Soil Organic Carbon Degradation Research Articles

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.

Cropping system partially offsets tillage-related degradation of soil organic carbon and aggregate properties in a 30-yr rainfed agroecosystem

Soil tillage increases the susceptibility of agricultural soils to erosion and organic carbon losses, but tillage effects could be mitigated through other management practices such as crop rotation. Here, we evaluated the 30-year impacts of tillage intensity and cropping system on surface soil bulk density, nutrient availability, dry aggregate size distribution, and water-stable aggregation. This study was established in 1980 in eastern Nebraska USA, and included six tillage treatments of varying intensity (no-till, ridge till, disk till, subsoil rip, chisel plow, moldboard plow) and four crop rotation treatments (continuous soybean [Glycine max (L.) Merr.]; soybean-corn [Zea mays L.]; corn-soybean, continuous corn) in a randomized block design with six replicates. Surface soils were sampled in 2011 and soil aggregate properties assessed, including occluded particulate organic matter (oPOM) in micro/macroaggregates (0.053–0.5 mm) and mega-aggregates (>2.0 mm). Because of significant treatment differences in bulk density, soil properties were converted to an equivalent soil mass (ESM) basis to more accurately assess management effects. After 30 years, only the main effects of tillage and crop rotation were significant for most measured soil properties. Surface soil organic carbon (SOC) stocks (ESM for ∼0−30 cm soil depth) decreased with tillage intensity, and stocks were higher when corn was included in the cropping system. Dry aggregate size distributions shifted towards smaller size classes as tillage intensity increased and whenever corn was included in the cropping system. As a result, aggregate mean weight diameter (mm) followed a similar trend. Soil stocks of water-stable mega-aggregates also decreased with increasing tillage intensity. In near-surface soils (0–7.5 cm), highly-erodible aggregate oPOM was highest in no-till soils and was more sensitive to tillage disturbance (56–69% loss) than mega-aggregate oPOM (5–35% loss). Even in no-till soils, highly-erodible aggregate oPOM concentrations decreased under continuous corn compared to rotated systems likely due to greater frequency of fertility management-related soil disturbances (i.e. fertilizer injection annually vs every two years). These results suggest that cropping systems that maximize plant carbon inputs can partially mitigate soil erosion risks due to long-term tillage, but that other crop management-related soil disturbances (i.e. method of fertilizer application) could limit the mitigating effect of cropping system.

Response of carbon and microbial properties to risk elements pollution in arctic soils

A 180-day incubation study was conducted to evaluate the effects of risk elements (REs) on organic carbon use and microbial activities in organic soils in the Arctic during the summer snowmelt period. Soils were artificially spiked with Cd, Pb, Cr, Ni, Cu, As, and a combination of these REs according to the levels measured in Arctic soils from REs-polluted industrial sites. During the incubation period, microbial activities and microbial biomass carbon (MBC) formation were inhibited, and microbial quotient (qCO2) values were relatively high in the spiked soils indicating that more energy was used by microbes for maintenance under REs stress. Meanwhile, microbial metabolism was significantly restrained. Microbial Specific phospholipid fatty acids (PLFAs) were reduced in RE spiked soils relative to the control, especially in the As- and multi-RE-spiked soils. The abundance of both fungi and bacteria was reduced in response to RE amendments by 14–24% and 1–55%, respectively. PLFA biomarkers indicated a shift in soil microbial community structure and activities influenced by REs, consequently having a negative effect on soil organic carbon degradation. This study addresses the knowledge gap regarding the alternation of biochemical reactions in Arctic soils under anthropogenic REs with relevant contamination levels.

