Labile Soil Organic Carbon Fractions Research Articles

Short-term effects of returning granulated straw on soil microbial community and organic carbon fractions in dryland farming.

We conducted a 2-year field experiment which was comprised of five treatments, namely no straw returning (CK), straw mulching (SM), straw plowed into the soil (SP), and straw returned in granulated form (SG). The aim of this study was to investigate the effects of different straw returning modes on soil bacterial and fungal community structure and their relationships to soil organic carbon (SOC) fractions at three different soil depths (0-20, 20-40, and 40-60 cm) in a dryland under maize cultivation in Northeast (NE) China. SM, SP, and SG treatments significantly increased SOC content. Compared with SM and SP treatments, SG treatment significantly increased the content of SOC and easily oxidizable carbon (EOC) in the topsoil (0-20 cm depth), and increased dissolved organic carbon (DOC) and SOC content of the light fraction (LFOC) in the 20-40 cm layer. Meanwhile, SG treatment exhibited the highest microbial biomass C (MBC) content in all of the three soil depths. SG treatment also enhanced bacterial richness as well as fungal richness and diversity in the upper 40 cm of soil. In addition, SG treatment increased the relative abundance of Proteobacteria in all depths, and had the highest relative abundance of Basidiomycota in the first 20 cm of soil. SP treatment showed the lowest soil organic carbon content in all fractions and soil microbial community composition. SM treatment exhibited similar results to SG treatment in SOC, DOC, and LFOC contents, and bacterial diversity in the topsoil and subsoil. As a whole, treatment SG improved soil quality and maize yield, hence we recommend returning granulated straw as the most effective practice for enhancing labile SOC fractions as well as maintaining soil diversity and microbial richness of arid farmlands in NE China.

Contrasting impacts of manure and inorganic fertilizer applications for nine years on soil organic carbon and its labile fractions in bulk soil and soil aggregates

Careful nutrient management to increase soil organic carbon (SOC) content is important in increasing agricultural productivity and maintaining ecosystem health. A field experiment was conducted for nine years to investigate the effects of manure (M) and inorganic fertilizer (NPK) on SOC and its labile fractions within the bulk soil and in soil aggregates in a wheat-maize rotation on the North China Plain. Nine treatments were included: control (CK) with no fertilization, cattle manure (M, applied at rates of 3000, 6000, 9000, and 12,000 kg ha−1 crop−1), and inorganic NPK fertilizer applied to give equivalent rates of N, P and K. Straw was returned to all plots. Results showed that fertilization significantly increased SOC sequestration and the concentrations of SOC and labile SOC fractions (cold water - extractable SOC, hot water-extractable SOC, microbial biomass C, and dissolved organic C within the bulk soil and soil aggregates). The values increased with increasing application rate of manure but not with increasing NPK fertilizer rate. The differences between manure and NPK fertilizer were apparent at rates equivalent to 9000 and 12,000 kg manure ha−1. Labile SOC fractions were linearly correlated with SOC within the bulk soil and aggregates and hot water-extractable C was the labile C fraction most sensitive to changes in SOC content. Aggregate stability was significantly positively correlated with SOC content and its labile fractions in both bulk soil and aggregates. The results indicate that straw return and/or combined application of fertilizers and manures may be feasible in achieving the ‘4 per 1000’ initiative, with manure application particularly effective. Manure application at a rate of 9000 kg ha−1 crop−1 may be the optimum strategy to sequester C and maintain high crop productivity. Periodic high application rates of manures should be integrated with appropriate inorganic fertilizer application rates to optimize nutrient management strategies on calcareous soils.

Effects of long-term fertiliser regime on soil organic carbon and its labile fractions under double cropping rice system of southern China

ABSTRACT This study was designed to explore changes in soil bulk density, soil organic carbon content, soil organic carbon stock and soil labile organic carbon fractions under 34 years fertilizer regime in the double-cropping rice system of southern China. The experiment including five fertilizer treatments: chemical fertilizer alone (MF), rice straw residue and chemical fertilizer (RF), 30% organic manure and 70% chemical fertilizer (LOM), 60% organic manure and 40% chemical fertilizer (HOM), and without fertilizer input as control (CK). The results showed that soil bulk density in the 0-20 cm soil layer with long-term fertilizer treatments were significantly decreased compared to CK treatment. The soil organic carbon content and stock with RF, LOM and HOM treatments were significantly higher than that of MF and CK treatments. These results confirmed by the soil carbon management index which were significantly increased with RF, LOM and HOM treatments. As a result, combined application of organic manure with chemical fertilizer is a benefit nutrient management for improving soil organic carbon content and stock, soil carbon management index in paddy field of southern China.

