Showing papers on "Organic matter published in 2009"
TL;DR: Due to the importance of rhizodeposition in regulating ecosystem functioning, it is critical that future research focuses on resolving the quantitative importance of the different C and N fluxes operating in the rhizosphere and the ways in which these vary spatially and temporally.
Abstract: The loss of organic and inorganic carbon from roots into soil underpins nearly all the major changes that occur in the rhizosphere. In this review we explore the mechanistic basis of organic carbon and nitrogen flow in the rhizosphere. It is clear that C and N flow in the rhizosphere is extremely complex, being highly plant and environment dependent and varying both spatially and temporally along the root. Consequently, the amount and type of rhizodeposits (e.g. exudates, border cells, mucilage) remains highly context specific. This has severely limited our capacity to quantify and model the amount of rhizodeposition in ecosystem processes such as C sequestration and nutrient acquisition. It is now evident that C and N flow at the soil–root interface is bidirectional with C and N being lost from roots and taken up from the soil simultaneously. Here we present four alternative hypotheses to explain why high and low molecular weight organic compounds are actively cycled in the rhizosphere. These include: (1) indirect, fortuitous root exudate recapture as part of the root’s C and N distribution network, (2) direct re-uptake to enhance the plant’s C efficiency and to reduce rhizosphere microbial growth and pathogen attack, (3) direct uptake to recapture organic nutrients released from soil organic matter, and (4) for inter-root and root–microbial signal exchange. Due to severe flaws in the interpretation of commonly used isotopic labelling techniques, there is still great uncertainty surrounding the importance of these individual fluxes in the rhizosphere. Due to the importance of rhizodeposition in regulating ecosystem functioning, it is critical that future research focuses on resolving the quantitative importance of the different C and N fluxes operating in the rhizosphere and the ways in which these vary spatially and temporally.
1,247 citations
TL;DR: Iron and manganese (hydr)oxides were found to be the main carriers for Cd, Zn and Ni under oxic conditions, whereas the organic fraction was most important for Cu.
987 citations
TL;DR: In this paper, the authors evaluated the impact of drying and wetting on the mineralization and fluxes of C and N in terrestrial soils and found that despite wetting pulses, cumulative mineralization is mostly smaller compared with soil with optimum moisture, indicating that dry soil cannot compensate for small mineralization rates during drought periods.
Abstract: In the next decades, many soils will be subjected to increased drying/wetting cycles or modified water availability considering predicted global changes in precipitation and evapotranspiration. These changes may affect the turnover of C and N in soils, but the direction of changes is still unclear. The aim of the review is the evaluation of involved mechanisms, the intensity, duration and frequency of drying and wetting for the mineralization and fluxes of C and N in terrestrial soils. Controversial study results require a reappraisal of the present understanding that wetting of dry soils induces significant losses of soil C and N. The generally observed pulse in net C and N mineralization following wetting of dry soil (hereafter wetting pulse) is short-lived and often exceeds the mineralization rate of a respective moist control. Accumulated microbial and plant necromass, lysis of live microbial cells, release of compatible solutes and exposure of previously protected organic matter may explain the additional mineralization during wetting of soils. Frequent drying and wetting diminishes the wetting pulse due to limitation of the accessible organic matter pool. Despite wetting pulses, cumulative C and N mineralization (defined here as total net mineralization during drying and wetting) are mostly smaller compared with soil with optimum moisture, indicating that wetting pulses cannot compensate for small mineralization rates during drought periods. Cumulative mineralization is linked to the intensity and duration of drying, the amount and distribution of precipitation, temperature, hydrophobicity and the accessible pool of organic substrates. Wetting pulses may have a significant impact on C and N mineralization or flux rates in arid and semiarid regions but have less impact in humid and subhumid regions on annual time scales. Organic matter stocks are progressively preserved with increasing duration and intensity of drought periods; however, fires enhance the risk of organic matter losses under dry conditions. Hydrophobicity of organic surfaces is an important mechanism that reduces C and N mineralization in topsoils after precipitation. Hence, mineralization in forest soils with hydrophobic organic horizons is presumably stronger limited than in grassland or farmland soils. Even in humid regions, suboptimal water potentials often restrict microbial activity in topsoils during growing seasons. Increasing summer droughts will likely reduce the mineralization and fluxes of C and N whereas increasing summer precipitation could enhance the losses of C and N from soils.
