Showing papers on "Organic matter published in 2010"

Priming effects : interactions between living and dead organic matter

Abstract: In this re-evaluation of our 10-year old paper on priming effects, I have considered the latest studies and tried to identify the most important needs for future research. Recent publications have shown that the increase or decrease in soil organic matter mineralization (measured as changes of CO 2 efflux and N mineralization) actually results from interactions between living (microbial biomass) and dead organic matter. The priming effect (PE) is not an artifact of incubation studies, as sometimes supposed, but is a natural process sequence in the rhizosphere and detritusphere that is induced by pulses or continuous inputs of fresh organics. The intensity of turnover processes in such hotspots is at least one order of magnitude higher than in the bulk soil. Various prerequisites for high-quality, informative PE studies are outlined: calculating the budget of labeled and total C; investigating the dynamics of released CO 2 and its sources; linking C and N dynamics with microbial biomass changes and enzyme activities; evaluating apparent and real PEs; and assessing PE sources as related to soil organic matter stabilization mechanisms. Different approaches for identifying priming, based on the assessment of more than two C sources in CO 2 and microbial biomass, are proposed and methodological and statistical uncertainties in PE estimation and approaches to eliminating them are discussed. Future studies should evaluate directions and magnitude of PEs according to expected climate and land-use changes and the increased rhizodeposition under elevated CO 2 as well as clarifying the ecological significance of PEs in natural and agricultural ecosystems. The conclusion is that PEs – the interactions between living and dead organic matter – should be incorporated in models of C and N dynamics, and that microbial biomass should regarded not only as a C pool but also as an active driver of C and N turnover.

Phenol oxidase, peroxidase and organic matter dynamics of soil

Abstract: Extracellular enzymes mediate the degradation, transformation and mineralization of soil organic matter. The activity of cellulases, phosphatases and other hydrolases has received extensive study and in many cases stoichiometric relationships and responses to disturbances are well established. In contrast, phenol oxidase and peroxidase activities, which are often uncorrelated with hydrolase activities, have been measured in only a small subset of soil enzyme studies. These enzymes are expressed for a variety of purposes including ontogeny, defense and the acquisition of carbon and nitrogen. Through excretion or lysis, these enzymes enter the environment where their aggegrate activity mediates key ecosystem functions of lignin degradation, humification, carbon mineralization and dissolved organic carbon export. Phenol oxidases and peroxidases are less stable in the environment than extracellular hydrolases, especially when associated with organic particles. Activities are also affected, positively and negatively, by interaction with mineral surfaces. High spatiotemporal variation obscures their relationships with environmental variables and ecological process. Across ecosystems, phenol oxidase and peroxidase activities generally increase with soil pH, a finding not predicted from the pH optima of purified enzymes. Activities associated with plant litter and particulate organic matter often correlate with decomposition rates and potential activities generally increase with the lignin and secondary compound content of the material. At the ecosystem scale, nitrogen amendment alters the expression of phenol oxidase and peroxidase enzymes more broadly than culture studies imply and these responses correlate with positive and negative changes in litter decomposition rates and soil organic matter content. At the global scale, N amendment of basidiomycete-dominated soils of temperate and boreal forest ecoystems often leads to losses of oxidative enzyme activity, while activities in grassland soils dominated by glomeromycota and ascomycetes show little net response. Land use that leads to loss of soil organic matter tends to increase oxidative activities. Across ecosystems, soil organic matter content is not correlated with mean potential phenol oxidase and peroxidase activities. A multiple regression model that includes soil pH, mean annual temperature, mean annual precipitation and potential phenol oxidase activity accounts for 37% of the variation in soil organic matter (SOM) content across ecosystems (n = 63); a similar model for peroxidase activity describes 32% of SOM variance (n = 43). Analysis of residual variation suggest that suites of interacting factors create both positive and negative feedbacks on soil organic matter storage. Soils with high oxygen availability, pH and mineral activity tend to be substrate limited: high in situ oxidative activities limit soil organic matter accumulation. Soils with opposing characteristics are activity limited: low in situ oxidative activities promote soil organic matter storage.