Reduction of Soil Cadmium Activity and Rice Cadmium Content by 4-year-consecutive Application of Organic Fertilizer

Because commercial organic fertilizers may contain cadmium (Cd) and may cause the dual effect of "inhibition" and "activation" on Cd availability in paddy soil with organic fertilizer input, the reduction of rice Cd following organic fertilizer application is still uncertain. Herewith, typical purple mud paddy fields were selected in the eastern Hunan Province. The effect of commercial organic fertilizer input on Cd reduction of double-rice paddy ecosystem was monitored for four consecutive years. The relationships between brown rice Cd content, soil available Cd, and soil factors (pH, soil labile organic carbon fractions, and iron oxide) at different growth stages in double-rice paddy fields were investigated. Results showed that the input of organic fertilizer reduced the Cd content in brown rice by 28%-56%. Meanwhile, the decrease of Cd content in brown rice of late rice (43%-56%) was higher than that of early rice (28%-45%), and the inter-annual fluctuation of the decrease was relatively small. On the one hand, soil available Cd content decreased by 6%-7% during several growth stages of double-rice (from tillering peak stage to full heading stage) with organic fertilizer input. Additionally, the content of soil exchangeable Cd was decreased by 11%, whereas the content of organic bound Cd was increased by 14%. This directly reflects the decrease of soil Cd availability. On the other hand, the soil pH value was steadily increased by 0.1-0.3 units following organic fertilizer input, which promoted the development of soil from acidity to slight acidity. Besides, the content of soil active organic carbon (light fraction organic carbon, coarse particulate organic carbon, and fine particulate organic carbon) was increased significantly (53%, 77%, and 107%, respectively). This indirectly reflects the decrease in soil Cd availability. This study implies that the decrease of soil Cd availability may be the primary driving force for the reduction of rice Cd content with consecutive organic fertilizer input in purple mud paddy fields.

Responses of Soil Organic Carbon Fractions to Land Use Types in Hilly Red Soil Regions, China

Land use type exerts important influences on soil organic carbon (SOC) and its fractions, and determines the stability of the carbon pool. Taking woodland as a reference, the content of SOC and its labile fractions[dissolved organic carbon (DOC), microbial biomass carbon (MBC), and particulate organic carbon (POC)] and non-labile fractions[mineral-associated organic carbon (MAOC)] in upland and paddy surface soils in hilly red soil regions were determined to explore the responses of SOC fractions to land use type. The results showed that the contents of SOC, MBC, POC, and MAOC ranked highest in paddy compared with upland and woodland. DOC content in woodland was significantly higher than in upland and paddy (P<0.001). The proportion of each SOC fraction, i.e. DOC/SOC, MBC/SOC, POC/SOC, and MAOC/SOC, was in the range of 0.22%-0.93%, 1.62%-2.70%, 31.08%-40.00%, and 43.22%-56.82%, respectively. The contents of labile fractions (MBC and POC) and their proportions (MBC/SOC and POC/SOC) were in the order of paddy > woodland > upland. MAOC content ranked the highest in paddy but the lowest in upland, while MAOC/SOC exhibited the opposite trend. The correlation suggested that the labile fractions (MBC and POC) and inert fraction (MAOC) were significantly positively correlated with SOC (P<0.001) in the three land use types, while no significant correlations were found between DOC and SOC and its fractions (P>0.05). There was a significant positive correlation between POC and MBC in upland and woodland (P<0.001). POC was significantly positively correlated with MAOC in the three land use types (P<0.001). MAOC and MBC in paddy and upland were significantly positively correlated (P<0.001). Therefore, compared with upland and woodland, SOC in paddy had a higher proportion of labile SOC fraction, but a lower proportion of inert fraction. Moreover, MBC content in paddy was not related to the accumulation of the labile fraction of POC, but positively related to the accumulation of the inert fraction of MAOC. In summary, agricultural land uses have great influence on SOC and its fractions in hilly red soil regions. Though paddy is beneficial for SOC sequestration, the proportions of labile fractions in its SOC are relatively higher, and thus it is vulnerable to loss due to improper agricultural management.