Excessive nitrogen addition accelerates N assimilation and P utilization by enhancing organic carbon decomposition in a Tibetan alpine steppe

High amounts of deposited nitrogen (N) dramatically influence the stability and functions of alpine ecosystems by changing soil microbial community functions, but the mechanism is still unclear. To investigate the impacts of increased N deposition on microbial community functions, a 2-year multilevel N addition (0, 10, 20, 40, 80 and 160 kg N ha−1 year−1) field experiment was set up in an alpine steppe on the Tibetan Plateau. Soil microbial functional genes (GeoChip 4.6), together with soil enzyme activity, soil organic compounds and environmental variables, were used to explore the response of microbial community functions to N additions. The results showed that the N addition rate of 40 kg N ha−1 year−1 was the critical value for soil microbial functional genes in this alpine steppe. A small amount of added N (≤40 kg N ha−1 year−1) had no significant effects on the abundance of microbial functional genes, while high amounts of added N (>40 kg N ha−1 year−1) significantly increased the abundance of soil organic carbon degradation genes. Additionally, the abundance of microbial functional genes associated with NH4+, including ammonification, N fixation and assimilatory nitrate reduction pathways, was significantly increased under high N additions. Further, high N additions also increased soil organic phosphorus utilization, which was indicated by the increase in the abundance of phytase genes and alkaline phosphatase activity. Plant richness, soil NO2−/NH4+ and WSOC/WSON were significantly correlated with the abundance of microbial functional genes, which drove the changes in microbial community functions under N additions. These findings help us to predict that increased N deposition in the future may alter soil microbial functional structure, which will lead to changes in microbially-mediated biogeochemical dynamics in alpine steppes on the Tibetan Plateau and will have extraordinary impacts on microbial C, N and P cycles.

Waterlogging increases organic carbon decomposition in grassland soils

Drainage increases soil organic carbon (SOC) mineralization, but waterlogging will not necessarily slow the SOC decomposition. We investigated the effects of water regime (anaerobic vs. aerobic conditions) on SOC mineralization in adjacent wetland and grassland soils with contrasting biochemical properties in the east Tibetan Plateau. Biochemical characteristics of SOC (compositions of non-cellulosic and amino polysaccharides) were analyzed, then soils were moistened to 70% of water-holding capacity (aerobic) or saturated (anaerobic) and incubated at 25 °C for 95 days. The non-cellulosic sugar content in SOC was similar in wetland and meadow soils, but the mass ratio of hexoses (galactose plus mannose or rhamnose plus fucose) to pentoses (arabinose plus xylose) was three-fold greater in meadows than wetlands. Furthermore, there was three times more accumulation of microbial residues (sum of glucosamine, mannosamine, galactosamine, and muramic acid) in SOC, and more fungal than bacterial necromass in the meadow than wetland soils. This suggests slower transformation of plant litter by microorganisms, and thus less accumulation of microbial products in the SOC of wetlands than meadows. Meadow soils had more microbial biomass and higher activities of β–glucosidase, phenol oxidase, and urease per unit of SOC than wetland soils. During the 95-day incubation, C loss (as CO2) per unit of SOC was 55–73% greater in anaerobic than aerobic meadow soils, but was 59–64% less from wetland soils under anaerobic conditions than aerobic conditions. Thus, SOC decomposition in anaerobic conditions was dependent on the degree of SOC degradation, which was related to the initial proportion of plant and microbial residues. We addressed the importance of biochemical nature in regulating SOC decomposition under anaerobic conditions. We conclude that both the wetland drainage and grassland waterlogging increase SOC mineralization (measured as CO2 evolution) in alpine areas.

Conversion behaviors of litter-derived organic carbon of two halophytes in soil and their influence on SOC stabilization of wetland in the Yangtze River Estuary.

Soil organic carbon (SOC) is both a product and a cause of soil development. Previous studies found that less carbon (C) is fixed by Phragmites communis than Spartina alterniflora in the Jiuduansha wetland of the Yangtze River Estuary. However, the P. communis zone presented higher contents of SOC and humus, which was mainly related to lower soil respiration (SR). It is not well known how different plants affect turnover of original SOC in the Jiuduansha wetland, and thus soil development and tidal flat evolution. In this study, in-field surveys and microcosm experiments were conducted to trace turnover of plant C and evaluate dynamics of SOC using stable C isotopic techniques. Spartina alterniflora decayed faster than P. communis, and more of its derived OC was lost through SR and leaching. Although S. alterniflora-derived OC suppressed the degradation of original SOC, it was consumed to a greater extent, making less supplementation to SOC. Phragmites communis-derived OC showed less degradability and accelerated the degradation of original SOC, but was more incorporated into new SOC and finally caused higher increase in SOC, specifically in bare tidal flat soil with poor original SOC. Overall, compared with S. alterniflora, P. communis-derived OC more effectively replaced the unstable original SOC, thereby improving the content and stability of SOC, especially for soil in early-development stages of tidal flats.

