Showing papers on "Soil organic matter published in 2013"

The Microbial Efficiency-Matrix Stabilization (MEMS) framework integrates plant litter decomposition with soil organic matter stabilization: do labile plant inputs form stable soil organic matter?

Abstract: The decomposition and transformation of above- and below-ground plant detritus (litter) is the main process by which soil organic matter (SOM) is formed. Yet, research on litter decay and SOM formation has been largely uncoupled, failing to provide an effective nexus between these two fundamental processes for carbon (C) and nitrogen (N) cycling and storage. We present the current understanding of the importance of microbial substrate use efficiency and C and N allocation in controlling the proportion of plant-derived C and N that is incorporated into SOM, and of soil matrix interactions in controlling SOM stabilization. We synthesize this understanding into the Microbial Efficiency-Matrix Stabilization (MEMS) framework. This framework leads to the hypothesis that labile plant constituents are the dominant source of microbial products, relative to input rates, because they are utilized more efficiently by microbes. These microbial products of decomposition would thus become the main precursors of stable SOM by promoting aggregation and through strong chemical bonding to the mineral soil matrix.

Biochar and its effects on plant productivity and nutrient cycling: a meta‐analysis

Abstract: Biochar is a carbon-rich coproduct resulting from pyrolyzing biomass. When applied to the soil it resists decomposition, effectively sequestering the applied carbon and mitigating anthropogenic CO2 emissions. Other promoted benefits of biochar application to soil include increased plant productivity and reduced nutrient leaching. However, the effects of biochar are variable and it remains unclear if recent enthusiasm can be justified. We evaluate ecosystem responses to biochar application with a meta-analysis of 371 independent studies culled from 114 published manuscripts. We find that despite variability introduced by soil and climate, the addition of biochar to soils resulted, on average, in increased aboveground productivity, crop yield, soil microbial biomass, rhizobia nodulation, plant K tissue concentration, soil phosphorus (P), soil potassium (K), total soil nitrogen (N), and total soil carbon (C) compared with control conditions. Soil pH also tended to increase, becoming less acidic, following the addition of biochar. Variables that showed no significant mean response to biochar included belowground productivity, the ratio of aboveground : belowground biomass, mycorrhizal colonization of roots, plant tissue N, and soil P concentration, and soil inorganic N. Additional analyses found no detectable relationship between the amount of biochar added and aboveground productivity. Our results provide the first quantitative review of the effects of biochar on multiple ecosystem functions and the central tendencies suggest that biochar holds promise in being a win-win-win solution to energy, carbon storage, and ecosystem function. However, biochar's impacts on a fourth component, the downstream nontarget environments, remain unknown and present a critical research gap.

Roots and associated fungi drive long-term carbon sequestration in boreal forest.

Abstract: Boreal forest soils function as a terrestrial net sink in the global carbon cycle. The prevailing dogma has focused on aboveground plant litter as a principal source of soil organic matter. Using C-14 bomb-carbon modeling, we show that 50 to 70% of stored carbon in a chronosequence of boreal forested islands derives from roots and root-associated microorganisms. Fungal biomarkers indicate impaired degradation and preservation of fungal residues in late successional forests. Furthermore, 454 pyrosequencing of molecular barcodes, in conjunction with stable isotope analyses, highlights root-associated fungi as important regulators of ecosystem carbon dynamics. Our results suggest an alternative mechanism for the accumulation of organic matter in boreal forests during succession in the long-term absence of disturbance.

Competition between roots and microorganisms for nitrogen: mechanisms and ecological relevance

Abstract: Demand of all living organisms on the same nutrients forms the basis for interspecific competition between plants and microorganisms in soils. This competition is especially strong in the rhizosphere. To evaluate competitive and mutualistic interactions between plants and microorganisms and to analyse ecological consequences of these interactions, we analysed 424 data pairs from 41 (15)N-labelling studies that investigated (15)N redistribution between roots and microorganisms. Calculated Michaelis-Menten kinetics based on K(m) (Michaelis constant) and V(max) (maximum uptake capacity) values from 77 studies on the uptake of nitrate, ammonia, and amino acids by roots and microorganisms clearly showed that, shortly after nitrogen (N) mobilization from soil organic matter and litter, microorganisms take up most N. Lower K(m) values of microorganisms suggest that they are especially efficient at low N concentrations, but can also acquire more N at higher N concentrations (V(max)) compared with roots. Because of the unidirectional flow of nutrients from soil to roots, plants are the winners for N acquisition in the long run. Therefore, despite strong competition between roots and microorganisms for N, a temporal niche differentiation reflecting their generation times leads to mutualistic relationships in the rhizosphere. This temporal niche differentiation is highly relevant ecologically because it: protects ecosystems from N losses by leaching during periods of slow or no root uptake; continuously provides roots with available N according to plant demand; and contributes to the evolutionary development of mutualistic interactions between roots and microorganisms.