Long-term effects of organic amendments on soil fertility. A review

Abstract: Common agricultural practices such as excessive use of agro-chemicals, deep tillage and luxury irrigation have degraded soils, polluted water resources and contaminated the atmosphere. There is increasing concern about interrelated environmental problems such as soil degradation, desertification, erosion, and accelerated greenhouse effects and climate change. The decline in organic matter content of many soils is becoming a major process of soil degradation, particularly in European semi-arid Mediterranean regions. Degraded soils are not fertile and thus cannot maintain sustainable production. At the same time, the production of urban and industrial organic waste materials is widespread. Therefore, strategies for recycling such organic waste in agriculture must be developed. Here, we review long-term experiments (3-60 years) on the effects of organic amendments used both for organic matter replenishment and to avoid the application of high levels of chemical fertilizers. The major points of our analysis are: (1) many effects, e.g. carbon sequestration in the soil and possible build-up of toxic elements, evolve slowly, so it is necessary to refer to long-term trials. (2) Repeated application of exogenous organic matter to cropland led to an improvement in soil biological functions. For instance, microbial biomass carbon increased by up to 100% using high-rate compost treatments, and enzymatic activity increased by 30% with sludge addition. (3) Long-lasting application of organic amendments increased organic carbon by up to 90% versus unfertilized soil, and up to 100% versus chemical fertilizer treatments. (4) Regular addition of organic residues, particularly the composted ones, increased soil physical fertility, mainly by improving aggregate stability and decreasing soil bulk density. (5) The best agronomic performance of compost is often obtained with the highest rates and frequency of applications. Furthermore, applying these strategies, there were additional beneficial effects such as the slow release of nitrogen fertilizer. (6) Crop yield increased by up to 250% by long-term applications of high rates of municipal solid waste compost. Stabilized organic amendments do not reduce the crop yield quality, but improve it. (7) Organic amendments play a positive role in climate change mitigation by soil carbon sequestration, the size of which is dependent on their type, the rates and the frequency of application. (8) There is no tangible evidence demonstrating negative impacts of heavy metals applied to soil, particularly when high-quality compost was used for long periods. (9) Repeated application of composted materials enhances soil organic nitrogen content by up to 90%, storing it for mineralization in future cropping seasons, often without inducing nitrate leaching to groundwater.

From Oil-Prone Source Rock to Gas-Producing Shale Reservoir - Geologic and Petrophysical Characterization of Unconventional Shale Gas Reservoirs

Abstract: Many currently producing shale-gas reservoirs are overmature oil-prone source rocks. Through burial and heating these reservoirs evolve from organic-matter-rich mud deposited in marine, lacustrine, or swamp environments. Key characterization parameters are: total organic carbon (TOC), maturity level (vitrinite reflectance), mineralogy, thickness, and organic matter type. Hydrogento-carbon (HI) and oxygen-to-carbon (OI) ratios are used to classify organic matter that ranges from oil-prone algal and herbaceous to gas-prone woody/coaly material. Although organic-matter-rich intervals can be hundreds of meters thick, vertical variability in TOC is high ( 50% of the total porosity, and these pores may be hydrocarbon wet, at least during most of the thermal maturation process. A full understanding of the relation of porosity and gas content will result in development of optimized processes for hydrocarbon recovery in shale-gas reservoirs.

Sewage Treatment with Anammox

Abstract: Organic matter must be removed from sewage to protect the quality of the water bodies that it is discharged to. Most current sewage treatment plants are aimed at removing organic matter only. They are energy-inefficient, whereas potentially the organic matter could be regarded as a source of energy. However, organic carbon is not the only pollutant in sewage: Fixed nitrogen such as ammonium (NH4+) and nitrate (NO3−) must be removed to avoid toxic algal blooms in the environment. Conventional wastewater treatment systems for nitrogen removal require a lot of energy to create aerobic conditions for bacterial nitrification, and also use organic carbon to help remove nitrate by bacterial denitrification (see the figure). An alternative approach is the use of anoxic ammonium-oxidizing (anammox) bacteria, which require less energy ( 1 ) but grow relatively slowly. We explore process innovations that can speed up the anammox process and use all organic matter as much as possible for energy generation.

Litter decomposition: what controls it and how can we alter it to sequester more carbon in forest soils?