977 citations
TL;DR: It is suggested that ecoenzymatic ratios reflect the equilibria between the elemental composition of microbial biomass and detrital organic matter and the efficiencies of microbial nutrient assimilation and growth.
Abstract: Biota can be described in terms of elemental composition, expressed as an atomic ratio of carbon:nitrogen:phosphorus (refs 1-3). The elemental stoichiometry of microoorganisms is fundamental for understanding the production dynamics and biogeochemical cycles of ecosystems because microbial biomass is the trophic base of detrital food webs. Here we show that heterotrophic microbial communities of diverse composition from terrestrial soils and freshwater sediments share a common functional stoichiometry in relation to organic nutrient acquisition. The activities of four enzymes that catalyse the hydrolysis of assimilable products from the principal environmental sources of C, N and P show similar scaling relationships over several orders of magnitude, with a mean ratio for C:N:P activities near 1:1:1 in all habitats. We suggest that these ecoenzymatic ratios reflect the equilibria between the elemental composition of microbial biomass and detrital organic matter and the efficiencies of microbial nutrient assimilation and growth. Because ecoenzymatic activities intersect the stoichiometric and metabolic theories of ecology, they provide a functional measure of the threshold at which control of community metabolism shifts from nutrient to energy flow.
917 citations
TL;DR: Based on the studied literature it appears that POU based CNT technology looks promising, that can possibly avoid difficulties of treating biological contaminants in conventional water treatment plants, and thereby remove the burden of maintaining the biostability of treated water in the distribution systems.
632 citations
TL;DR: In this paper, the authors examined the character of riverine dissolved organic matter in 34 watersheds along a gradient of agricultural land use and found that the structural complexity of organic matter decreases as the ratio of continuous croplands to wetlands increases.
Abstract: Nearly 40% of the Earth’s ice-free surface area is cropland or pasture. Measurements of dissolved organic matter along a gradient of agricultural land use suggest that agricultural watersheds contain less complex, more microbially derived dissolved organic matter than natural wetlands. Nearly 40% of the Earth’s ice-free surface area is cropland or pasture1. Agricultural land use can increase the delivery of nutrients such as nitrogen and phosphorus to fluvial ecosystems2, but the impact of farming on riverine dissolved organic carbon is still largely unknown, despite increasing recognition that rivers act as important modifiers in the global carbon cycle3,4. Here, we examine the character of riverine dissolved organic matter in 34 watersheds along a gradient of agricultural land use. We show that changes in the character of dissolved organic matter are related to agricultural land use, nitrogen loading and wetland loss. Specifically, we find that the structural complexity of dissolved organic matter decreases as the ratio of continuous croplands to wetlands increases. At the same time, the amount of microbially derived dissolved organic matter increases with greater agricultural land use. Furthermore, we find that periods of soil dryness are associated with a decrease in the structural complexity of dissolved organic matter. We suggest that these effects of land use and climate on the character of riverine dissolved organic matter have important implications for global carbon cycling, owing to their potential to control rates of microbial carbon processing (for example, uptake, retention and outgassing) in agricultural systems.
620 citations
TL;DR: The formation of small aggregates of about 170 nm and surface coating of several nanometers of SRHA on iron oxide NPs confirm the role of NOM in the disaggregation process and indicate that NPs might mimic the behavior of natural colloids.
514 citations
TL;DR: It is found that aromatic pi-systems within organic compounds have the capacity to adsorb to minerals and organic soil and sediment components such as natural organic matter and fire-derived black carbon through specific sorptive forces other than hydrophobic interactions.
Abstract: This review intends to deepen our understanding of mechanisms by which molecules with aromatic moieties attach to organic and mineral components of terrestrial environments. We present published evidence for the existence of specific, sorptive interactions of aromatic moieties with environmental sorbents. We find that aromatic pi-systems within organic compounds have the capacity to adsorb to minerals and organic soil and sediment components such as natural organic matter (NOM) and fire-derived black carbon (BC) through specific sorptive forces other than hydrophobic interactions. Polar interactions of aromatic pi-donor and -acceptor compounds show adsorption energies between 4 and 167 kJ mol(-1). Bonding strengths of cation-pi interactions and pi-pi electron donor-acceptor (EDA) interactions appear to be larger than H bonding strengths and comparable to inner- and outer-sphere complex formation. We conclude that, in analogy to polar and ionizable functional groups, components with aromatic pi-donor and -acceptor systems equip organic molecules with a substantial sorptive potential. This observation has important implications for the fate and transport of aromatic contaminants. The resulting sorptive interactions might also play a yet-overlooked functional role in the complex chain of processes which preserve NOM against decomposition.
