Soil Organic Carbon Degradation Research Articles

Splash erosion-induced soil aggregate turnover and associated organic carbon dynamics

The splash erosion-induced soil aggregate turnover is important to understand the mechanisms of soil degradation and soil organic carbon (SOC) cycling. However, the relationship between aggregate turnover and rainfall impact has not been explored directly because of methodological difficulties. The soil samples, developed from Quaternary red clay, were adopted from three land use types: natural forest (NF), garden/shrubs (AS), and abandoned meadow (AM). By using rare earth oxides to trace the transformation in soil aggregates of four size-fractions (>2 mm, large macroaggregate; 0.25–2 mm, small macroaggregate; 0.053–0.25 mm, microaggregate; <0.053 mm, silt and clay fraction), we investigated aggregate turnover based on splash experiments at four different rainfall kinetic energies (501.95, 326.00, 252.50 and 153.39 J·m−2·h−1). After splash erosion, the rare earth oxides concentrations and SOC content in different soil aggregate fractions were measured. Our results indicated that the soil aggregates mainly followed the direction of breakdown, with higher cumulative breakdown rates than formation rates. Specifically, with the rainfall kinetic energy increased to 501.95 J·m−2·h−1, the three soil samples (NF, AS, AM) exhibited the highest breakdown rates of 48.35%, 31.95% and 39.29% in large macroaggregates, respectively. Meanwhile, the formation pathway was mainly in the direction of forming small macroaggregates. Compared to soil aggregate from NF, AS and AM aggregates had higher proportion of silt and clay fraction transformed into larger aggregates. Moreover, the changes of SOC varied in aggregates during splash process. With the increase of rainfall kinetic energy, the organic matter content gradually decreased in large aggregates and microaggregates in AM and AS (P < 0.05), in contrast to a trend of first increasing and then decreasing (P < 0.05) in their small aggregates. In the clay particles, with the increase of rainfall kinetic energy, the SOC content gradually decreased in NF (P < 0.05) and fluctuated in AS, with both their organic matter content lower than before the splash test (P < 0.05). The loss of SOC in total soil was linearly related to the net breakdown rate of soil aggregates (P < 0.05, R2 =0.635). Therefore, we proposed a framework to highlight the quantitative relationship between aggregate turnover and SOC degradation under splash, which may enrich the mechanisms of soil erosion and soil degradation in the positions where erosion takes place.

Effects of soil organic carbon metabolism on electro-bioremediation of petroleum-contaminated soil

Soil organic carbon (SOC) potentially interacts with microbial metabolism and may affect the degradation of petroleum-derived carbon (PDC) in the electro-bioremediation of petroleum-contaminated soil. This study evaluated the interactions among organic carbon, soil properties, and microbial communities to explore the role of SOC during the electro-bioremediation process. The results showed that petroleum degradation exerted superposition and synergistic electrokinetic and bioremediation effects, as exemplified by the EB and EB-PR tests, owing to the maintenance and enhancement of SOC utilization (P/S value), respectively. The highest P/S value (2.0–2.4) was found in the electrochemical oxidation zone due to low SOC consumption. In the biological oxidation zones, electric stimulation enhanced the degradation of PDC and SOC, with higher average P/S values than those of the Bio test. Soil pH, Eh, inorganic ions, and bioavailable petroleum fractions were the main factors reshaping the microbial communities. SOC metabolism effectively buffered the stress of environmental factors and pollutants while maintaining functional bacterial abundance, microbial alpha diversity, and community similarity, thus saving the weakened PDC biodegradation efficiency in the EB and EB-PR tests. The study of the effect of SOC metabolism on petroleum biodegradation contributes to the development of sustainable low-carbon electro-bioremediation technology.

Responses of soil organic carbon cycle to land degradation by isotopically tracing in a typical karst area, southwest China.