Responses of soil heterotrophic respiration to moisture availability: An exploration of processes and models

Abstract: Soil moisture strongly affects the dynamics of soil organic matter and is an important environmental variable in all models predicting changes in soil carbon stocks from site to global scales. Despite this, the mechanisms determining the response of heterotrophic soil respiration to soil moisture remain poorly quantified, being represented in most current carbon cycle models as simple empirical functions. With the aim of providing an overview and new insights into the mechanisms involved, here we review the observations and theory behind the respiration-moisture relationship. We start by calculating best estimates of average empirical relationships using published data and comparing the results for contrasting soil types. The theoretical linkages between substrate and gas diffusivity in soil pores and heterotrophic respiration are then explored as a function of temperature and textural properties. Based on this theoretical model we interpret the variability of moisture effects observed in previous empirical studies. Transient CO2 efflux–moisture relationships are discussed next, reviewing the theory and models developed in the last years with particular emphasis on the ‘Birch effect’. We continue by giving an overview of recent pore-scale models and consider how these can be used to gain a more mechanistic understanding of carbon storage and stabilization in variably saturated soils. From this review we conclude that current empirical models are useful but limited approximations, with questionable predictive capacity. Significant efforts are still necessary to represent the full range of soil moisture responses in a unifying model with a sound theoretical basis that can help identify common underlying processes. Equations present here, combining diffusion, texture and substrate to model respiration, are a step forward in this direction.

The temperature response of soil microbial efficiency and its feedback to climate

Abstract: Soils are the largest repository of organic carbon in the terrestrial biosphere. Nevertheless, relatively little is known about the factors controlling the efficiency with which microbial communities utilize carbon, and its effect on soil–atmosphere CO2 exchange. Now research using long-term experimental plots suggests that climate warming could alter the decay dynamics of more stable organic-matter compounds with implications for carbon storage in soils and ultimately climate warming. Soils are the largest repository of organic carbon (C) in the terrestrial biosphere and represent an important source of carbon dioxide (CO2) to the atmosphere, releasing 60–75 Pg C annually through microbial decomposition of organic materials1,2. A primary control on soil CO2 flux is the efficiency with which the microbial community uses C. Despite its critical importance to soil–atmosphere CO2 exchange, relatively few studies have examined the factors controlling soil microbial efficiency. Here, we measured the temperature response of microbial efficiency in soils amended with substrates varying in lability. We also examined the temperature sensitivity of microbial efficiency in response to chronic soil warming in situ. We find that the efficiency with which soil microorganisms use organic matter is dependent on both temperature and substrate quality, with efficiency declining with increasing temperatures for more recalcitrant substrates. However, the utilization efficiency of a more recalcitrant substrate increased at higher temperatures in soils exposed to almost two decades of warming 5 °C above ambient. Our work suggests that climate warming could alter the decay dynamics of more stable organic matter compounds, thereby having a positive feedback to climate that is attenuated by a shift towards a more efficient microbial community in the longer term.

Long-term fate of nitrate fertilizer in agricultural soils.

Abstract: Increasing diffuse nitrate loading of surface waters and groundwater has emerged as a major problem in many agricultural areas of the world, resulting in contamination of drinking water resources in aquifers as well as eutrophication of freshwaters and coastal marine ecosystems. Although empirical correlations between application rates of N fertilizers to agricultural soils and nitrate contamination of adjacent hydrological systems have been demonstrated, the transit times of fertilizer N in the pedosphere–hydrosphere system are poorly understood. We investigated the fate of isotopically labeled nitrogen fertilizers in a three–decade-long in situ tracer experiment that quantified not only fertilizer N uptake by plants and retention in soils, but also determined to which extent and over which time periods fertilizer N stored in soil organic matter is rereleased for either uptake in crops or export into the hydrosphere. We found that 61–65% of the applied fertilizers N were taken up by plants, whereas 12–15% of the labeled fertilizer N were still residing in the soil organic matter more than a quarter century after tracer application. Between 8–12% of the applied fertilizer had leaked toward the hydrosphere during the 30-y observation period. We predict that additional exports of 15N-labeled nitrate from the tracer application in 1982 toward the hydrosphere will continue for at least another five decades. Therefore, attempts to reduce agricultural nitrate contamination of aquatic systems must consider the long-term legacy of past applications of synthetic fertilizers in agricultural systems and the nitrogen retention capacity of agricultural soils.