486 citations
TL;DR: In this article, a review explores the application of this underutilized tool, with an emphasis on conceptual advances made using the state-factor approach and on detecting processes causing abrupt change in soil C stores.
Abstract: Research over the past several decades has clarified the mechanisms and timescales involved in stabilizing organic matter in soils, but we still lack process-based understanding sufficient for predicting how vulnerable soil carbon (C) is, given climatic or environmental change across a range of soil types and landscapes. Part of the problem is the emphasis on short-term studies and processes that dominate C balance at the point or soil profile scale, whereas other processes that dominate over longer timescales and larger spatial scales may actually be more important for determining the carbon balance of soils in a region. Radiocarbon is one of the only tools to study the dynamics of C in soils on decadal to millennial timescales. It provides a means for directly testing models of organic matter dynamics in ecosystems and, when measured in respired CO2 or dissolved organic carbon (DOC), provides evidence of shifts in microbial metabolism. This review explores the application of this underutilized tool, with an emphasis on conceptual advances made using the state-factor approach and on detecting processes causing abrupt change in soil C stores.
468 citations
TL;DR: Streamwater DOM is characterized from 11 coastal watersheds on the Gulf of Alaska that vary widely in glacier coverage and the bioavailability of DOM to marine microorganisms is significantly correlated with increasing 14C age, indicating that glacial runoff is a quantitatively important source of labile reduced carbon to marine ecosystems.
Abstract: Riverineorganicmattersupportsoftheorderofone-fifthofestuarine metabolism 1 . Coastal ecosystems are therefore sensitive to alteration of both the quantity and lability of terrigenous dissolved organic matter (DOM) delivered by rivers. The lability of DOM is thought to vary with age, with younger, relatively unaltered organic matter being more easily metabolized by aquatic heterotrophs than older, heavily modified material 2–4 . This view is developed exclusively from workinwatershedswhereterrestrialplantandsoilsourcesdominate streamwaterDOM.Here wecharacterize streamwaterDOMfrom11 coastal watersheds on the Gulf of Alaska that vary widely in glacier coverage (0–64 per cent). In contrast to non-glacial rivers, we find that the bioavailability of DOM to marine microorganisms is significantlycorrelatedwithincreasing 14 Cage.Moreover,themostheavily glaciated watersheds are the source of the oldest ( 4kyr 14 C age) and mostlabile(66percentbioavailable)DOM.Theseglacialwatersheds haveextreme runoffrates,inpart becausetheyare subject tosomeof the highest rates of glacier volume loss on Earth 5 .W e estimate the cumulative flux of dissolved organic carbon derived from glaciers contributing runoff to the Gulf of Alaska at 0.13 60.01Tgyr 21 (1Tg 510 12 g), of which 0.10Tg is highly labile. This indicates that glacial runoff is a quantitatively important source of labile reduced carbon to marine ecosystems. Moreover, because glaciers and ice sheets represent the second largest reservoir of water in the global hydrologic system, our findings indicate that climatically driven changes in glacier volume could alter the age, quantity and reactivity of DOM entering coastal oceans. Biogeochemicalcycling in coastal marginsnear riverine outflows is dominated by the influx of terrestrial organic matter and nutrients. The effect of anthropogenic increases in nutrient export on sensitive systemsiswell-documentedinregionssuchastheGulfofMexicozone of hypoxia 6 . It is much less clear how climate-induced shifts in the exportofterrigenousDOMwillaffectcoastalenvironments,although the reactivity of this carbon will be key as the extent of its incorporationintomarinefoodwebsdependslargelyonitschemicalcharacter 7 .
465 citations
TL;DR: In this paper, the authors studied the effect of soil fertility, crop, manuring, and management on changes in soil organic matter under temperate climatic conditions and found that the amount of organic matter in soil depends on the input of organic material, its rate of decomposition, the rate at which existing soil organic material is mineralized, soil texture, and climate.
