth of 30 cm in 2011. Contents and delta C-13 values of bulk SOC were measured, as well as three particle-size fractions: macroparticulate organic C (MacroPOC, > 250 mu m), microparticulate organic C (MicroPOC, 53-250 mu m) and mineral-associated organic C (MAOC, 250 mu m soil MacroPOC, and the C lost in the medium or large N treatments was from the > 53 mu m POC fraction. Five years of ammonium-N addition increased significantly the surface soil POC:MAOC ratio and increased the instability of soil organic matter (SOM). These results suggest that exogenous N input within the critical load level will benefit C sequestration in the alpine meadow soils on the Qinghai-Tibetan Plateau over the short term.