The loss of soil organic carbon (SOC) under land degradation threatens crop production and reduces soil fertility and stability, which is more reflected in eco-sensitive environments. However, fewer studies simultaneously compared SOC variations and δ13CSOC compositions under diverse land uses, especially in karst areas. Soil profiles from two agricultural lands and a secondary forest land were selected to analyze SOC contents and their stable isotope composition (δ13CSOC) in a typical karst area located in southwest China to understand the response of the SOC cycle to land degradation. Moreover, the relationships between SOC contents and mean weight diameter (MWD) and soil erodibility (K) factor were comprehensively analyzed for assessing the response of SOC to soil degradation risk. The mean SOC content was found to be the lowest in abandoned cropland (6.91 g/kg), followed by secondary forest land (9.31 g/kg) and grazing shrubland (34.80 g/kg), respectively. Meanwhile, the δ13CSOC values exhibited the following trend: secondary forest land (mean: -23.79‰) ≈abandoned cropland (mean: -23.76‰) >shrubland (mean: -25.33‰). The isotopic tracing results suggested that plant litter was the main contributor to SOC in the secondary forest land. Whereas abundant nitrogen from goat feces enhanced plant productivity and resulted in additional accumulation of SOC in the grazing shrubland. Conversely, long-term cultivation led to the depletion of SOC sequestration by the loss of calcium. In surface soils, the fractionation of δ13CSOC were considerably affected by the decomposition of SOC by soil microorganisms and covered vegetation rather than agricultural influences. The findings indicate that the cycling of SOC and soil stability in the calcareous soil of southwest China are largely regulated by different land uses and the presence of vegetation cover. The depletion of SOC and soil physical degradation pose significant challenges for abandoned cropland, particularly in the karst area, where land degradation is inevitable. Nevertheless, moderate grazing enhances SOC levels, which is beneficial to the land fertility maintenance in the karst region. Therefore, more emphasis should be placed on the cultivation methods and management strategies for abandoned cropland in the karst area.

Anthropogenic N input increases global warming potential by awakening the “sleeping” ancient C in deep critical zones

Even a small net increase in soil organic carbon (SOC) mineralization will cause a substantial increase in the atmospheric CO2 concentration. It is widely recognized that the SOC mineralization within deep critical zones (2 to 12m depth) is slower and much less influenced by anthropogenic disturbance when compared to that of surface soil. Here, we showed that 20 years of nitrogen (N) fertilization enriched a deep critical zone with nitrate, almost doubling the SOC mineralization rate. This result was supported by corresponding increases in the expressions of functional genes typical of recalcitrant SOC degradation and enzyme activities. The CO2 released and the SOC had a similar 14C age (6000 to 10,000years before the present). Our results indicate that N fertilization of crops may enhance CO2 emissions from deep critical zones to the atmosphere through a previously disregarded mechanism. This provides another reason for markedly improving N management in fertilized agricultural soils.

Reactive iron, not fungal community, drives organic carbon oxidation potential in floodplain soils

Wetlands host ∼20% of terrestrial organic carbon and serve as a major sink for atmospheric carbon. Anoxic soils and sediments accrue soil organic carbon (SOC) partly by hampering the activity of extracellular oxidative enzymes that break down phenolic polymers. Upon aeration, fungal-driven oxidative enzymatic depolymerization and microbial respiration of released monomers ensue. Redox-active metals can simultaneously catalyze abiotic nonspecific oxidation of SOC, notable examples including Mn(III) or Fe(II) through Fenton-like, hydrogen peroxide-catalyzed oxidative radical production. However, the extent of reactive metal contributions to biotic and abiotic SOC degradation is not understood in the context of natural environments with diverse redox chemistry. We tested the relative contributions of fungi, Mn(III) and Fe(II) to phenolic substrate (L-DOPA) oxidation in floodplain soils representing a range of transient redox conditions driven by permanent vs. periodic flooding. Phenol oxidative potential was highest in permanently flooded soils with fewer fungal taxa known for observed (per)oxidase activity and instead correlated with HCl-extractable Fe(II), Fe(total) and Fe(II)/Fe(total), suggesting a specific role of Fe(II). Fe(II) additions enhanced phenol oxidative potential in sterilized and non-sterilized soils in the presence of hydrogen peroxide, indicating abiotic Fe-mediated radical chemistry could significantly enhance wetland SOC oxidative depolymerization throughout redox-active floodplain soils. Fungal community composition did not correlate to phenol oxidative potential overall and only more oxic soils adjacent to the river with diverse fungal communities showed declining oxidative potential after sterilization. Mn(III) addition did not significantly enhance phenol oxidative potential across all soils, although it appeared to drive fungal-mediated oxidative potential in the most aerated floodplain soils. Understanding how metals mediate SOC depolymerization as abiotic oxidants or microbially-harnessed enzyme cofactors and substrates in soils under variable hydrologic controls will improve our ability to represent depolymerization in terrestrial carbon models in wetland and other frequently saturated soils.