Projected trends of soil organic carbon stocks in Meghalaya state of Northeast Himalayas, India. Implications for a policy perspective

Agricultural and forestry activities can affect soil organic carbon (SOC) levels and CO2 emissions from terrestrial ecosystems due to land use changes. In Northeast Himalayas, studies on the effects of forest conversion to temporary agricultural lands (jhum) on the loss of SOC and soil quality degradation have received the attention of policy makers and scientific research. Presently, local communities are now oriented towards the settled plantations systems with modern cash crops such as tea and rubber, that could act as potential SOC sinks. However, no information on SOC dynamics and simulation studies after land-use change from temporary agricultural lands (jhum) to settled cultivations and under climate change (CC) conditions are available for the Meghalaya state. Applying the RothC model, we focused on four different scenarios including the conversion from jhum to settled cultivation (rubber plantations and tea gardens), as well as continuous jhum cultivation and jhum to jhum with a period of secondary succession. Simulations under CC conditions indicated that SOC stocks significantly increased by 1.20 t C ha−1 yr−1 in tea gardens compared to rubber and jhum scenarios. Conversely, SOC stocks slightly decreased by 0.07 t C ha−1 yr−1 in rubber plantations, while the regrowth of a natural vegetation cover as secondary succession following the abandonment of the jhum fields, showed a lower SOC decrease (0.18 t C ha−1 yr−1) compared to the continuous jhum cultivation (0.24 t C ha−1 yr−1). Thus, for CC mitigation in a policy perspective, tea gardens could represent the best land use to store increasing amounts of SOC in the long-term perspective and optimize farmers' incomes, while in rubber plantations SOC storage is limited in time. Jhum cultivation can benefit in terms of productivity and profitability by extending the duration of the secondary succession period.

Aerobic respiration of mineral-bound organic carbon in a soil

Associations with minerals can potentially augment soil organic carbon (SOC) stability by reducing the bioavailability and degradation of SOC. However, few studies have directly measured aerobic respiration of mineral-bound SOC. In this study, we investigated the microbial aerobic respiration and bioavailability of ferrihydrite-sorbed glucose (Fh-GLU) and ferrihydrite-sorbed formic acid (Fh-FA) by adding 13C-labeled compounds to a soil. During an 11-day incubation, 30.2% of free, non-Fh-sorbed glucose (GLU) and 61.8% of free formic acid (FA) were respired, whereas 4.2% and 27.9% of Fh-GLU and Fh-FA were respired, respectively. Our results demonstrated that Fh-bound GLU/FA had lower bioavailability compared to free organic compounds. Associations with Fh led to greater inhibition in the bioavailability of GLU than that for FA. The priming effects of added compounds on the respiration of native SOC were decreased by their association with Fh. Our results demonstrated that the bioavailability and priming effect of organic compounds depend on their interactions with minerals.

Soil organic carbon depletion and degradation in surface soil after long-term non-growing season warming in High Arctic Svalbard