Soil Erosion Threatens Food Production

Abstract: Since humans worldwide obtain more than 99.7% of their food (calories) from the land and less than 0.3% from the oceans and aquatic ecosystems, preserving cropland and maintaining soil fertility should be of the highest importance to human welfare. Soil erosion is one of the most serious threats facing world food production. Each year about 10 million ha of cropland are lost due to soil erosion, thus reducing the cropland available for world food production. The loss of cropland is a serious problem because the World Health Organization and the Food and Agricultural Organization report that two-thirds of the world population is malnourished. Overall, soil is being lost from agricultural areas 10 to 40 times faster than the rate of soil formation imperiling humanity’s food security.

Soil compaction impact and modelling. A review

Abstract: Compaction of agricultural soils is a concern for many agricultural soil scientists and farmers since soil compaction, due to heavy field traffic, has resulted in yield reduction of most agronomic crops throughout the world. Soil compaction is a physical form of soil degradation that alters soil structure, limits water and air infiltration, and reduces root penetration in the soil. Consequences of soil compaction are still underestimated. A complete understanding of processes involved in soil compaction is necessary to meet the future global challenge of food security. We review here the advances in understanding, quantification, and prediction of the effects of soil compaction. We found the following major points: (1) When a soil is exposed to a vehicular traffic load, soil water contents, soil texture and structure, and soil organic matter are the three main factors which determine the degree of compactness in that soil. (2) Soil compaction has direct effects on soil physical properties such as bulk density, strength, and porosity; therefore, these parameters can be used to quantify the soil compactness. (3) Modified soil physical properties due to soil compaction can alter elements mobility and change nitrogen and carbon cycles in favour of more emissions of greenhouse gases under wet conditions. (4) Severe soil compaction induces root deformation, stunted shoot growth, late germination, low germination rate, and high mortality rate. (5) Soil compaction decreases soil biodiversity by decreasing microbial biomass, enzymatic activity, soil fauna, and ground flora. (6) Boussinesq equations and finite element method models, that predict the effects of the soil compaction, are restricted to elastic domain and do not consider existence of preferential paths of stress propagation and localization of deformation in compacted soils. (7) Recent advances in physics of granular media and soil mechanics relevant to soil compaction should be used to progress in modelling soil compaction.

Life in the ‘charosphere’ - Does biochar in agricultural soil provide a significant habitat for microorganisms?

Abstract: Biochar application has become a novel and emergent technology for sequestering C, improving soil quality and crop production, and is a potential win–win strategy for ecosystem service delivery. Biochar addition can also stimulate soil microbial activity, and although it is unclear exactly why biochar should benefit soil microorganisms, it is thought that the large surface area and volume of pores provide a significant habitat for microbes. The aim of this study was to determine the level of microbial colonisation of wood-derived biochar that had been buried in an agricultural soil for three years. We have examined the level of colonisation on the internal and external surfaces of field-aged biochar by scanning electron microscopy, and used 14C-labelled glucose to quantify the rates of microbial activity in different spatial niches of the biochar and the surrounding soil. Microbial colonisation of field-aged biochar was very sparse, with no obvious differences between the external and internal surfaces. At the high field application rate of 50 t ha−1, biochar contributed only 6.52 ± 0.11% of the total soil pore space and 7.35 ± 0.81% of the total soil surface area of the topsoil (0–30 cm). Further, 17.46 ± 0.02% of the biochar pores were effectively uninhabitable for most microbes, being

Biochar Impacts on Soil Physical Properties and Greenhouse Gas Emissions

Abstract: Biochar, a co-product of a controlled pyrolysis process, can be used as a tool for sequestering C in soil to offset greenhouse gas (GHG) emissions, and as a soil amendment. Whereas the impacts of biochar application on soil chemical properties are widely known, the research information on soil physical properties is scarce. The objectives of this review are to (i) synthesize available data on soil physical properties and GHG emissions, (ii) offer possible mechanisms related to the biochar-amended soil processes, and (iii) identify researchable priorities. Application rates of 1%–2% (w/w) of biochar can significantly improve soil physical quality in terms of bulk density (BD), and water holding capacity (WHC). However, little data are available on surface area (SA), aggregation stability, and penetration resistance (PR) of biochar-amended soil. While biochar amendment can initially accentuate the flux of carbon dioxide (CO2), the emission of GHGs may be suppressed over time. A 2-phase complexation hypothesis is proposed regarding the mechanisms of the interaction between soil and biochar.