Sources and chemical stability of soil organic carbon in natural and created coastal marshes of Louisiana

Coastal marshes are globally important for sequestering carbon, yet sea-level rise and anthropogenic stressors can reduce their capacity as carbon sinks. Marsh restoration can offset a portion of carbon loss through the degradation of natural marshes, but potential differences in the sources and stability of soil organic carbon (SOC) between created and natural marshes may affect their function as a long-term carbon sink. Here, we examine the sources and chemical stability of SOC in natural and created marshes across the Gulf coast of Louisiana, USA. Marshes were examined along an estuarine salinity gradient in a former interdistributary basin of the Mississippi River Delta and in six created marshes across a 32-year chronosequence and a natural reference marsh (n = 6) in the Chenier Plain. Carbon source was assessed using δ13C analysis and chemical stability was determined through an acid hydrolysis digestion that removed labile carbon (LC). Soil δ13C values suggested that the local vegetation dominated SOC in all natural marshes although brackish marshes had a mix of sources and degradation of SOC. Recalcitrant carbon (RC) was 72.2 ± 0.5 % of SOC across fresh, brackish and saline marshes. The depth-averaged RC accumulation rate was almost three times greater than LC accumulation rate, yet both contributed significantly to accretion and long-term SOC accumulation (124–132 g m−2 y−1 in natural marshes). RC and SOC accumulation rate increased with mineral sediment accumulation rate. For the created marshes, SOC became increasingly recalcitrant due to an increase in in-situ plant inputs, but accumulation rates were lower than the natural marshes. Overall, this study illustrates that natural marshes have a large stock of RC from the vegetation while dredge sediment created marshes have no plant-derived carbon initially, which accumulates slowly thereafter. Restoration practices may be improved by preserving and augmenting these deteriorating but carbon-rich natural marshes.

Effects of decadal nitrogen and phosphorus fertilization on microbial taxonomic and functional attributes associated with soil organic carbon decomposition and concentration in an alpine meadow

Microorganisms drive many important soil processes and ecosystem functioning; however, the soil microbial community can respond to environmental change, which can alter decomposition and storage of soil organic carbon (SOC). We assessed soil microbial taxonomic and functional attributes linked to SOC decomposition and concentration based on a grassland experiment fertilized with nitrogen (N) and phosphorus (P). There were divergent responses at both the taxonomic and functional level to fertilization, with a greater quantitative response in terms of the taxonomic attributes of bacteria and fungi than the abundance of SOC degradation genes. There was a negative correlation between SOC concentration and the abundance of SOC degradation genes. This indicated that N and P fertilization had the potential to stimulate SOC decomposition through modification of the soil microbial community, particularly by increasing the abundance of SOC degradation genes. Redundancy and variation partitioning analyses, as well as structural equation models revealed that soil total P was a pivotal factor shaping the microbial community both for taxonomic and functional gene attributes in the alpine meadow. The abundance of SOC degrading genes had stronger effects on SOC concentrations than did taxonomic attributes. Quantification of the abundance of SOC degradation genes can provide more direct and effective understanding of SOC decomposition processes and prediction of shifts in soil function as organic C pool in grasslands than a focus on the taxonomy of microbial communities. Given the important role of microbial community in regulating SOC decomposition and lower SOC concentrations under N and P fertilization, we recommend that the alpine meadow should be protected from mineral fertilizers, particularly P alone fertilization, to sustain high grassland ecosystem function as SOC sink in this region.

Adsorption of soil invertase to goethite, gibbsite and their organic complexes and the effects on enzyme catalytic performance