Abstract: Soil organic matter is important in relation to soil fertility, sustainable agricultural systems, and crop productivity, and there is concern about the level of organic matter in many soils, particularly with respect to global warming. Long-term experiments since 1843 at Rothamsted provide the longest data sets on the effect of soil, crop, manuring, and management on changes in soil organic matter under temperate climatic conditions. The amount of organic matter in soil depends on the input of organic material, its rate of decomposition, the rate at which existing soil organic matter is mineralized, soil texture, and climate. All four factors interact so that the amount of soil organic matter changes, often slowly, toward an equilibrium value specific to the soil type and farming system. For any one cropping system, the equilibrium level of soil organic matter in a clay soil will be larger than that in a sandy soil, and for any one soil type the value will be larger with permanent grass than with continuous arable cropping. Trends in long-term crop yields show that as yield potential has increased, yields are often larger on soils with more organic matter compared to those on soils with less. The effects of nitrogen, improvements in soil phosphorus availability, and other factors are discussed. Benefits from building up soil organic matter are bought at a cost with large losses of both carbon and nitrogen from added organic material. Models for the buildup and decline of soil organic matter, the source and sink of carbon dioxide in soil, are presented.
TL;DR: In this article, a review of the available knowledge about the potential temperature sensitivity and actual temperature sensitivity of decomposition in situ, which ultimately depends on substrate availability, is presented, and the contradictory results demonstrate a need to focus research on biological and physicochemical controls of SOM stabilisation and destabilisation processes as a basis for understanding strictly causal relationships and kinetic properties of key processes that determine pool sizes and turnover rates of functional SOM pools.
Abstract: Soil organic matter (SOM) represents one of the largest reservoirs of carbon on the global scale. Thus, the temperature sensitivity of bulk SOM and of different SOM fractions is a key factor determining the response of the terrestrial carbon balance to climatic warming. We condense the available knowledge about the potential temperature sensitivity and the actual temperature sensitivity of decomposition in situ, which ultimately depends on substrate availability. We review and evaluate contradictory results of estimates of the temperature sensitivity of bulk SOM and of different SOM fractions. The contradictory results demonstrate a need to focus research on biological and physicochemical controls of SOM stabilisation and destabilisation processes as a basis for understanding strictly causal relationships and kinetic properties of key processes that determine pool sizes and turnover rates of functional SOM pools. The current understanding is that temperature sensitivity of SOM mineralisation is governed by the following factors: (1) the stability of SOM, (2) the substrate availability, which is determined by the balance between input of organic matter, stabilisation and mineralisation of SOM, (3) the physiology of the soil microflora, its efficiency in substrate utilisation and its temperature optima and (4) physicochemical controls of destabilisation and stabilisation processes, like pH and limitation of water, oxygen and nutrient supply. As soil microflora is functionally omnipotent and most SOM is of high age and stability, the temperature dependence of stable SOM pools is the central question that determines C stocks and stock changes under global warming.
TL;DR: Coronaviruses die off rapidly in wastewater, with T99.9 values of between 2 and 4 days, while poliovirus survived longer than coronavirus in all test waters, except the 4°C tap water.
Abstract: The advent of severe acute respiratory syndrome and its potential environmental transmission indicates the need for more information on the survival of coronavirus in water and wastewater. The survival of representative coronaviruses, feline infectious peritonitis virus, and human coronavirus 229E was determined in filtered and unfiltered tap water (4 and 23°C) and wastewater (23°C). This was compared to poliovirus 1 under the same test conditions. Inactivation of coronaviruses in the test water was highly dependent on temperature, level of organic matter, and presence of antagonistic bacteria. The time required for the virus titer to decrease 99.9% (T99.9) shows that in tap water, coronaviruses are inactivated faster in water at 23°C (10 days) than in water at 4°C (>100 days). Coronaviruses die off rapidly in wastewater, with T99.9 values of between 2 and 4 days. Poliovirus survived longer than coronaviruses in all test waters, except the 4°C tap water.
TL;DR: In this article, the authors compared the burial efficiency of organic carbon (buried OC: deposited OC) in a diverse set of 27 different sediments from 11 lakes, focusing on the potential effects of organic matter source, oxygen exposure, and protective sorption of OC onto mineral surfaces.