Abstract: Key recent developments in litter decomposition research are reviewed. Long-term inter-site experiments indicate that temperature and moisture influence early rates of litter decomposition primarily by determining the plants present, suggesting that climate change effects will be small unless they alter the plant forms present. Thresholds may exist at which single factors control decay rate. Litter decomposes faster where the litter type naturally occurs. Elevated CO2 concentrations have little effect on litter decomposition rates. Plant tissues are not decay-resistant; it is microbial and biochemical transformations of materials into novel recalcitrant compounds rather than selective preservation of recalcitrant compounds that creates stable organic matter. Altering single characteristics of litter will not substantially alter decomposition rates. Nitrogen addition frequently leads to greater stabilization into humus through a combination of chemical reactions and enzyme inhibition. To sequester more C in soil, we need to consider not how to slow decomposition, but rather how to divert more litter into humus through microbial and chemical reactions rather than allowing it to decompose. The optimal strategy is to have litter transformed into humic substances and then chemically or physically protected in mineral soil. Adding N through fertilization and N-fixing plants is a feasible means of stimulating humification.

Effect of halide ions and carbonates on organic contaminant degradation by hydroxyl radical-based advanced oxidation processes in saline waters.

Abstract: Advanced oxidation processes (AOPs) generating nonselective hydroxyl radicals (HO*) provide a broad-spectrum contaminant destruction option for the decontamination of waters. Halide ions are scavengers of HO* during AOP treatment, such that treatment of saline waters would be anticipated to be ineffective. However, HO* scavenging by halides converts HO* to radical reactive halogen species (RHS) that participate in contaminant destruction but react more selectively with electron-rich organic compounds. The effects of Cl-, Br-, and carbonates (H2CO3+HCO3-+CO3(2-)) on the UV/H2O2 treatment of model compounds in saline waters were evaluated. For single target organic contaminants, the impact of these constituents on contaminant destruction rate suppression at circumneutral pH followed the order Br->carbonates>Cl-. Traces of Br- in the NaCl stock had a greater effect than Cl- itself. Kinetic modeling of phenol destruction demonstrated that RHS contributed significantly to phenol destruction, mitigating the impact of HO* scavenging. The extent of treatment efficiency reduction in the presence of halides varied dramatically among different target organic compounds. Destruction of contaminants containing electron-poor reaction centers in seawater was nearly halted, while 17beta-estradiol removal declined by only 3%. Treatment of mixtures of contaminants with each other and with natural organic matter (NOM) was evaluated. Although NOM served as an oxidant scavenger, conversion of nonselective HO* to selective radicals due to the presence of anions enhanced the efficiency of electron-rich contaminant removal in saline waters by focusing the oxidizing power of the system away from the NOM toward the target contaminant. Despite the importance of contaminant oxidation by halogen radicals, the formation of halogenated byproducts was minimal.

A review of allochthonous organic matter dynamics and metabolism in streams

Abstract: The role of allochthonous organic matter in lotic ecosystems has been an important research topic among aquatic ecologists since the seminal work by Lindeman was published in 1942. Since 1986, studies on organic matter budgets, ecosystem metabolism, and decomposition published in J-NABS have made significant contributions to the overall understanding of organic matter dynamics in streams. In this review, we summarize the utility of organic matter budgets, cover the major advances in research on ecosystem metabolism, and describe the intrinsic and extrinsic factors influencing organic matter decomposition. We also discuss future directions and current applications of research and highlight the need for additional studies on the role of land use and climate change, as well as continued use of organic matter processing as a functional metric in biomonitoring studies. We emphasize the need for continued data synthesis into comprehensive organic matter budgets. Such comparative studies can elucidate important drivers of organic matter dynamics and can assist in the understanding of large continental/ global changes that might be occurring now and in the near future. In general, continued emphasis on synthesizing information into a larger framework for streams and rivers will improve our overall understanding of the importance of organic matter in lotic ecosystems.

Effect of fertilizer application on soil heavy metal concentration.

Abstract: A large amount of chemicals is annually applied at the agricultural soils as fertilizers and pesticides. Such applications may result in the increase of heavy metals particularly Cd, Pb, and As. The objective of this study was to investigate the variability of chemical applications on Cd, Pb, and As concentrations of wheat-cultivated soils. Consequently, a study area was designed and was divided into four subareas (A, B, C, and D). The soil sampling was carried out in 40 points of cultivated durum wheat during the 2006-2007 periods. The samples were taken to the laboratory to measure their heavy metal concentration, soil texture, pH, electrical conductivity, cationic exchange capacity, organic matter, and carbonate contents. The result indicated that Cd, Pb, and As concentrations were increased in the cultivated soils due to fertilizer application. Although the statistical analysis indicates that these heavy metals increased significantly (P value<0.05), the lead and arsenic concentrations were increased dramatically compared to Cd concentration. This can be related to overapplication of fertilizers as well as the pesticides that are used to replant plant pests, herbs, and rats.