Soil invertase plays important role in the enzyme-driven degradation of soil organic carbon, and influenced by the adsorption of minerals or clays. We studied the adsorption effects of goethite, gibbsite and their organic complexes (with tannic acid) on invertase, aiming to explore the influence on the mobility and catalytic behavior of invertase. Results showed that both minerals and mineral–tannic acid complexes (henceforth ‘complexes’) had considerable adsorption capacity for invertase and restricted its mobility. The catalytic performance of invertase was characterized. Under optimum conditions, the kinetics of the enzyme was reduced markedly after adsorption, indicating the decreased enzyme-substrate affinity. Adsorbed invertases retained 30.9–46.1% of the original activity and the mineral-adsorbed enzymes showed lower activities than the complex-adsorbed enzymes. All minerals and complexes had similar patterns on the influence on the pH and thermal properties of invertase, in which the adsorption improved the pH stability of the enzyme at various pH values, lowered the pH sensitivity and shifted the optimum pH towards the acid sites; while decreased thermal stability at various temperature values and higher temperature sensitivity of the adsorbed invertase were observed. In a 150-h incubation under soil–like conditions, all adsorbed invertases showed greater long–term stability than free invertase, and higher total activities (sum of adsorbed activities and dissolved activities) in adsorption system were observed after the incubation. These results implied that, in a soil microenvironment, the goethite, gibbsite and their organic complexes have a protection effect on invertase through adsorption.

Relationship Between Irrigation Thresholds and Potato Tuber Depth in Sandy Soil

Soil disturbance resulting from tuber crop harvesting is a major threat to soil health. The depth of soil intervention is a critical factor that further strengthens the effects of such disturbance and makes harvest one of the most harmful cropping operations. In the case of potato, soil moisture is a determining factor for root and stolon development, hence, a deeper soil intervention may be required at harvest. While potato ranks as the fourth most cultivated crop worldwide, the impact of soil moisture on potato tuber vertical and horizontal distribution has received very little attention. The objective of this study was to evaluate the effects of four soil matric potential thresholds (SMPTs; –10, –20, –30, and –45 kPa) on the spatial (vertical and horizontal) distribution of potato tubers grown in plastic containers filled with sandy soil using an X-ray computed tomography scanner. The results of the experiments conducted in a greenhouse environment suggest that the horizontal distribution of the tubers did not differ significantly across the irrigation treatments. However, a linear relationship between SMPT, and therefore irrigation threshold, and potato tuber depth was observed. In addition, the deepest tuber position was observed under the –10 kPa SMPT, while the tubers were closer to the soil surface under the –45 kPa SMPT, which could lead to a greater preponderance of tuber diseases such as late blight or greening. Thus, potato irrigation events implementing a SMPT between –20 and –30 kPa could reduce the harvest depth, hence, decreasing the negative impacts of soil disturbance on soil structural stability and soil organic carbon degradation while mitigating the impacts of disease as well as reducing fuel costs, greenhouse gas emissions, soil loss and erosion.

Soil, vine, climate change; the challenge of predicting soil carbon changes and greenhouse gas emissions in vineyards and is the 4 per 1000 goal realistic?

The evaluation of the current and future impact of climate change on viticulture requires an integrated view on a complex interacting system within the soil-plant-atmospheric continuum under continuous change. Aside of the globally observed increase in temperature in almost all viticulture regions for at least four decades, we observe several clear trends at the regional level in the ratio of precipitation to potential evapotranspiration. Additionally the recently published 6th assessment report of the IPCC (The physical science basis) shows case-dependent further expected shifts in climate patterns which will have substantial impacts on the way we will conduct viticulture in the decades to come.Looking beyond climate developments, we observe rising temperatures in the upper soil layers which will have an impact on the distribution of microbial populations, the decay rate of organic matter or the storage capacity for soil organic carbon (SOC). All this influences the emission of greenhouse gases (GHGs) and the viscosity of water in the soil-plant pathway, altering the transport of water. Interactions between micro-organisms in the rhizosphere, the grapevine root system, degradation and fixation processes of SOC are complex and poorly understood but respond to environmental factors (such as increased soil temperatures), the plant material (rootstock for instance), and the cultivation system (for example bio-organic versus conventional, cover crop use versus open tillage). Increasing SOC stocks is discussed as a measure to reduce soil GHG emissions with the potential to improve the balance between GHG emissions and carbon removal from the atmosphere. Yet it is difficult to deduct the impact of climatic changes and cultivation practices on patterns of carbon storage or losses from soils. This paper presents a first attempt to quantify these potential impacts on SOC for a vineyard location using the RothC-model (Coleman and Jenkinson, 2005) in combination with the Geisenheim long-term (&gt; 100-year) soil temperature record and climate predictions by the STAR II-model of the Potsdam Institute of Climate Impact using a medium realization run (Orlowsky et al., 2008).It is shown that retaining pruning wood and using a full cover crop yielded a SOC increase of 16.2 t C ha-1 over time. However, CO2 emissions over the simulated time span were only slightly less than C-storage in the soil. It is concluded that cover crops in vineyards helps to achieve CO2-neutrality but additional measures are required to make vineyards a significant C-sink.