Abstract: We compared the burial efficiency of organic carbon (buried OC: deposited OC) in a diverse set of 27 different sediments from 11 lakes, focusing on the potential effects of organic matter source, oxygen exposure, and protective sorption of OC onto mineral surfaces. Average OC burial efficiency was high (mean 48%), and it was particularly high in sediments receiving high input of allochthonous organic matter (mean 67%). Further, OC burial efficiency was strongly negatively related to the oxygen exposure time, again particularly so in sediments receiving high allochthonous loads. On the other hand, OC burial efficiency was not related to the mineral surface area, which was used as a proxy of the sorption capacity of the mineral phase for OC. The high OC burial efficiency in many lake sediments can thus be attributed to the frequent and significant input of allochthonous organic matter to lakes, as well as to a strong dependence of OC burial efficiency on oxygen exposure time. This study demonstrates that the carbon sink in lake sediments alters the OC export from the continents to the sea and that the fate of OC in lake sediments (burial vs. mineralization to carbon dioxide and methane) is highly sensitive to environmental conditions.
TL;DR: In this paper, the authors measured litter and soil organic matter (SOM) decomposition and microbial enzyme activity in a long-term N fertilization experiment at eight forested and grassland sites in central Minnesota, USA, to determine variation among sites in enzyme activity, and variation in the response of enzymes, litter decomposition, and soil respiration to added N.
Abstract: Long-term nitrogen (N) addition experiments have found positive, negative, and neutral effects of added N on rates of decomposition. A leading explanation for this variation is differential effects of N on the activity of microbially produced extracellular enzymes involved in decomposition. Specifically, it is hypothesized that adding N to N-limited ecosystems increases activity of cellulose degrading enzymes and decreases that of lignin degrading enzymes, and that shifts in enzyme activity in response to added N explain the decomposition response to N fertilization. We measured litter and soil organic matter (SOM) decomposition and microbial enzyme activity in a long-term N fertilization experiment at eight forested and grassland sites in central Minnesota, USA, to determine (1) variation among sites in enzyme activity, (2) variation in the response of enzymes, litter decomposition, and soil respiration to added N, and (3) whether changes in enzyme activity in response to added N explained variability among sites in the effect of N on litter and SOM decomposition. Site differences in pH, moisture, soil carbon, and microbial biomass explained much of the among-site variation in enzyme activity. Added N generally stimulated activities of cellulose degrading and N- and phosphorus-acquiring enzymes in litter and soil, but had no effect on lignin degrading enzyme activity. In contrast, added N generally had negative or neutral effects on litter and SOM decomposition in the same sites, with no correspondence between effects of N on enzyme activity and decomposition across sites.
TL;DR: In this paper, the authors performed a meta-analysis of the literature comparing the environmental impacts of organic and conventional farming and linking these to differences in management practices. And they concluded that organic farming contributes positively to agro-biodiversity (breeds used by the farmers) and natural biodiversity.
Abstract: Purpose – This paper aims to perform a meta‐analysis of the literature comparing the environmental impacts of organic and conventional farming and linking these to differences in management practises. The studied environmental impacts are related to land use efficiency, organic matter content in the soil, nitrate and phosphate leaching to the water system, greenhouse gas emissions and biodiversity.Design/methodology/approach – The theoretic framework uses the driver‐state‐response framework and literature data were analysed using meta‐analysis methodology. Meta‐analysis is the statistical analysis of multiple study results. Data were obtained by screening peer reviewed literature.Findings – From the paper's meta‐analysis it can conclude that soils in organic farming systems have on average a higher content of organic matter. It can also conclude that organic farming contributes positively to agro‐biodiversity (breeds used by the farmers) and natural biodiversity (wild life). Concerning the impact of the o...
TL;DR: In this article, three different tillage practices for monoculture of winter wheat (Triticum aestivum L.) were evaluated after 11 years, and the authors concluded that both variants of conservation tillage (NT and ST) increase SOC stock in the rainfed farming areas of northern China and are therefore more sustainable practices than those currently being used.