Substantial nitrogen acquisition by arbuscular mycorrhizal fungi from organic material has implications for N cycling

Abstract: Arbuscular mycorrhizal (AM) fungi are obligate biotrophs that acquire carbon (C) solely from host plants. AM fungi can proliferate hyphae in, and acquire nitrogen (N) from, organic matter. Although they can transfer some of that N to plants, we tested the hypothesis that organic matter is an important N source for the AM fungi themselves. We grew pairs of plants with and without the AM fungus Glomus hoi in microcosms that allowed only the fungus access to a 15N/13C-labeled organic patch; in some cases, one plant was shaded to reduce C supply to the fungus. The fungal hyphae proliferated vigorously in the patch, irrespective of shading, and increased plant growth and N content; ~3% of plant N came from the patch. The extraradical mycelium of the fungus was N-rich (3–5% N) and up to 31% of fungal N came from the patch, confirming the hypothesis. The fungus acquired N as decomposition products, because hyphae were not 13C-enriched. In a second experiment, hyphae of both G. hoi and Glomus mosseae that exploited an organic material patch were also better able to colonize a new host plant, demonstrating a fungal growth response. These findings show that AM fungi can obtain substantial amounts of N from decomposing organic materials and can enhance their fitness as a result. The large biomass and high N demand of AM fungi means that they represent a global N pool equivalent in magnitude to fine roots and play a substantial and hitherto overlooked role in the nitrogen cycle.

Illuminated darkness: molecular signatures of Congo River dissolved organic matter and its photochemical alteration as revealed by ultrahigh precision mass spectrometry.

Abstract: Congo River water was filtered and then irradiated for 57 d in a solar simulator, resulting in extensive photodegradation of dissolved organic matter (DOM). Whole-water (i.e., unfractionated) DOM was analyzed pre- and post-irradiation using ultrahigh resolution Fourier transform ion cyclotron mass spectrometry (FT-ICR MS), revealing the following three pools of DOM classified based upon their photoreactivity: (1) photo-resistant, (2) photo-labile, and (3) photo-produced. Photo-resistant DOM was heterogeneous, with most molecular classes represented, although only a small number of aromatics and no condensed aromatics were identified. The photoproduced pool was dominated by aliphatic compounds, although it included a small number of aromatics, including condensed aromatics. Aromatic compounds were the most photoreactive, with . 90% being lost upon irradiation. Photochemistry also resulted in a significant drop in the number of molecules identified and a decrease in their structural diversity. The FT-ICR MS signatures of two classes of refractory organic matter, black carbon and carboxylic-rich alicyclic molecules (CRAM), were present in the sample prior to irradiation, indicating that the Congo River could be a significant exporter of recalcitrant DOM to the ocean. All black carbon–like molecules identified in the initial sample were lost during irradiation. Molecular signatures consistent with CRAM were also highly photo-labile, demonstrating that environmental solar irradiation levels are capable of removing these refractory compounds from aquatic systems. Irradiation also shifted the molecular signature of terrestrial DOM toward that of marine DOM, thereby complicating the task of tracking terrestrial DOM in the ocean. The transfer of terrestrial dissolved organic matter (TDOM) via rivers to the oceans is a significant component in the global and oceanic carbon budgets (Schlesinger and Melack 1981; Hedges 1992). The , 0.25 Pg of dissolved organic carbon (DOC) discharged from rivers annually can account for the mean radiocarbon-based turnover times of oceanic DOC (, 4000–6000 yr; Williams and Gordon 1970). However, only very small amounts of TDOM have been identified in seawater using both organic biomarker and stable carbon isotopic approaches (Meyers-Schulte and Hedges 1986; Moran et al. 1991; Opsahl and Benner 1997). Estimates of TDOM contributions to the oceans have been based on comparisons of the biochemical and isotopic compositions of open-ocean dissolved organic matter (DOM) to freshwater riverine end-member counterparts (Meyers-Schulte and Hedges 1986; Hernes and Benner 2006) and have not typically accounted for changes during transit within rivers, estuaries, or the ocean (Cole and Caraco 2001; Raymond and Bauer 2001; Benner 2002). Without a better understanding of the types and magnitudes of modifications that components of TDOM undergo in lower rivers, estuaries, and the ocean, we may be misinterpreting the effective molecular signatures of TDOM in the ocean. Three massive tropical rivers, the Congo, Amazon, and Orinoco, are responsible for over a quarter of global DOC input to the oceans (Coynel et al. 2005). The Congo is second only to the Amazon as a DOC conduit between the terrestrial and marine biogeochemical cycles, exporting , 12.4 Tg DOC yr21, equivalent to , 5% of the global