Effect of Shrub Encroachment on Alpine Grass Soil Microbial Community Assembly

Shrub encroachment is a common phenomenon in grasslands all over the world. However, little is known about the consequences of shrub encroachment on soil microbial community structure in different layers. We investigated the effects of three common shrub encroachment (Potentilla fruticosa, Spiraea alpina, and Caragana microphylla) on grassland soil bacterial communities at the surface and deep layers in Qinghai–Tibetan Plateau. 16S rRNA gene sequencing was used to investigate the bacterial communities, and Fourier translation infrared spectroscopy (FTIR) was conducted to assess the soil organic carbon (SOC) chemical composition in surface and deep layers of shrub-encroached alpine grassland. Shrub encroachment has significantly increased SOC degradation in deep layer. After shrub invasion, the bacterial alpha-diversity in the surface and deep soil was higher than in grassland soil (except for the surface layer of C. microphylla). Factors driving bacterial community changes in soil surface and deep layer were different. Among the soil properties that were measured, SOC content was the primary factor that altered soil bacterial community composition in surface soil, while SOC chemical composition (aromatic and polysaccharides) was the main driver in the deep layer. A total of 39 and 42 biomarkers were found by linear discriminant analysis (LDA) effect size (LEfSe) in the surface and deep soil layer among the four sampling groups, respectively. Phylogenetic investigation of communities by reconstruction of unobserved states (PICRUSt) showed that the most abundant predicted functional genes belonged to categories of metabolism (52.83%) in the primary metabolic pathway. Redundancy analysis (RDA) results also showed that the key factors affecting bacterial metabolic function appear to be SOC, pH, and aromatics, which are largely consistent with those affecting community composition. We suggest that shrub encroachment affect the structure, diversity, and predicted functions of bacterial communities, thus affecting the C cycle in this region.

Evaluation of Land Degradation Neutrality in Inner Mongolia Combined with Ecosystem Services

Currently, the internationally recognized land degradation neutrality (LDN) effort is evaluated using three indicators: land use/cover, land productivity, and carbon stocks. However, these three indicators may not completely capture the factors influencing LDN, and some evaluation rules are not in line with the land restoration goals of China. Therefore, this study introduces the ecosystem service value (ESV) indicator, assesses the differences in connotation and evaluation methods between ESV and LDN, and puts forward an evaluation rule that integrates their advantages, so as to carry out an evaluation of LDN in Inner Mongolia. The conclusions are as follows: (a) The ESVs of the Inner Mongolia Autonomous Region in 2000, 2005, 2010, 2015, and 2020 were USD 287.49, 286.04, 285.72, 286.38, and 287.90 billion, respectively, which presents a slight trend of decrease and then increase over time. (b) The modified LDN evaluation rule mainly includes the following changes to the LUCC evaluation rule: (1) the original degradation of cropland to grassland is considered as restoration, (2) water bodies participate in the transformation evaluation between land use types, and (3) the evaluation of transformed secondary land use types is added. The evaluation of net primary productivity (NPP) and soil organic carbon (SOC) still follow the method formulated by the United Nations Convention to Combat Desertification (UNCCD). (c) The proportion of degraded, stable, and restored land area within the LUCC were 11.31%, 77.34%, and 11.35%, respectively. The proportion of restored area is greater than the proportion of degraded land, which indicates that LDN has been achieved in Inner Mongolia according to the LUCC evaluation. The areas of degradation, stability, and restoration for NPP accounted for 0.10%, 40.52%, and 59.38% of the total area, respectively, with the restored area being much larger than the degraded area. The areas of SOC degradation, stability, and restoration accounted for 13.06%, 74.82%, and 12.11% of the total area, respectively, and the degraded area was slightly larger than the restored area. (d) The LDN evaluation results showed that the proportions of degraded, stable, and restored areas were 21.80%, 27.25%, and 50.96%, respectively. From these results, it is clear that Inner Mongolia has achieved the LDN target. Compared with the rules formulated by the UNCCD, for the LDN evaluation results implementing the modified rule, the proportion of degraded land increased by 2.44%, the proportion of stable land decreased by 1.52%, and the proportion of restored land decreased by 0.92%. In the future, Inner Mongolia should strengthen the implementation of a series of ecological restoration projects to obtain greater ecological benefits.