Rhizosphere priming: a nutrient perspective.

Abstract: Rhizosphere priming is the change in decomposition of soil organic matter (SOM) caused by root activity. Rhizosphere priming plays a crucial role in soil carbon (C) dynamics and their response to global climate change. Rhizosphere priming may be affected by soil nutrient availability, but rhizosphere priming itself can also affect nutrient supply to plants. These interactive effects may be of particular relevance in understanding the sustained increase in plant growth and nutrient supply in response to a rise in atmospheric CO2 concentration. We examined how these interactions were affected by elevated CO2 in two similar semiarid grassland field studies. We found that an increase in rhizosphere priming enhanced the release of nitrogen (N) through decomposition of a larger fraction of SOM in one study, but not in the other. We postulate that rhizosphere priming may enhance N supply to plants in systems that are N limited, but that rhizosphere priming may not occur in systems that are phosphorus (P) limited. Under P limitation, rhizodeposition may be used for mobilisation of P, rather than for decomposition of SOM. Therefore, with increasing atmospheric CO2 concentrations, rhizosphere priming may play a larger role in affecting C sequestration in N poor than in P poor soils.

Rhizosphere Priming: a Nutrient Perspective

Abstract: Rhizosphere priming is the change in decomposition of soil organic matter (SOM) caused by root activity. Rhizosphere priming plays a crucial role in soil carbon (C) dynamics and their response to global climate change. Rhizosphere priming may be affected by soil nutrient availability, but rhizosphere priming itself can also affect nutrient supply to plants. These interactive effects may be of particular relevance in understanding the sustained increase in plant growth and nutrient supply in response to a rise in atmospheric CO2 concentration. We examined how these interactions were affected by elevated CO2 in two similar semiarid grassland field studies. We found that an increase in rhizosphere priming enhanced the release of nitrogen (N) through decomposition of a larger fraction of SOM in one study, but not in the other. We postulate that rhizosphere priming may enhance N supply to plants in systems that are N limited, but that rhizosphere priming may not occur in systems that are phosphorus (P) limited. Under P limitation, rhizodeposition may be used for mobilisation of P, rather than for decomposition of SOM. Therefore, with increasing atmospheric CO2 concentrations, rhizosphere priming may play a larger role in affecting C sequestration in N poor than in P poor soils.

Soil nitrous oxide emissions following crop residue addition: a meta‐analysis

Abstract: Annual production of crop residues has reached nearly 4 billion metric tons globally. Retention of this large amount of residues on agricultural land can be beneficial to soil C sequestration. Such potential impacts, however, may be offset if residue retention substantially increases soil emissions of N(2)O, a potent greenhouse gas and ozone depletion substance. Residue effects on soil N(2)O emissions have gained considerable attention since early 1990s; yet, it is still a great challenge to predict the magnitude and direction of soil N(2)O emissions following residue amendment. Here, we used a meta-analysis to assess residue impacts on soil N(2)O emissions in relation to soil and residue attributes, i.e., soil pH, soil texture, soil water content, residue C and N input, and residue C : N ratio. Residue effects were negatively associated with C : N ratios, but generally residue amendment could not reduce soil N(2)O emissions, even for C : N ratios well above ca. 30, the threshold for net N immobilization. Residue effects were also comparable to, if not greater than, those of synthetic N fertilizers. In addition, residue effects on soil N(2)O emissions were positively related to the amounts of residue C input as well as residue effects on soil CO(2) respiration. Furthermore, most significant and stimulatory effects occurred at 60-90% soil water-filled pore space and soil pH 7.1-7.8. Stimulatory effects were also present for all soil textures except sand or clay content ≤10%. However, inhibitory effects were found for soils with >90% water-filled pore space. Altogether, our meta-analysis suggests that crop residues played roles beyond N supply for N(2)O production. Perhaps, by stimulating microbial respiration, crop residues enhanced oxygen depletion and therefore promoted anaerobic conditions for denitrification and N(2)O production. Our meta-analysis highlights the necessity to connect the quantity and quality of crop residues with soil properties for predicting soil N(2)O emissions.

Soil health: looking for suitable indicators. What should be considered to assess the effects of use and management on soil health?