Abstract: The Loess Plateau in northwest China is one of the most eroded landscapes in the world, and it is urgent that alternative practices be evaluated to control soil erosion. Our objective was to determine how three different tillage practices for monoculture of winter wheat ( Triticum aestivum L.) affected soil organic carbon (SOC) and N content after 11 years. Conventional tillage with residue removal (CT), shallow tillage with residue cover (ST), and no-tillage with residue cover (NT) were investigated. Carbon and N in various aggregate-size classes and various labile organic C fractions in the 0–15- and 15–30-cm soil layers were evaluated. The ST and NT treatments had 14.2 and 13.7% higher SOC stocks and 14.1 and 3.7% higher total N (N t ) stocks than CT in the upper 15 cm, respectively. Labile C fractions: particulate organic C (POC), permanganate oxidizable C (KMnO 4 -C), hot-water extractable C (HWC), microbial biomass C (MBC) and dissolved organic C (DOC) were all significantly higher in NT and ST than in CT in the upper 15 cm. KMnO 4 -C, POC and HWC were the most sensitive fractions to tillage changes. The portion of 0.25–2 mm aggregates, mean weight diameter (MWD) and geometric mean diameter (GMD) of aggregates from ST and NT treatments were larger than from CT at both 0–15- and 15–30-cm soil depths. The ST and NT treatments had significantly higher SOC and N t in the 0.25–2 mm fraction at both depths and significantly higher N t content in the upper 15 cm. Positive significant correlations were observed between SOC, labile organic C fractions, MWD, GMD, and macroaggregate (0.25–2 mm) C within the upper 15 cm. We conclude that both variants of conservation tillage (NT and ST) increase SOC stock in the rainfed farming areas of northern China and are therefore more sustainable practices than those currently being used.
TL;DR: In this article, the authors measured the potential activity of six hydrolytic enzymes at 4 and 20°C on four sampling dates in tussock, intertussock and shrub organic soils at Toolik Lake, Alaska.
Abstract: Arctic soils contain large amounts of organic matter due to very slow rates of detritus decomposition. The first step in decomposition results from the activity of extracellular enzymes produced by soil microbes. We hypothesized that potential enzyme activities are low relative to the large stocks of organic matter in Arctic tundra soils, and that enzyme activity is low at in situ temperatures. We measured the potential activity of six hydrolytic enzymes at 4 and 20 °C on four sampling dates in tussock, intertussock, shrub organic, and shrub mineral soils at Toolik Lake, Alaska. Potential activities of N-acetyl glucosaminidase, β-glucosidase, and peptidase tended to be greatest at the end of winter, suggesting that microbes produced enzymes while soils were frozen. In general, enzyme activities did not increase during the Arctic summer, suggesting that enzyme production is N-limited during the period when temperatures would otherwise drive higher enzyme activity in situ. We also detected seasonal variations in the temperature sensitivity (Q10) of soil enzymes. In general, soil enzyme pools were more sensitive to temperature at the end of the winter than during the summer. We modeled potential in situβ-glucosidase activities for tussock and shrub organic soils based on measured enzyme activities, temperature sensitivities, and daily soil temperature data. Modeled in situ enzyme activity in tussock soils increased briefly during the spring, then declined through the summer. In shrub soils, modeled enzyme activities increased through the spring thaw into early August, and then declined through the late summer and into winter. Overall, temperature is the strongest factor driving low in situ enzyme activities in the Arctic. However, enzyme activity was low during the summer, possibly due to N-limitation of enzyme production, which would constrain enzyme activity during the brief period when temperatures would otherwise drive higher rates of decomposition.
TL;DR: In this article, an increase in 14C-based mean residence time (MRT) and a shift from plant to microbial origin was observed in soil organic matter (OM) accumulations.