Arctic tundra active-layer soils are at risk of soil organic carbon (SOC) depletion and degradation upon global climate warming because they are in a stage of relatively early decomposition. Non-growing season (NGS) warming is particularly pronounced, and observed increases of CO2 emissions during experimentally warmed NGSs give concern for great SOC losses to the atmosphere. Here, we used snow fences in Arctic Spitsbergen dwarf shrub tundra to simulate 1.86 °C NGS warming for 9 consecutive years, while growing season temperatures remained unchanged. In the snow fence treatment, the 4-11 cm thick A-horizon had a 2% lower SOC concentration and a 0.48 kg C m−2 smaller pool size than the controls, indicating SOC pool depletion. The snow fence treatment's A-horizon's alkyl/O-alkyl ratio was also significantly increased, indicating an advance of SOC degradation. The underlying 5 cm of B/C-horizon did not show these effects. Our results support the hypothesis that SOC depletion and degradation are connected to the long-term transience of observed ecosystem respiration (ER) increases upon soil warming. We suggest that the bulk of warming induced ER increases may originate from surface and not deep active layer or permafrost horizons. The observed losses of SOC might be significant for the ecosystem in question, but are in magnitude comparatively small relative to anthropogenic greenhouse gas enrichment of the atmosphere. We conclude that a positive feedback of carbon losses from surface soils of Arctic dwarf shrub tundra to anthropogenic forcing will be minor, but not negligible.

The impacts of four potential bioenergy crops on soil carbon dynamics as shown by biomarker analyses and DRIFT spectroscopy

AbstractPerennial bioenergy crops accumulate carbon (C) in soils through minimally disturbing management practices and large root inputs, but the mechanisms of microbial control over C dynamics under bioenergy crops have not been clarified. Root‐derived C inputs affect both soil microbial contribution to and degradation of soil organic matter resulting in differing soil organic carbon (SOC) concentrations, storage, and stabilities under different vegetation regimes. Here, we measured biomarker amino sugars and neutral sugars and used diffuse reflectance mid‐infrared Fourier transform spectroscopy (DRIFTS) to explore microbial C contributions, degradation ability, and SOC stability, respectively, under four potential bioenergy crops, M.×giganteus (Miscanthus × giganteus), switchgrass (Panicum virgatum L.), a mixed prairie, and a maize (Zea mays L.)–maize–soybean (Glycine max(L.) Merr.) (MMS) rotation over six growing seasons. Our results showed that SOC concentration (g/kg) increased by 10.6% in mixed prairie over the duration of this experiment and SOC storage (Mg/ha) increased by 17.0% and 15.6% in switchgrass and mixed prairie, respectively. Conversion of row crops to perennial grasses maintained SOC stability and increased bacterial residue contribution to SOC in M.×giganteus and switchgrass by 20.0% and 15.0%, respectively, after 6 years. Degradation of microbe‐derived labile SOC was increased in M.×giganteus, and degradation of both labile and stable SOC increased in MMS rotation. These results demonstrate that microbial communities under perennial grasses maintained SOC quality, while SOC quantity increased under switchgrass and mixed prairie. Annual MMS rotation displayed decreases in aspects of SOC quality without changes in SOC quantity. These findings have implications for understanding microbial control over soil C quantity and quality under land‐use shift from annual to perennial bioenergy cropping systems.

Molecular Insights into Arctic Soil Organic Matter Degradation under Warming.

Molecular composition of the Arctic soil organic carbon (SOC) and its susceptibility to microbial degradation are uncertain due to heterogeneity and unknown SOC compositions. Using ultrahigh-resolution mass spectrometry, we determined the susceptibility and compositional changes of extractable dissolved organic matter (EDOM) in an anoxic warming incubation experiment (up to 122 days) with a tundra soil from Alaska (United States). EDOM was extracted with 10 mM NH4HCO3 from both the organic- and mineral-layer soils during incubation at both -2 and 8 °C. Based on their O:C and H:C ratios, EDOM molecular formulas were qualitatively grouped into nine biochemical classes of compounds, among which lignin-like compounds dominated both the organic and the mineral soils and were the most stable, whereas amino sugars, peptides, and carbohydrate-like compounds were the most biologically labile. These results corresponded with shifts in EDOM elemental composition in which the ratios of O:C and N:C decreased, while the average C content in EDOM, molecular mass, and aromaticity increased after 122 days of incubation. This research demonstrates that certain EDOM components, such as amino sugars, peptides, and carbohydrate-like compounds, are disproportionately more susceptible to microbial degradation than others in the soil, and these results should be considered in SOC degradation models to improve predictions of Arctic climate feedbacks.