Soil Organic Carbon Pool and its Storage in Arial Beel Wetland Soils of Bangladesh

The actual quantity of Soil Organic Carbon (SOC) stored in wetlands can only be estimated within a broad range of uncertainty. An accurate assessment of the size and distribution of the SOC storages in wetland resources is very difficult to obtain, therefore, the proposed research objective is to measure SOC storage and its pool on wetland soils of Arial beel in Bangladesh. Initial results of Arial beel soil profiles indicates SOC concentrations are high in surface soils ranges from 1.67 to 1.95% but its concentrations are decreasing with depth whereas SOC stock in kg C m-2 is increased with depth due to increse soil bulk density with depth. However, carbon in deeper layers may be more stable than that in surface soils due to difference in source, composition and environmental conditions. Soil organic C stored in the three different locations of wetlands soils to 1 m depth such as 16.47, 18.27 and 17.22 kg C m-2, respectively with an average of 17.32 kg C m-2. On the other hand, SOC stored in upland soils to 1m depth such as 11.24 kg C m-2, significantly less than the wetland soils, which indicates that wetland soils serve as a major source of SOC. However, this SOC act as a conditioner to enhance fertility status while combating with climatic extremes, not only that it is a vital component of soil with important effects on the functioning of terrestial ecosystems. For SOC pool, different extraction methods are used such as, highly labile fraction of SOC extracted with hot water (about 3-8% of toal SOC), water soluble fraction of SOC extracted with water (about 1% of total SOC), labile fraction is extracted using CaCl2 (about 1% of total SOC), moderately labile fraction extracted by pyrophosphate (about 4-10% of total SOC), polyaromatic SOC is extracted using toluene + methanol (trace amount of total SOC), microbial biomass C extracted by K2SO4 (about 2-5%) and the resistant fraction remaining after extraction. However, the SOC concentration is high in surface layer but with depth concentration decreases. In addition, soil bulk density and thickness values increase with depth, as a result deeper layers stored more carbon than surface layer in Arial beel soils. There is increasing evidence from the results that wetlands have an important and under-estimated role in carbon storage and its pool the regualation of greenhouse gas emission. Some types of wetlands play a particularly key role as C stores, these include forested wetlands and vegetated inter-tidal wetlands and hence, Sundarban mangrove forest and Tengarchar SOC stocks and pools measurement are an urgent issue for the Climate Change researchers and policy makers.

Application of Hydrochar Altered Soil Microbial Community Composition and the Molecular Structure of Native Soil Organic Carbon in a Paddy Soil.

The benefits and disadvantages of hydrochar incorporation into soil have been heavily researched. However, the effect of hydrochar application on the soil microbial communities and the molecular structure of native soil organic carbon (SOC) has not been thoroughly elucidated. This study conducted an incubation experiment at 25 °C for 135 days using a soil column with 0.5 and 1.5% hydrochar-amended paddy soil to explore the interconnections between changes in soil properties and microbial communities and shifts in native SOC structure using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS) and NMR after hydrochar application. Hydrochar addition decreased the labile SOC fraction by 15.6-33.6% and increased the stable SOC fraction by 10.3-27.0%. These effects were significantly stronger for 1.5% hydrochar-treated soil. Additionally, hydrochar addition induced the native SOC with 1.0-3.0% more carbon and 6.0-13.0% higher molecular weight. The SOC in hydrochar-amended soil contained more aromatic compounds but fewer carbohydrates and lower polarity. This was resulted by a statistically significant reduction in Sphingobacterium, which was active in polycyclic aromatic hydrocarbon degradation, and an increase in Flavobacterium, Anaerolinea, Penicillium, and Acremonium, which were the efficient decomposers of labile SOC. These findings will help elucidate the potential influence of hydrochar on the carbon biogeochemical cycle in the soil.

Spectral responses to labile organic carbon fractions as useful soil quality indicators across a climatic gradient

Abstract Light fraction (LF) and permanganate-oxidizable C (POXC) demonstrate high reliability as indicators for monitoring soil functioning in response to changes in soil organic carbon (SOC). However, mechanisms affecting the amount and composition of labile fractions and their relationship with SOC content at regional scales have not been thoroughly studied. The aim of this study was to examine the spectral features associated with these labile organic matter fractions in samples collected from 75 sites under different soil types, land use and climatic conditions in Chile. Topsoil was analyzed for total C and N content, aggregate stability, and texture. Additionally, the spectral properties of LF material and whole soils were analyzed using diffuse reflectance mid-infrared spectroscopy (MidIR). Our results show that LF shared a similar spectral composition but with different band intensities across climatic regimes. LF spectra were associated with O-alkyl C in cool and rainy areas, whereas a relative accumulation of aromatic structures was found in warmer areas. Whole soils spectra showed that SOC, POXC and aggregability were related to the prevalence of aliphatic and polysaccharides compounds in colder areas. While in warm arid areas, the stabilization of aliphatic compounds was found to be related to clay minerals. Furthermore, we found that POXC and SOC content were closely related and changes in POXC were affected by variations in climate conditions. The understanding of spectral features linked to labile SOC fractions on at larger geographical scale will contribute to the development of sustainable land management options for the prevention of land degradation in the context of adaptation to climate change.