Abstract: Sequential density fractionation separated soil particles into “light” predominantly mineral-free organic matter vs. increasingly “heavy” organo-mineral particles in four soils of widely differing mineralogy. With increasing particle density C concentration decreased, implying that the soil organic matter (OM) accumulations were thinner. With thinner accumulations we saw evidence for both an increase in 14C-based mean residence time (MRT) of the OM and a shift from plant to microbial origin.Evidence for the latter included: (1) a decrease in C/N, (2) a decrease in lignin phenols and an increase in their oxidation state, and (3) an increase in δ13C and δ15N. Although bulk-soil OM levels varied substantially across the four soils, trends in OM composition and MRT across the density fractions were similar. In the intermediate density fractions (~1.8–2.6 g cm−3), most of the reactive sites available for interaction with organic molecules were provided by aluminosilicate clays, and OM characteristics were consistent with a layered mode of OM accumulation. With increasing density (lower OM loading) within this range, OM showed evidence of an increasingly microbial origin. We hypothesize that this microbially derived OM was young at the time of attachment to the mineral surfaces but that it persisted due to both binding with mineral surfaces and protection beneath layers of younger, less microbially processed C. As a result of these processes, the OM increased in MRT, oxidation state, and degree of microbial processing in the sequentially denser intermediate fractions. Thus mineral surface chemistry is assumed to play little role in determining OM composition in these intermediate fractions. As the separation density was increased beyond ~2.6 g cm−3, mineralogy shifted markedly: aluminosilicate clays gave way first to light primary minerals including quartz, then at even higher densities to various Fe-bearing primary minerals. Correspondingly, we observed a marked drop in δ15N, a weaker decrease in extent of microbial processing of lignin phenols, and some evidence of a rise in C/N ratio. At the same time, however, 14C-based MRT time continued its increase. The increase in MRT, despite decreases in degree of microbial alteration, suggests that mineral surface composition (especially Fe concentration) plays a strong role in determining OM composition across these two densest fractions.
TL;DR: In this paper, controlled incubations of a wetland soil were performed under oxic and anoxic conditions to investigate the extent to which the following processes account for this phenomenon: i) production of organic metabolites by microbes during soil reduction, ii) release of organic matter (OM) from Mn- and Fe-oxyhydroxides that undergo reductive dissolution, and iii) desorption of OM from soil minerals due to pH changes.
TL;DR: In this paper, a 57-day irradiation experiment was conducted to examine the fate of terrigenous DOM derived from tropical ecosystems, and it was shown that these commonly used measurements for examination of DOM in the oceans have a higher rate of photochemical decay than the bulk DOC pool.
Abstract: [1] Photochemical degradation of Congo River dissolved organic matter (DOM) was investigated to examine the fate of terrigenous DOM derived from tropical ecosystems. Tropical riverine DOM receives greater exposure to solar radiation, particularly in large river plumes discharging directly into the open ocean. Initial Congo River DOM exhibited dissolved organic carbon (DOC) concentration and compositional characteristics typical of organic rich blackwater systems. During a 57 day irradiation experiment, Congo River DOM was shown to be highly photoreactive with a decrease in DOC, chromophoric DOM (CDOM), lignin phenol concentrations (S8) and carbon-normalized yields (L8), equivalent to losses of � 45, 85–95, >95 and >95% of initial values, respectively, and a +3.1 % enrichment of the d 13 C-DOC signature. The loss of L8 and enrichment of d 13 C-DOC during irradiation was strongly correlated (r = 0.99, p < 0.01) indicating tight coupling between these biomarkers. Furthermore, the loss of CDOM absorbance was correlated to the loss of L8 (e.g., a355 versus L8; r = 0.98, p < 0.01) and d 13 C-DOC (e.g., a355 versus d 13 C; r = 0.97, p < 0.01), highlighting the potential of CDOM absorbance measurements for delineating the photochemical degradation of lignin and thus terrigenous DOM. It is apparent that these commonly used measurements for examination of terrigenous DOM in the oceans have a higher rate of photochemical decay than the bulk DOC pool. Further process-based studies are required to determine the selective removal rates of these biomarkers for advancement of our understanding of the fate of this material in the ocean.
TL;DR: The transformation of organic matter during anaerobic digestion of mixtures of energetic crops, cow slurry, agro-industrial waste and organic fraction of municipal solid waste was studied by analysing different samples at diverse points during the anaer aerobic digestion process in a full-scale plant.
TL;DR: In this article, the authors evaluated the soil carbon and nitrogen contents in different soil organic matter (SOM) pools (light and heavy fractions), the role of light and heavy-fraction C in SOC sequestration, and culturable microbial counts in the surface (0-20 cm) of a fluvo-aquic soil after 18 years of fertilization treatments under a wheat-maize cropping system in the North China Plain.
TL;DR: In this article, the level of rhizosphere-dependent organic matter decomposition with the C and N balance of the whole plant-soil system was investigated, and it was shown that the effect of the priming effect on SOM decomposition varied widely, from zero to more than 380% of the unplanted control.