Examining soil organic carbon distribution and dynamic change in a hickory plantation region with Landsat and ancillary data

Soil organic carbon (SOC) is an important soil property relating to soil formation, structure, and water-holding capacity. Remote sensing has been used for predicting SOC but remains a challenge due to the complex and indirect relationship between SOC and remote sensing variables. In this research we explored the approaches of predicting SOC distribution in a hickory plantation region using random forest (RF) and geographically weighted regression (GWR) based on Landsat and ancillary data, analyzed SOC spatial distribution and dynamic change between 2008 and 2013 through a thresholding approach, and examined major factors that resulted in SOC degradation in the young and mature hickory plantations using a logistic regression. The results showed that RF outperformed GWR in the prediction of SOC and provided stable and reliable SOC predictions with root mean squared errors of 4.6 g kg−1 in 2008 and 4.4 g kg−1 in 2013. A large area of hickory plantation was experiencing SOC decrease. The analysis of major factors causing SOC degradation indicated that steep slope and high proportion of silt component in soil resulted in SOC decrease in the young hickory plantations, and high elevation, high proportion of silt component in the soil, and the increase of soil fraction in the ground cover led to SOC decrease in the mature hickory plantations. This research provides valuable approaches to spatially predict SOC and identify major factors driving SOC degradation, which will be useful for adopting better measures to improve management of hickory plantations.

Degradability of raw and post-processed chars in a two-year field experiment

The object of the present work was to analyse the degradation dynamics of four chars and a digestate applied to a sandy soil as well as possible initial priming effects on the mineralisation of soil organic carbon in a two-year field experiment. For that purpose, soil carbon content, soil respiration and the corresponding carbon isotopic abundances were repeatedly measured throughout two consecutive vegetation periods. In order to quantify and separate the amount and the degradation of the substrate-derived carbon and to assess soil priming effects, isotopic mixing models were applied to soil-derived and substrate-derived carbon, and to the respired CO2.Pyrolysis char was degraded with decreasing intensity over time with an estimated half-life of about 80years. HTC (HydroThermal Carbonisation) char showed a high degradation during the first year but, during the second year, the remaining recalcitrant pool was degraded much slower with a half-life between 49 and 61years. Digestate was degraded at a constant intensity with a half-life of about 14years. When the chars were fermented before being applied to the soil, the initial degradation of HTC char was reduced, but on the two-year scale, the degradation of both chars was higher than for untreated chars, yielding a half-life between 11 and 15years, comparable to digestate. The results showed considerable stability of the untreated pyrolysis and HTC chars under field conditions, and moreover, no net influence of chars as well as of digestate on the degradation of soil organic carbon after two years.

A Bayesian belief network framework to predict SOC dynamics of alternative management scenarios

Understanding the key drivers that affect a decline of soil organic carbon (SOC) stock in agricultural areas is of major concern since leading to a decline in service provision from soils and potentially carbon release into the atmosphere. Despite an increasing attention is given to SOC depletion and degradation processes, SOC dynamics are far from being completely understood because they occur in the long term and are the result of a complex interaction between management and pedo-climatic factors. In order to improve our understanding of SOC reduction phenomena in the mineral soils of Veneto region, this study aimed to adopt an innovative probabilistic Bayesian belief network (BBN) framework to model SOC dynamics and identify management scenarios that maximise its accumulation and minimise GHG emissions.Results showed that the constructed BBN framework was able to describe SOC dynamics of the Veneto region, predicting probabilities of general accumulation (11.0%) and depletion (55.0%), similar to those already measured in field studies (15.3% and 50%, respectively). A general enhancement in the SOC content was observed where a minimum soil disturbance was adopted. This outcome suggested that management strategies of conversion from croplands to grasslands, no tillage and conservation agriculture are the most promising management strategies to reverse existing SOC reduction dynamics. Moreover, measures implying SOC stocks were also those providing major benefits in terms of GHGs reduction emissions. Finally, climate change scenarios slightly affected management practice. Advancements in our BBN framework might include more detailed classes at higher resolution as well as any socio-cultural or economic aspect that should improve the evaluation of prediction scenarios.