High molecular diversity of extraterrestrial organic matter in Murchison meteorite revealed 40 years after its fall

Abstract: Numerous descriptions of organic molecules present in the Murchison meteorite have improved our understanding of the early interstellar chemistry that operated at or just before the birth of our solar system. However, all molecular analyses were so far targeted toward selected classes of compounds with a particular emphasis on biologically active components in the context of prebiotic chemistry. Here we demonstrate that a nontargeted ultrahigh-resolution molecular analysis of the solvent-accessible organic fraction of Murchison extracted under mild conditions allows one to extend its indigenous chemical diversity to tens of thousands of different molecular compositions and likely millions of diverse structures. This molecular complexity, which provides hints on heteroatoms chronological assembly, suggests that the extraterrestrial chemodiversity is high compared to terrestrial relevant biological- and biogeochemical-driven chemical space.

Contaminant Removal Processes in Subsurface-Flow Constructed Wetlands: A Review

Abstract: The main contaminant removal processes occurring in subsurface-flow constructed wetlands treating wastewater are reviewed. Redox conditions prevailing in the wetlands are analyzed and linked to contaminant removal mechanisms. The removal of organic matter and its accumulation in the granular medium of the wetlands are evaluated with regard to particulate and dissolved components and clogging processes. The main biological processes linked to organic matter transformation—aerobic respiration, denitrification, acid fermentation, sulfate reduction, and methanogenesis—are reviewed separately. The processes of removal of surfactants, pesticides and herbicides, emergent contaminants, nutrients, heavy metals and faecal organisms are analyzed. Advances in wetland modeling are presented as a powerful tool for understanding multiple interactions occurring in subsurface-flow constructed wetlands during the removal of contaminants.

Catechol and Humic Acid Sorption onto a Range of Laboratory-Produced Black Carbons (Biochars)

Abstract: Although the major influence of black carbon (BC) on soil and sediment organic contaminant sorption is widely accepted, an understanding of the mechanisms and natural variation in pyrogenic carbon interaction with natural organic matter (NOM) is lacking. The sorption of a phenolic NOM monomer (catechol) and humic acids (HA) onto BC was examined using biochars made from oak, pine, and grass at 250, 400, and 650 degrees C. Catechol sorption equilibrium occurred after 14 d and was described by a diffusion kinetic model, while HA required only 1 d and followed pseudo-second-order kinetics. Catechol sorption capacity increased with increasing biochar combustion temperature, from pine < oak < grass and from coarse < fine particle size. At lower catechol concentrations, sorption affinity (Freundlich constant, K(f)) was directly related to micropore surface area (measured via CO(2) sorptometry) indicating the predominance of specific adsorption. In contrast, HA exhibited an order of magnitude less sorption (0.1% versus 1%, by weight) due to its exclusion from micropores. Greater sorption of both catechol and HA occurred on biochars with nanopores, i.e. biochars made at higher temperatures. These findings suggest that addition of BC to soil, via natural fires or biochar amendments, will sequester abundant native OM through sorption.