Optimization of plant harvest and management patterns to enhance the carbon sink of reclaimed wetland in the Yangtze River estuary

Estuarine wetlands are often located in economically developed and densely populated estuarine deltas, which are frequently disturbed and threatened by human activities. Reclamation, as an important way to alleviate the demand for local land resources, can lead to habitat destruction of natural coastal wetlands and weakening of ecological service functions, including carbon sink capacity. Research has shown that poor plant growth and weakened carbon fixation were the main reasons for the reduced carbon sequestration in a reclaimed wetland. This study aimed to examine the impacts of plant management on the improvement or restoration of carbon sink function in Chongming Dongtan reclaimed wetland, located in the Yangtze River Estuary, China. A management pattern that could effectively enhance the carbon sink function of the reclaimed wetland was selected based on analyses of the effects of different plant harvesting and management patterns (no harvesting, harvesting without returning to the field, direct straw return, and charred straw return) on the plant growth, carbon fixation, and soil respiration, combined with whole-life-cycle carbon footprint evaluation from straw harvest to field return. Compared with no harvesting, the aboveground biomass of direct straw return and charred straw return increased by about 12.3% and 15.5%, respectively (P < 0.05). Simultaneously, straw charring released the least amount of CO2 (1.94 μmol m−2 s−1) and inhibited degradation of soil organic carbon through affecting its microbial community structure. Moreover, considering the carbon budget of different patterns, the charred straw return pattern also most effectively enhanced the carbon sink function and thus could be used for subsequent improvement of carbon sequestration in reclaimed wetlands.

Inclusion of biochar in a C dynamics model based on observations from an 8-year field experiment

Abstract. Biochar production and application as soil amendment is a promising carbon (C)-negative technology to increase soil C sequestration and mitigate climate change. However, there is a lack of knowledge about biochar degradation rate in soil and its effects on native soil organic carbon (SOC), mainly due to the absence of long-term experiments performed in field conditions. The aim of this work was to investigate the long-term degradation rate of biochar in an 8-year field experiment in a poplar short-rotation coppice plantation in Piedmont (Italy), and to modify the RothC model to assess and predict how biochar influences soil C dynamics. The RothC model was modified by including two biochar pools, labile (4 % of the total biochar mass) and recalcitrant (96 %), and the priming effect of biochar on SOC. The model was calibrated and validated using data from the field experiment. The results confirm that biochar degradation can be faster in field conditions in comparison to laboratory experiments; nevertheless, it can contribute to a substantial increase in the soil C stock in the long term. Moreover, this study shows that the modified RothC model was able to simulate the dynamics of biochar and SOC degradation in soils in field conditions in the long term, at least in the specific conditions examined.

Evaluation of long-term carbon sequestration of biochar in soil with biogeochemical field model

Return of biomass-derived biochar (BC) into soil has been considered as one of the carbon sequestration (CS) methods. It is important to evaluate the long-term biochar CS potential by integrating the complex physical interferences and biochemical reactions in real soil. This study incorporated biochar into a biogeochemical field model and established a daily-resolution simulator to assess 5-, 50-, 500-year CS potential upon Soil-Biochar-Plant interaction. Through the scenario simulation of burying 7.5–75 t/ha BC-C in a 50 cm-depth rainfed cropland soil with corn planted, we found biochar could retain 483–557 kg C/t BC-C after 500 years' natural decomposition, although soil pedoturbation and plant erosion accelerated its mineralization. Moreover, biochar provided labile-C to compensate microbial decomposition and modified long-term soil climate, resulting in a decrease in soil organic carbon degradation of 44–265 kg C/t BC-C. Furthermore, biochar promoted plant photosynthetic performance by offering exogenous nutrients, equivalent to capturing 66–1039 kg C/t BC-C over 50 years. But biochar limited endogenous nutrient release and inhibited plant growth after exogenous nutrients exhausted, so total CS decreases yearly after reaching an upper limit (1030–1722 kg C/t BC-C). A total of 651–725 kg C/t BC-C could be sequestered after 500 years. And biochar is more potential in infertile and arid soils. Overall, this study indicates the necessity of taking the biogeochemical reactions into consideration to assess biochar long-term CS, and it further demonstrates biochar soil implementation is a prospective carbon-negative strategy.