Abstract: Soil Health refers to the ecological equilibrium and the functionality of a soil and its capacity to maintain a well balanced ecosystem with high biodiversity above and below surface, and productivity. To understand and use soil health as a tool for sustainability, physical, chemical, and biological properties must be employed to verify which respond to the soil use and management within a desired timescale. Attributes with a rapid response to natural or anthropogenic actions are considered good indicators of soil health. Among the physical indicators, soil texture, aggregation, moisture, porosity, and bulk density have been used, while among chemical indicators total C and N, mineral nutrients, organic matter, cation exchange capacity, among others are well established. However, most of them generally have a slow response, when compared to the biological ones, such as microbial biomass C and N, biodiversity, soil enzymes, soil respiration, etc., in addition to macro and mesofauna. Thus, a systemic approach based on different kinds of indicators (physical, chemical and biological) in assessing soil health would be safer than using only one kind of attribute. Many human activities have caused desertification, loss of biodiversity, disruption of aggregates, loss of organic matter and nutrients, among others. Today, it is imperious to maintain soil health and productivity with increasing emphasis on reforestation and recuperation of degraded areas through the use of organic amendments, reintroduction of plants, soil fauna and microorganisms. This review focused on an integrative view on indicators of soil health to be used as tools for prediction of sustainability in production systems.

Environmental and physical controls on northern terrestrial methane emissions across permafrost zones

Abstract: Methane (CH4) emissions from the northern high-latitude region represent potentially significant biogeochemical feedbacks to the climate system. We compiled a database of growing-season CH4 emissions from terrestrial ecosystems located across permafrost zones, including 303 sites described in 65 studies. Data on environmental and physical variables, including permafrost conditions, were used to assess controls on CH4 emissions. Water table position, soil temperature, and vegetation composition strongly influenced emissions and had interacting effects. Sites with a dense sedge cover had higher emissions than other sites at comparable water table positions, and this was an effect that was more pronounced at low soil temperatures. Sensitivity analysis suggested that CH4 emissions from ecosystems where the water table on average is at or above the soil surface (wet tundra, fen underlain by permafrost, and littoral ecosystems) are more sensitive to variability in soil temperature than drier ecosystems (palsa dry tundra, bog, and fen), whereas the latter ecosystems conversely are relatively more sensitive to changes of the water table position. Sites with near-surface permafrost had lower CH4 fluxes than sites without permafrost at comparable water table positions, a difference that was explained by lower soil temperatures. Neither the active layer depth nor the organic soil layer depth was related to CH4 emissions. Permafrost thaw in lowland regions is often associated with increased soil moisture, higher soil temperatures, and increased sedge cover. In our database, lowland thermokarst sites generally had higher emissions than adjacent sites with intact permafrost, but emissions from thermokarst sites were not statistically higher than emissions from permafrost-free sites with comparable environmental conditions. Overall, these results suggest that future changes to terrestrial high-latitude CH4 emissions will be more proximately related to changes in moisture, soil temperature, and vegetation composition than to increased availability of organic matter following permafrost thaw.

Prediction of Soil Organic Carbon at the European Scale by Visible and Near InfraRed Reflectance Spectroscopy

Abstract: Soil organic carbon is a key soil property related to soil fertility, aggregate stability and the exchange of CO2 with the atmosphere. Existing soil maps and inventories can rarely be used to monitor the state and evolution in soil organic carbon content due to their poor spatial resolution, lack of consistency and high updating costs. Visible and Near Infrared diffuse reflectance spectroscopy is an alternative method to provide cheap and high-density soil data. However, there are still some uncertainties on its capacity to produce reliable predictions for areas characterized by large soil diversity. Using a large-scale EU soil survey of about 20,000 samples and covering 23 countries, we assessed the performance of reflectance spectroscopy for the prediction of soil organic carbon content. The best calibrations achieved a root mean square error ranging from 4 to 15 g C kg−1 for mineral soils and a root mean square error of 50 g C kg−1 for organic soil materials. Model errors are shown to be related to the levels of soil organic carbon and variations in other soil properties such as sand and clay content. Although errors are ∼5 times larger than the reproducibility error of the laboratory method, reflectance spectroscopy provides unbiased predictions of the soil organic carbon content. Such estimates could be used for assessing the mean soil organic carbon content of large geographical entities or countries. This study is a first step towards providing uniform continental-scale spectroscopic estimations of soil organic carbon, meeting an increasing demand for information on the state of the soil that can be used in biogeochemical models and the monitoring of soil degradation.