Are land use and short time climate change effective on soil carbon compositions and their relationships with soil properties in alpine grassland ecosystems on Qinghai-Tibetan Plateau?

Fencing and grass plantation are two key interventions to preserve the degraded grassland on the Qinghai-Tibetan Plateau (QTP). Climate warming and N deposition have substantially affected the alpine grassland ecosystems. However, molecular composition of soil organic carbon (SOC), the indicator of degradation of SOC, and its responses to climate change are still largely unclear. In this study, we conducted the experiments in three types of land use on the QTP: alpine meadow (AM), alpine steppe (AS), and cultivated grassland (CG) under 2°C climatic warming, 5 levels of nitrogen deposition rates at 8, 24, 40, 56, and 72kg N ha−1year−1, as well as a combination of climatic warming and N deposition (8kg N ha−1year−1). Our findings indicate that all three types of land use were dominated by O-alkyl carbon. The alkyl/O-alkyl ratio, aromaticity and hydrophobicity index of the CG were larger than those of the AM and AS, and this difference was generally stable under different treatments. Most of the SOC in the alpine grasslands was derived from fresh plants, and the carbon in the CG was more stable than that in the AM and AS. The compositions of all the alpine ecosystems were stable under short-term climatic changes, suggesting the short-term climate warming and nitrogen deposition likely did not affect the molecular composition of the SOC in the restored grasslands.

Cultivation ofOpuntia ficus‐indicaunder different soil management practices: A possible sustainable agricultural system to promote soil carbon sequestration and increase soil microbial biomass and activity

AbstractMonitoring of soil organic carbon (SOC) content, SOC pools, and enzymes involved in SOC degradation and mineralization is needed to elucidate the impact of conversion from forest to agriculture on SOC and properties, and how and which crop system and management practices can contribute to C sequestration, thus climate change mitigation. This study compared the impact of forest conversion to maize (Zea maysL.) and cactus (Opuntia ficus‐indica[L.] Mill.) fields with different farming management on SOC content and pools (recalcitrant, labile, and extractable), soil microbial biomass carbon, and enzyme activities involved in the soil C cycle. For this purpose, a site under strong land use changes in Central Mexico with high demographic density was selected. Forest conversion to maize fields decreased the original SOC and the C recalcitrance index (RI). However, land use change from forest to cactus systems maintained SOC content, C RI, and arylesterase activity near the values observed in the original forest. Cactus systems promoted higher soil extractable organic carbon content, related to increases in microbial biomass and β‐glucosidase activity. Additionally, cactus cultivation can maintain SOC and a C RI at a similar level to the forest soil, which contributes to sustainability of the agroecosystem and climate change mitigation. Therefore, it could comprise a promising viable option for sustainable crop production systems under temperate subhumid conditions, in turn promoting socioeconomic development and rural growth.

The effects of maize ( Zea mays L.) hybrid and harvest date on above- and belowground biomass dynamics, forage yield and quality – A trade-off for carbon inputs?