Abstract: Understanding soil organic matter (SOM) decomposition and its interaction with rhizosphere processes is a crucial topic in soil biology and ecology. Using a natural 13C tracer method to separately measure SOM-derived CO2 from root-derived CO2, this study aims to connect the level of rhizosphere-dependent SOM decomposition with the C and N balance of the whole plant–soil system, and to mechanistically link the rhizosphere priming effect to soil microbial turnover and evapotranspiration. Results indicated that the magnitude of the rhizosphere priming effect on SOM decomposition varied widely, from zero to more than 380% of the unplanted control, and was largely influenced by plant species and phenology. Balancing the extra soil C loss from the strong rhizosphere priming effect in the planted treatments with C inputs from rhizodeposits and root biomass, the whole plant–soil system remained with a net carbon gain at the end of the experiment. The increased soil microbial biomass turnover rate and the enhanced evapotranspiration rate in the planted treatments had clear positive relationships with the level of the rhizosphere priming effect. The rhizosphere enhancement of soil carbon mineralization in the planted treatments did not result in a proportional increase in net N mineralization, suggesting a possible de-coupling of C cycling with N cycling in the rhizosphere.
TL;DR: A review of the application of thermal analysis techniques in soil science can be found in this paper, where the authors summarize the historical development of this technique and present the emerging application for the characterization of soil organic matter.
TL;DR: In this paper, the authors used the V:Mo ratio in the marine fraction of the Kimmeridge Clay Formation to hindcast the hydrography and biogeochemical conditions of deposition of a black shale of Late Jurassic age from its trace metal and organic carbon content.
TL;DR: The effects of ash weathering and organic matter accumulation over time on the chemical, physical and biological properties of the developing ash-derived soil are not well understood and require further study.
TL;DR: In this article, the authors estimate nutrient production rates using the standard estuarine model and a non-steady-state box model, separate nutrient fluxes associated with fresh and saline submarine groundwater discharge (SGD), and estimate offshore fluxes from radium isotope distributions.
TL;DR: The concepts that have been employed to interpret N mineralization–immobilization in soil are described and discussed, and how N turnover is related to the characteristics of organic N and the biota conducting the transformations are discussed.
Abstract: The aim of this review is to describe and discuss the concepts that have been employed to interpret N mineralization–immobilization in soil, and how N turnover is related to the characteristics of organic N and the biota conducting the transformations. A brief survey of the period before the arrival of electronic searches became available provides access to the classical literature that can help interpret today's challenges. Classical (acid hydrolysis) and modern spectrometry and spectroscopy techniques indicate that protein N is the prevalent component of organic N in soil. The presence of heterocyclic N can indicate its abiotic, partial synthesis as in fire-affected soils. Clays and pedogenic oxides can protect organic N against microbial degradation. The evidence for such protection is mostly based on in vitro studies involving pure clays, and proteins and their relevance to field conditions requires further work. The proteomic approach, with extraction and characterization of proteins stabilised by soil colloids (structural proteomics) might give further insights into this area. Functional proteomics can improve our understanding of the degradation of organic pollutants and organic debris as well as identifying the molecular colloquia between microorganisms and between soil biota and plant roots. Subdivision of organic N into sub-pools has helped to interpret mechanistic studies and modelling of N dynamics. Uncomplexed organic matter, obtained by physical fractionation procedures, is considered a labile pool. The interpretation of N mineralization measurements is affected by immobilization during microbial attack especially in high-C environments. Transfer of materials among particle size fractions and changes in microbiological properties of aggregates also can occur during fractionation procedures. Classical mineralization–immobilization turnover (MIT) does not always occur since microorganisms (and plants) can take up amino acid N with intracellular deamination. Protozoa, due to their grazing activities, can influence not only N mineralization but also the composition of rhizosphere–plant growth stimulating communities. Differences between N-poor and N-rich microsites, occurring in the same soil, can markedly affect the competition for N between plants and microorganisms especially the nitrifiers. The use of molecular techniques has allowed the identification of unculturable microorganisms and functional genes in the N cycle. Archeae are probably capable of oxidising NH4+ to NO3− and anerobic ammonia oxidation (Ammonox) bacteria have been identified in biofilms and probably also occur in soils. The use of nitrate as an electron acceptor is encoded by specific gene clusters but nitrate reduction also occurs in dissimilatory nitrate reduction.