Relative contributions of plant traits and soil microbial properties to mountain grassland ecosystem services

Abstract: Summary 1. Plant functional diversity and soil microbial community composition are tightly coupled. Changes in these interactions may influence ecosystem functioning. Links between plant functional diversity, soil microbial communities and ecosystem functioning have been demonstrated in experiments using plant monocultures and mixtures, using broad plant and microbial functional groups, but have not been examined in diverse natural plant communities. 2. We quantified the relative effects of plant and microbial functional properties on key ecosystem functions. We measured plant functional diversity, soil microbial community composition and parameters associated with nitrogen (N) cycling and key nutrient cycling processes at three grassland sites in different parts of Europe. Because plant structure and function strongly influence soil microbial communities, we determined relationships between ecosystem properties, plant traits and soil community characteristics following a sequential approach in which plant traits were fitted first, followed by the additional effects of soil micro-organisms. 3. We identified a continuum from standing green biomass and standing litter, linked mostly with plant traits, to potential N mineralization and potential leaching of soil inorganic N, linked mostly with microbial properties. Plant and microbial functional parameters were equally important in explaining % organic matter content in soil. A parallel continuum ran from plant height, linked with above-ground biomass, to plant quality effects captured by the leaf economics spectrum, which were linked with the recycling of carbon (C) and N. 4. More exploitative species (higher specific leaf area, leaf N concentrations and lower leaf dry matter content) and taller swards, along with soil microbial communities dominated by bacteria, with rapid microbial activities, were linked with greater fodder production, but poor C and N retention. Conversely, dominance by conservative species (with opposite traits) and soil microbial communities dominated by fungi, and bacteria with slow activities, were usually linked with low production, but greater soil C storage and N retention. 5. Synthesis – Grassland production, C sequestration and soil N retention are jointly related to plant and microbial functional traits. Managing grasslands for selected, or multiple, ecosystem services will thus require a consideration of the joint effects of plant and soil communities. Further understanding of the mechanisms that link plant and microbial functional traits is essential to achieve this.

Thermal adaptation of decomposer communities in warming soils.

Abstract: Temperature regulates the rate of biogeochemical cycles. One way it does so is through control of microbial metabolism. Warming effects on metabolism change with time as physiology adjusts to the new temperature. I here propose that such thermal adaptation is observed in soil microbial respiration and growth, as the result of universal evolutionary trade-offs between the structure and function of both enzymes and membranes. I review the basis for these trade-offs and show that they, like substrate depletion, are plausible mechanisms explaining soil respiration responses to warming. I argue that controversies over whether soil microbes adapt to warming stem from disregarding the evolutionary physiology of cellular metabolism, and confusion arising from the term thermal acclimation to represent phenomena at the organism- and ecosystem-levels with different underlying mechanisms. Measurable physiological adjustments of the soil microbial biomass reflect shifts from colder- to warmer-adapted taxa. Hypothesized declines in the growth efficiency of soil microbial biomass under warming are controversial given limited data and a weak theoretical basis. I suggest that energy spilling (aka waste metabolism) is a more plausible mechanism for efficiency declines than the commonly invoked increase in maintenance-energy demands. Energy spilling has many fitness benefits for microbes and its response to climate warming is uncertain. Modeled responses of soil carbon to warming are sensitive to microbial growth efficiency, but declines in efficiency mitigate warming-induced carbon losses in microbial models and exacerbate them in conventional models. Both modeling structures assume that microbes regulate soil carbon turnover, highlighting the need for a third structure where microbes are not regulators. I conclude that microbial physiology must be considered if we are to have confidence in projected feedbacks between soil carbon stocks, atmospheric CO2, and climate change.

Carbon-nutrient stoichiometry to increase soil carbon sequestration

Abstract: The more stable fine fraction pool of soil organic matter (FF-SOM; <0.4 mm) has more nitrogen, phosphorus and sulphur (N, P, S) per unit of carbon (C) than the plant material from which it originates and has near constant ratios of C:N:P:S. Consequently, we hypothesised that the sequestration of C-rich crop residue material into the FF-SOM pool could be improved by adding supplementary nutrients to the residues based on these ratios. Here we report on the effect of N, P and S availability on the net humification efficiency (NHE), the change in the size of the FF-SOM pool (as estimated by fine fraction C (FF-C)), following incubation of soil with wheaten straw. Four diverse soils were subjected to seven consecutive incubation cycles, with wheaten straw (10 t ha equivalent) added at the beginning of each cycle, with and without inorganic N, P, S addition (5 kg N, 2 kg P and 1.3 kg S per tonne of straw). This nutrient addition doubled the mean NHE in all soils (from 7% to 15%) and when applied at twice the rate increased NHE further (up to 29%) for the two soils that received this treatment. The FF-N, -P and -S levels increased in concert with FF-C levels in all soils in close agreement with published stoichiometric ratios (C:N:P:S = 10,000:833:200:143). Microbial biomass-C (MB-C) levels were estimated during one incubation cycle and found to increase in parallel with FF-C from 448 μg MB-C g soil (no nutrient addition) to 727 μg MB-C g soil (plus nutrients) and 947 μg MB-C g soil (plus 2× nutrients). There was a significant relationship between MB-C and the change in FF-C during that incubation cycle, providing evidence of a close relationship between the microbial biomass and FF-SOM formation. The two to four-fold increases in NHE achieved with nutrient addition demonstrated that inorganic nutrient availability is critical to sequester C into the more stable FF-SOM pool irrespective of soil type and C input. This has important implications for strategies to build soil fertility or mitigate climate change via increased soil organic C, as the availability and value of these nutrients must be considered.