Effect of long-term fertilization on bacterial community in a sandy loam soil and its relation to organic carbon accumulation

Fertilization can affect the transformation of soil organic carbon (SOC) and soil microbial community composition. However, thus far, how SOC accumulation in association with bacterial community is still unclear. We collected arable soils (aquic inceptisol) from a long-term fertilization experiment (20 years) including compost (CM), inorganic nitrogen + phosphorus + potassium (NPK), half compost N plus half inorganic fertilizer N (HCM), NP, NK, PK, and untreated (Control). We investigated the relationship between the SOC accumulation rate and bacterial community composition measured by high-throughput sequencing. The highest SOC accumulation rate was observed in the compost treatments. Furthermore, compost and balanced NPK treatments increased soil carbohydrate content significantly (P &lt; 0.05), while no such enhancement was observed following NK and PK application. Compared with the Control, fertilization substantially reduced the effective diffusion coefficient of oxygen in soil. Meanwhile, fertilization lowered the relative abundance (RA) of Bacteroidetes but increased the RA of Proteobacteria. Compost application increased the RA of Firmicutes, while inorganic fertilizers reduced it. The RA of Proteobacteria and Firmicutes were significantly and positively correlated with the SOC and carbohydrate content and the SOC accumulation rate (P &lt; 0.05). SOC accumulation was also accompanied with the reduction in the effective diffusion coefficient of oxygen in soil. Our results indicated that poor aeration may induce a shift in the microbial community composition and a transition from aerobic to anaerobic degradation of SOC, thereby favoring SOC accumulation.

Effects of microplastics on soil carbon dioxide emissions and the microbial functional genes involved in organic carbon decomposition in agricultural soil

The accumulation of microplastics (MPs) in agricultural fields can not only disguise soil organic carbon (SOC) storage but also affect the production of carbon dioxide (CO2) by microbial decomposition. However, little is known about the impact of this emerging pollutant on soil CO2 emissions and the functional genes related to SOC degradation. In the present study, a short-term (30-day) microcosm experiment was performed to investigate the effects of virgin and aged low-density polyethylene (LDPE) MPs on soil CO2 emissions. We also measured functional gene abundances related to starch (sga), hemicellulose (abfA, manB and xylA), cellulose (cex) and lignin (lig and mnp) degradation through a high-throughput quantitative-PCR-based chip. Compared with the soils without MPs, low doses (0.01% and 0.1%) of both virgin and aged MPs had negligible effects on SOC decomposition, whereas a high dose (1.0%) of these two MPs significantly (p < 0.05) accelerated the production of CO2 in soils by 15–17%, showing a dose-dependent effect. The presence of MPs did not significantly affect soil dissolved organic carbon or microbial biomass carbon. A higher metabolic quotient at 1.0% MP concentration indicated that the microbes were stressed and needed more substrates and energy during their metabolic process, which could likely explain the increase in CO2 emission induced by this dose of MPs. Exposure to virgin MPs significantly reduced the functional genes related to hemicellulose (abfA and manB) degradation, whereas increasing the aged MPs concentrations significantly decreased the abundances of functional genes encoding starch (sga), hemicellulose (abfA, manB and xylA), and cellulose (cex) hydrolysis. Overall, we conclude that the low dose (<0.1%) of MPs in the soils has a negligible effect on the production of CO2, but this factor should be considered in evaluating the global C budget in future research as this contaminant reaches a certain threshold (1.0%).

Bacterial Number and Genetic Diversity in a Permafrost Peatland (Western Siberia): Testing a Link with Organic Matter Quality and Elementary Composition of a Peat Soil Profile