Priming effect: bridging the gap between terrestrial and aquatic ecology

Abstract: Understanding how ecosystems store or release carbon is one of ecology's greatest challenges in the 21st century. Organic matter covers a large range of chemical structures and qualities, and it is classically represented by pools of different recalcitrance to degradation. The interaction effects of these pools on carbon cycling are still poorly understood and are most often ignored in global-change models. Soil scientists have shown that inputs of labile organic matter frequently tend to increase, and often double, the mineralization of the more recalcitrant organic matter. The recent revival of interest for this phenomenon, named the priming effect, did not cross the frontiers of the disciplines. In particular, the priming effect phenomenon has been almost totally ignored by the scientific communities studying marine and continental aquatic ecosystems. Here we gather several arguments, experimental results, and field observations that strongly support the hypothesis that the priming effect is a general phenomenon that occurs in various terrestrial, freshwater, and marine ecosystems. For example, the increase in recalcitrant organic matter mineralization rate in the presence of labile organic matter ranged from 10% to 500% in six studies on organic matter degradation in aquatid ecosystems. Consequently, the recalcitrant organic matter mineralization rate may largely depend on labile organic matter availability, influencing the CO2 emissions of both aquatic and terrestrial ecosystems. We suggest that (1) recalcitrant organic matter may largely contribute to the CO2 emissions of aquatic ecosystems through the priming effect, and (2) priming effect intensity may be modified by global changes, interacting with eutrophication processes and atmospheric CO2 increases. Finally, we argue that the priming effect acts substantially in the carbon and nutrient cycles in all ecosystems. We outline exciting avenues for research, which could provide new insights on the responses of ecosystems to anthropogenic perturbations and their feedbacks to climatic changes.

Carbon dynamics in topsoil and in subsoil may be controlled by different regulatory mechanisms

Abstract: It is estimated that in excess of 50% of the soil carbon stock is found in the subsoil (below 20–30 cm). Despite this very few studies have paid attention to the subsoil. Although surface and subsurface horizons differ in pedological, environmental and physicochemical features, which are all likely to affect the mechanisms and biological actors involved, models of carbon dynamics tend to assume that the underlying processes are identical in all horizons, but with lower gross fluxes in the subsurface. The aim of this study was to test this assumption by analysing factors governing organic matter decomposition in topsoil (from depths of 5–10 cm) and subsoil (from depths of 80–100 cm). To this end, we established incubations that lasted 51 days, in which factors that were thought to control organic matter mineralization were altered: oxygen concentration, soil structure and the energetic and nutritional status. At the end of the incubation period, the microbial biomass was measured and the community level physiological profiles established. The mineralization per unit organic carbon proved to be as important in the subsoil as it was in surface samples, in spite of lower carbon contents and different catabolic profiles. Differences in the treatment effects indicated that the controls on C dynamics were different in topsoil and subsoil: disrupting the structure of the subsoil caused a 75% increase in mineralization while the surface samples remained unaffected. On the other hand, a significant priming affect was found in the topsoil but not in the subsoil samples. Spatial heterogeneity in carbon content, respiration and microbial communities was greater in subsoil than in topsoil at the field scale. These data suggest greater attention should be paid to the subsoil if global C dynamics is to be fully understood.

What is recalcitrant soil organic matter

Abstract: Environmental context.On a global scale, soils store more carbon than plants or the atmosphere. The cycling of this vast reservoir of reduced carbon is closely tied to variations in environmental conditions, but robust predictions of climate–carbon cycle feedbacks are hampered by a lack of mechanistic knowledge regarding the sensitivity of organic matter decomposition to rising temperatures. This text provides a critical discussion of the practice to conceptualise parts of soil organic matter as intrinsically resistant to decomposition or ‘recalcitrant’. Abstract.The understanding that some natural organic molecules can resist microbial decomposition because of certain molecular properties forms the basis of the biogeochemical paradigm of ‘intrinsic recalcitrance’. In this concept paper I argue that recalcitrance is an indeterminate abstraction whose semantic vagueness encumbers research on terrestrial carbon cycling. Consequently, it appears to be advantageous to view the perceived ‘inherent resistance’ to decomposition of some forms of organic matter not as a material property, but as a logistical problem constrained by (i) microbial ecology; (ii) enzyme kinetics; (iii) environmental drivers; and (iv) matrix protection. A consequence of this view would be that the frequently observed temperature sensitivity of the decomposition of organic matter must result from factors other than intrinsic molecular recalcitrance.