Abstract There are two major environmental constraints of silage maize (Zea mays L.) production: loss of soil organic carbon (SOC) and nitrogen (N) leaching to groundwater. Winter catch crops (CC) increase carbon inputs and can accumulate residual N, but only if the preceding maize is harvested timely to allow high above- and belowground biomass and N accumulation. To quantify the combined effects of hybrid maize variety and harvest date on forage yield and quality, as well as on root dynamics, root turnover and carbon input, a two-year field experiment (April 2012–October 2013) was conducted in northern Germany. Early maturing Suleyka was harvested at 10 (hd1) and 20 (hd2) September and mid-early Ronaldinio at 30 September (hd3) and 15 October (hd4). Maize hybrids showed no consistent differences of aboveground dry matter (DM) and N accumulation dynamics. Only the very early harvest (hd1) resulted in 11–13% lower DM yields. Sufficiently high DM and starch contents, however, were not achieved at harvest dates prior to hd3 (30 September). Similar to shoot yield, the final accumulated root biomass, carbon (C) and N were not affected by hybrid/harvest date, and on average amounted 2.48 t DM, 1.2 t C and 52.9 kg N ha‐1 in the upper soil depth of 30 cm, which represented between 77 and 75% of the DM, C and N accumulation to 60 cm soil depth. Root turnover during the growing season reached up to 65, 75 and 69% of root DM, N and C. Stubble DM and C left on the field, however, was 31% lower at early harvest (hd1) compared to later harvests, resulting in a trade-off to total C inputs compared to delayed harvest. The quantified C inputs are discussed by two C balance approaches, which indicate that silage maize cultivation is not necessarily associated with SOC degradation, especially if winter catch crops are introduced.

Tetrachloroethene Release and Degradation During Combined ERH and Sodium Persulfate Oxidation

AbstractElectrical resistance heating (ERH) is a thermal treatment technology that involves passing electrical current through soil to increase subsurface temperatures. In addition to volatizing and recovering contaminant mass in the gas phase, heating the subsurface has the potential to decompose contaminants by increasing the rate of degradation reactions. Prior laboratory studies using convective heating demonstrated that the rate of tetrachloroethene (PCE) degradation was not sufficient to cause substantial in situ PCE mass destruction. However, similar experiments have not been performed using ERH, which has the potential to degrade PCE in reaction with the heating electrodes and electrochemically. Thus, the objective of this study was to determine the extent of PCE degradation during thermal treatment of PCE‐contaminated soil using a bench‐scale ERH system. The contaminated soil, a silty clay loam, was collected from a single borehole at a former dry cleaning facility prior to undergoing ERH treatment. After 30 days of ERH, 52% of the initial PCE mass was recovered, potentially indicating that 48% of the PCE was degraded during ERH. Although potential degradation products such as carbon dioxide were observed, their presence was attributed to the degradation of soil organic carbon and carbonates rather than PCE destruction. A second ERH experiment was conducted to assess the potential benefit of adding the heat‐activated oxidant, sodium‐persulfate, during treatment. After 19 days of ERH and three persulfate injections, 93% of the initial PCE was recovered, with 3% PCE destruction based on chloride evolution. However, the difference in mass recovery between the first and second experiments could have been due to differences in the initial mass of PCE, even though soil from the same core was used in both experiments. The results of this work suggest that the majority of mass recovered during ERH of the PCE‐contaminated soil at the former dry cleaning facility will be due to volatilization and gas phase extraction rather than abiotic degradation, even with the addition of sodium persulfate.

Microbial Community and Functional Gene Changes in Arctic Tundra Soils in a Microcosm Warming Experiment.

Microbial decomposition of soil organic carbon (SOC) in thawing Arctic permafrost is important in determining greenhouse gas feedbacks of tundra ecosystems to climate. However, the changes in microbial community structure during SOC decomposition are poorly known. Here we examine these changes using frozen soils from Barrow, Alaska, USA, in anoxic microcosm incubation at −2 and 8°C for 122 days. The functional gene array GeoChip was used to determine microbial community structure and the functional genes associated with SOC degradation, methanogenesis, and Fe(III) reduction. Results show that soil incubation after 122 days at 8°C significantly decreased functional gene abundance (P < 0.05) associated with SOC degradation, fermentation, methanogenesis, and iron cycling, particularly in organic-rich soil. These observations correspond well with decreases in labile SOC content (e.g., reducing sugar and ethanol), methane and CO2 production, and Fe(III) reduction. In contrast, the community functional structure was largely unchanged in the −2°C incubation. Soil type (i.e., organic vs. mineral) and the availability of labile SOC were among the most significant factors impacting microbial community structure. These results demonstrate the important roles of microbial community in SOC degradation and support previous findings that SOC in organic-rich Arctic tundra is highly vulnerable to microbial degradation under warming.