Storage and stability of organic carbon in soils as related to depth, occlusion within aggregates, and attachment to minerals

Abstract: Conceptual models suggest that stability of or- ganic carbon (OC) in soil depends on the source of plant litter, occlusion within aggregates, incorporation in organo- mineral complexes, and location within the soil profile. Den- sity fractionation is a useful tool to study the relevance of OC stabilization in aggregates and in association with min- erals, but it has rarely been applied to full soil profiles. We aim to determine factors shaping the depth profiles of phys- ically unprotected and mineral associated OC and test their relevance for OC stability across a range of European soils that vary in vegetation, soil types, parent material, and land use. At each of the 12 study sites, 10 soil cores were sam- pled to 60 cm depth and subjected to density separation. Bulk soil samples and density fractions (free light fractions - fLF, occluded light fractions - oLF, heavy fractions - HF) were analysed for OC, total nitrogen (TN), 13 C, and 1 14 C. Bulk samples were also incubated to determine CO2 evolution per g OC in the samples (specific mineralization rates) as an in- dicator for OC stability. Depth profiles of OC in the light fraction (LF-OC) matched those of roots for undisturbed grassland and forest sites, suggesting that roots are shaping the depth distribution of LF-OC. Organic C in the HF declined less with soil depth than LF-OC and roots, especially at grassland sites. The de- crease in 1 14 C (increase in age) of HF-OC with soil depth was related to soil pH as well as to dissolved OC fluxes. This indicates that dissolved OC translocation contributes to the formation of subsoil HF-OC and shapes the 1 14 C profiles. The LF at three sites were rather depleted in 14 C, indi- cating the presence of fossil material such as coal and lig- nite, probably inherited from the parent material. At the other sites, modern 1 14 C signatures and positive correlations be- tween specific mineralization rates and fLF-OC indicate the fLF is a potentially available energy and nutrient source for subsurface microorganisms throughout the profile. Declining specific mineralization rates with soil depth confirm greater stability of OC in subsoils across sites. The overall impor- tance of OC stabilization by binding to minerals was demon- strated by declining specific mineralization rates with in- creasing contributions of HF-OC to bulk soil OC, and the low 1 14 C values of HF-OC. The stability of HF-OC was greater in subsoils than in topsoils; nevertheless, a portion of HF-OC was active throughout the profile. While quanti- tatively less important than OC in the HF, consistent older ages of oLF-OC than fLF-OC suggest that occlusion of LF- OC in aggregates also contributes to OC stability in subsoils. Overall, our results indicate that association with minerals is the most important factor in stabilization of OC in soils, irre- spective of vegetation, soil type, and land use.

Stimulation of Different Functional Groups of Bacteria by Various Plant Residues as a Driver of Soil Priming Effect

Abstract: The turnover of organic matter in soil depends on the activity of microbial decomposers. However, little is known about how modifications of the diversity of soil microbial communities induced by fresh organic matter (FOM) inputs can regulate carbon cycling. Here, we investigated the decomposition of two 13C labeled crop residues (wheat and alfalfa) and the dynamics of the genetic structure and taxonomic composition of the soil bacterial communities decomposing 13C labeled FOM and native unlabeled soil organic matter (SOM), respectively. It was achieved by combining the stable isotope probing method with molecular tools (DNA genotyping and pyrosequencing of 16S rDNA). Although a priming effect (PE) was always induced by residue addition, its intensity increased with the degradability of the plant residue. The input of both wheat and alfalfa residues induced a rapid dynamics of FOM-degrading communities, corresponding to the stimulation of bacterial phyla which have been previously described as copiotrophic organisms. However, the dynamics and the identity of the bacterial groups stimulated depended on the residue added, with Firmicutes dominating in the wheat treatment and Proteobacteria dominating in the alfalfa treatment after 3 days of incubation. In both treatments, SOM-degrading communities were dominated by Acidobacteria, Verrucomicrobia, and Gemmatimonadetes phyla which have been previously described as oligotrophic organisms. An early stimulation of SOM-degrading populations mainly belonging to Firmicutes and Bacteroidetes groups was observed in the alfalfa treatment whereas no change occurred in the wheat treatment. Our findings support the hypothesis that the succession of bacterial taxonomic groups occurring in SOM- and FOM-degrading communities during the degradation process may be an important driver of the PE, and consequently of carbon dynamics in soil.