13C fractionation at the root-microorganisms-soil interface: A review and outlook for partitioning studies

Abstract: Natural variations of the 13 C/ 12 C ratio have been frequently used over the last three decades to trace C sources and fluxes between plants, microorganisms, and soil. Many of these studies have used the natural- 13 C-labelling approach, i.e. natural δ 13 C variation after C 3 –C 4 vegetation changes. In this review, we focus on 13 C fractionation in main processes at the interface between roots, microorganisms, and soil: root respiration, microbial respiration, formation of dissolved organic carbon, as well as microbial uptake and utilization of soil organic matter (SOM). Based on literature data and our own studies, we estimated that, on average, the roots of C 3 and C 4 plants are 13 C enriched compared to shoots by +1.2 ± 0.6‰ and +0.3 ± 0.4‰, respectively. The CO 2 released by root respiration was 13 C depleted by about −2.1 ± 2.2‰ for C 3 plants and −1.3 ± 2.4‰ for C 4 plants compared to root tissue. However, only a very few studies investigated 13 C fractionation by root respiration. This urgently calls for further research. In soils developed under C 3 vegetation, the microbial biomass was 13 C enriched by +1.2 ± 2.6‰ and microbial CO 2 was also 13 C enriched by +0.7 ± 2.8‰ compared to SOM. This discrimination pattern suggests preferential utilization of 13 C-enriched substances by microorganisms, but a respiration of lighter compounds from this fraction. The δ 13 C signature of the microbial pool is composed of metabolically active and dormant microorganisms; the respired CO 2 , however, derives mainly from active organisms. This discrepancy and the preferential substrate utilization explain the δ 13 C differences between microorganisms and CO 2 by an ‘apparent’ 13 C discrimination. Preferential consumption of easily decomposable substrates and less negative δ 13 C values were common for substances with low C/N ratios. Preferential substrate utilization was more important for C 3 soils because, in C 4 soils, microbial respiration strictly followed kinetics, i.e. microorganisms incorporated heavier C (∆ = +1.1‰) and respired lighter C (∆ = −1.1‰) than SOM. Temperature and precipitation had no significant effect on the 13 C fractionation in these processes in C 3 soils. Increasing temperature and decreasing precipitation led, however, to increasing δ 13 C of soil C pools. Based on these 13 C fractionations we developed a number of consequences for C partitioning studies using 13 C natural abundance. In the framework of standard isotope mixing models, we calculated CO 2 partitioning using the natural- 13 C-labelling approach at a vegetation change from C 3 to C 4 plants assuming a root-derived fraction between 0% and 100% to total soil CO 2 . Disregarding any 13 C fractionation processes, the calculated results deviated by up to 10% from the assumed fractions. Accounting for 13 C fractionation in the standard deviations of the C 4 source and the mixing pool did not improve the exactness of the partitioning results; rather, it doubled the standard errors of the CO 2 pools. Including 13 C fractionations directly into the mass balance equations reproduced the assumed CO 2 partitioning exactly. At the end, we therefore give recommendations on how to consider 13 C fractionations in research on carbon flows between plants, microorganisms, and soil.

Widespread coupling between the rate and temperature sensitivity of organic matter decay

Abstract: Soils comprise the largest terrestrial carbon store on the planet. Soil respiration measurements suggest that the more biogeochemically recalcitrant the soil organic matter, the greater the temperature sensitivity of soil respiration.

Soil carbon dynamics in saline and sodic soils: a review.

Abstract: Soil salinity (high levels of water-soluble salt) and sodicity (high levels of exchangeable sodium), called collectively salt-affected soils, affect approximately 932 million ha of land globally. Saline and sodic landscapes are subjected to modified hydrologic processes which can impact upon soil chemistry, carbon and nutrient cycling, and organic matter decomposition. The soil organic carbon (SOC) pool is the largest terrestrial carbon pool, with the level of SOC an important measure of a soil's health. Because the SOC pool is dependent on inputs from vegetation, the effects of salinity and sodicity on plant health adversely impacts upon SOC stocks in salt-affected areas, generally leading to less SOC. Saline and sodic soils are subjected to a number of opposing processes which affect the soil microbial biomass and microbial activity, changing CO fluxes and the nature and delivery of nutrients to vegetation. Sodic soils compound SOC loss by increasing dispersion of aggregates, which increases SOC mineralisation, and increasing bulk density which restricts access to substrate for mineralisation. Saline conditions can increase the decomposability of soil organic matter but also restrict access to substrates due to flocculation of aggregates as a result of high concentrations of soluble salts. Saline and sodic soils usually contain carbonates, which complicates the carbon (C) dynamics. This paper reviews soil processes that commonly occur in saline and sodic soils, and their effect on C stocks and fluxes to identify the key issues involved in the decomposition of soil organic matter and soil aggregation processes which need to be addressed to fully understand C dynamics in salt-affected soils. © 2009 The Authors. Journal compilation