Permafrost peatlands, containing a sizable amount of soil organic carbon (OC), play a pivotal role in soil (peat) OC transformation into soluble and volatile forms and greatly contribute to overall natural CO2 and CH4 emissions to the atmosphere under ongoing permafrost thaw and soil OC degradation. Peat microorganisms are largely responsible for the processing of this OC, yet coupled studies of chemical and bacterial parameters in permafrost peatlands are rather limited and geographically biased. Towards testing the possible impact of peat and peat pore water chemical composition on microbial population and diversity, here we present results of a preliminary study of the western Siberia permafrost peatland discontinuous permafrost zone. The quantitative evaluation of microorganisms and determination of microbial diversity along a 100 cm thick peat soil column, which included thawed and frozen peat and bottom mineral horizon, was performed by RT-PCR and 16S rRNA gene-based metagenomic analysis, respectively. Bacteria (mainly Proteobacteria, Acidobacteria, Actinobacteria) strongly dominated the microbial diversity (99% sequences), with a negligible proportion of archaea (0.3–0.5%). There was a systematic evolution of main taxa according to depth, with a maximum of 65% (Acidobacteria) encountered in the active layer, or permafrost boundary (50–60 cm). We also measured C, N, nutrients and ~50 major and trace elements in peat (19 samples) as well as its pore water and dispersed ice (10 samples), sampled over the same core, and we analyzed organic matter quality in six organic and one mineral horizon of this core. Using multiparametric statistics (PCA), we tested the links between the total microbial number and 16S rRNA diversity and chemical composition of both the solid and fluid phase harboring the microorganisms. Under climate warming and permafrost thaw, one can expect a downward movement of the layer of maximal genetic diversity following the active layer thickening. Given a one to two orders of magnitude higher microbial number in the upper (thawed) layers compared to bottom (frozen) layers, an additional 50 cm of peat thawing in western Siberia may sizably increase the total microbial population and biodiversity of active cells.

Influences of temperature and moisture on abiotic and biotic soil CO2 emission from a subtropical forest

BackgroundSoil CO2 efflux is considered to mainly derive from biotic activities, while potential contribution of abiotic processes has been mostly neglected especially in productive ecosystems with highly active soil biota. We collected a subtropical forest soil to sterilize for incubation under different temperature (20 and 30 °C) and moisture regimes (30%, 60 and 90% of water holding capacity), aiming to quantify contribution of abiotic and biotic soil CO2 emission under changing environment scenarios.Main findings:Results showed that abiotic processes accounted for a considerable proportion (15.6−60.0%) of CO2 emission in such a biologically active soil under different temperature and moisture conditions, and the abiotic soil CO2 emission was very likely to derive from degradation of soil organic carbon via thermal degradation and oxidation of reactive oxygen species. Furthermore, compared with biotically driving decomposition processes, abiotic soil CO2 emission was less sensitive to changes in temperature and moisture, causing reductions in proportion of the abiotic to total soil CO2 emission as temperature and moisture increased.ConclusionsThese observations highlight that abiotic soil CO2 emission is unneglectable even in productive ecosystems with high biological activities, and different responses of the abiotic and biotic processes to environmental changes could increase the uncertainty in predicting carbon cycling.

Loss of soil carbon and nitrogen rather than variations in soil particle size distribution decreased microbial residues during conversion of grassland to croplands

AbstractConversion of grassland into cropland always induces a great degradation of soil organic carbon (SOC). However, during this process information on microbial residue C (MR‐C) and its contribution to SOC with increasing conversion age remain scarce. The relative contribution of soil properties to the accumulation of amino sugars is still poorly understood. Here, we studied variations in soil amino sugars and MR‐C, and their contribution to SOC with increasing conversion age (5 (M05), 16 (M16) and 34 (M34) years, respectively) from grassland to cropland. In contrast to the contents of glucosamine, fungal glucosamine, galactosamine, muramic acid, total amino sugars and MR‐C in grassland, those in M05 decreased in the range of 14.8–37.5%, and those in M34 decreased in the range of 31.8–65.1%, respectively. The contents of amino sugars and MR‐C did not differ between M16 and M34. The similar contribution of MR‐C to SOC (44.6–46.3%) in grassland, M16 and M34 is likely to result from a balance between microbial and non‐degraded plant residues. Decreasing ratios of fungal to bacterial residue C with conversion age indicated a decreased relative contribution of fungal residues to SOC, which probably resulted from the decreased percentage of macroaggregates. MR‐C and SOC decreased and reached a steady status in a few years (≧16 a) after conversion of grassland to cropland. The redundancy analysis model revealed that total nitrogen (TN), pH, SOC and available nitrogen (AN) played a bigger role than particle size distribution, in terms of explaining the contribution of amino sugars to soil carbon.Highlights Conversion of grassland into cropland decreased amino sugars and microbial residues. The contribution of MR‐C to SOC peaked in M05. The relative contribution of fungal residues to SOC decreased with increasing conversion age. TN and SOC contributed more to the accumulation of amino sugars than particle size distribution.