Global drivers and patterns of microbial abundance in soil

Abstract: Aim While soil microorganisms play key roles in Earth’s biogeochemical cycles, methodological constraints and sparse data have hampered our ability to describe and understand the global distribution of soil microbial biomass. Here, we present a comprehensive quantification of the environmental drivers of soil microbial biomass. Location Global. Methods We used a comprehensive global dataset of georeferenced soil microbial biomass estimates and high-resolution climatic and soil data. Results We show that microbial biomass carbon (CMic) is primarily driven by moisture availability, with this single variable accounting for 34% of the global variance. For the microbial carbon-to-soil organic carbon ratio (CMic/COrg), soil nitrogen content was an equally important driver as moisture. In contrast, temperature was not a significant predictor of microbial biomass patterns at a global scale, while temperature likely has an indirect effect on microbial biomass by influencing rates of evapotranspiration and decomposition. As our models explain an unprecedented 50% of the global variance of CMic and CMic/COrg ,w e were able to leverage gridded environmental information to build the first spatially explicit global estimates of microbial biomass and quantified the global soil microbial carbon pool to equal 14.6 Pg C. Main Conclusions Our unbiased models allowed us to build the first global spatially explicit predictions of microbial biomass. These patterns show that soil microbial biomass is not primarily driven by temperature, but instead, biomass is more heterogeneous through the effects of moisture availability and soil nutrients. Our global estimates provide important data for integration into large-scale carbon and nutrient models that may imply a major step forward in our ability to predict the global carbon balance, now and in a future climate.

The critical soil P levels for crop yield, soil fertility and environmental safety in different soil types

Abstract: Sufficient soil phosphorus (P) is important for achieving optimal crop production, but excessive soil P levels may create a risk of P losses and associated eutrophication of surface waters. The aim of this study was to determine critical soil P levels for achieving optimal crop yields and minimal P losses in common soil types and dominant cropping systems in China. Four long-term experiment sites were selected in China. The critical level of soil Olsen-P for crop yield was determined using the linear-plateau model. The relationships between the soil total P, Olsen-P and CaCl2-P were evaluated using two-segment linear model to determine the soil P fertility rate and leaching change-point. The critical levels of soil Olsen-P for optimal crop yield ranged from 10.9 mg kg−1 to 21.4 mg kg−1, above which crop yield response less to the increasing of soil Olsen-P. The P leaching change-points of Olsen-P ranged from 39.9 mg kg−1 to 90.2 mg kg−1, above which soil CaCl2-P greatly increasing with increasing soil Olsen-P. Similar change-point was found between soil total P and Olsen-P. Overall, the change-point ranged from 4.6 mg kg−1 to 71.8 mg kg−1 among all the four sites. These change-points were highly affected by crop specie, soil type, pH and soil organic matter content. The three response curves could be used to access the soil Olsen-P status for crop yield, soil P fertility rate and soil P leaching risk for a sustainable soil P management in field.

An equivalent soil mass procedure for monitoring soil organic carbon in multiple soil layers

Abstract: Summary Soil carbon stocks are commonly quantified at fixed depths as the product of soil bulk density, depth and organic carbon (OC) concentration. However, this method systematically overestimates OC stocks in treatments with greater bulk densities such as minimum tillage, exaggerating their benefits. Its use has compromised estimates of OC change where bulk densities differed between treatments or over time periods. We argue that its use should be discontinued and a considerable body of past research re-evaluated. Accurate OC estimations must be based on quantification in equivalent soil masses (ESMs). The objective of this publication is to encourage accurate quantification of changes in OC stocks and other soil properties using ESM procedures by developing a simple procedure to quantify OC in multiple soil layers. We explain errors inherent in fixed depth procedures and show how these errors are eliminated using ESM methods. We describe a new ESM procedure for calculating OC stocks in multiple soil layers and show that it can be implemented without bulk density sampling, which reduces sampling time and facilitates evaluations at greater depths, where bulk density sampling is difficult. A spreadsheet has been developed to facilitate calculations. A sample adjustment procedure is described to facilitate OC quantification in a single equivalent soil mass layer from the surface, when multiple-layer quantification is not necessary.