Soil microbial community responses to labile organic carbon fractions in relation to soil type and land use along a climate gradient

There has been a growing interest in studying the labile C pool in order to promote the sequestration and stabilization of soil organic carbon (SOC). Although labile SOC fractions have emerged as standardized indicators because of their potential to detect early SOC trends over time, the relationships between microbial attributes and labile SOC remains poorly understood. In this study, we explored the influence of labile SOC fractions on the topsoil bacteria-archaea community across 28 sites with different land use, climate aridity, and soil types across a wide range of SOC content (0.6–12%) in central Chile. We applied Illumina sequencing to the 16S rRNA to examine shifts in the diversity and composition of these soil microbial communities. Additionally, labile SOC fractions such as the permanganate oxidizable carbon (POXC) and light fraction organic matter (LFOM), along with the soil physicochemical properties were analyzed. The results demonstrated that among all of the environmental factors tested, the pH, POXC/SOC ratio and LFOM were key drivers of microbial community structure (β-diversity). The α-diversity metrics exhibited a decreasing trend when aridity increased, and community structure was found to vary, with high POXC/SOC in sites associated with drier conditions. In addition, POXC/SOC ratios and LFOM were clearly related to shifts in the relative abundances of specific taxonomic groups at genera level. When there was high POXC/SOC and low LFOM content, members of Bacteroidetes (Adhaeribacter, Flavisolibacter, and Niastella), Proteobacteria (Skermanella, Ramlibacter, and Sphingomonas), and Archaea (Thaumarchaeota) were found to be the most dominant groups; however, the microbial taxa responded differently to both labile C fraction types. These results have implications for understanding how labile C content can potentially be used to predict shifts in the microbial community, thus facilitating the development of predictive ecosystem models, as well as early warning indicators for soil degradation.

Soil organic carbon and its labile fractions in paddy soil as influenced by water regimes and straw management

Understanding the effects of water regimes and straw management on soil organic carbon (SOC) pools is necessary to improve soil C sequestration and soil quality. Based on a long-term field experiment (15 years), we examined impacts of water regimes and straw management methods on total soil organic C (TOC), particulate organic C (POC) and potassium permanganate-oxidizable C (KMnO4–C) at two soil depths. Water regimes consisted of flooding cultivation and non-flooded cultivation. In non-flooded cultivation, limited irrigation was employed to keep the soil moist condition without standing water covering the field during the rice-growing season. It is substantially different from conventional flooded rice cultivation. The two straw management methods were straw mulching and straw removal. The results showed that water management had greater effects on TOC and KMnO4–C than did straw management at 0–10 cm soil layer. Compared to the non-flooded treatment, flooding cultivation resulted in 17.4% higher TOC and 31.4% higher KMnO4–C contents. Correspondingly, straw management showed dominant effects on POC. Straw mulching significantly increased POC content by 13.0% as compared to the straw removal treatment. However, at deeper soil depth (10–20 cm), labile SOC fractions were only affected by water regimes. The higher carbon management index (CMI) values at 0–20 cm were recorded in the flooding condition with straw mulching treatment. This was attributed to the increased carbon pool index (CPI), which contributed to the formation of more stable organic compounds that collectively act as a soil reservoir. The combined non-flooded cultivation and straw mulching method (NFC–SM) produced a similar 15–year average rice grain yield and TOC content in 2016 compared to the traditional flooding cultivation with straw removal treatment (FC–SR). Our data indicated that NFC–SM could be an ideal strategy not only to save water but also to maintain soil fertility.

Responses of soil labile organic carbon fractions and stocks to different vegetation restoration strategies in degraded karst ecosystems of southwest China

The effective restoration of degraded areas, resulting in the sequestration of significant amounts of soil organic carbon (SOC) and labile soil organic carbon fractions (LOCF), is essential for mitigating global climatic changes. It is therefore important to improve our understanding of the responses and magnitudes of SOC and LOCF during restoration processes in degraded land like karst ecosystems. In this study, we selected four different restoration strategies: (1) not subjected to any management practices, mainly bare land (BL); (2) Pinus yunnanensis plantation (PY), native and characterized by slow growth and high drought resistance; (3) Eucalyptus maideni plantation (EM), non-native and characterized by rapid growth and high adaptability; (4) natural regeneration of secondary forest (SF). The SOC concentrations, SOC stocks, sensitive indicators of LOCF (microbial biomass C (MBC), permanganate oxidizable C (KMnO4-C), dissolved organic C (DOC), water soluble organic C (WSOC)), non-labile C (NLC), and the carbon pool management index (CPMI) were investigated. The different reforestation strategies had significant effects (p < 0.05) on SOC stocks; compared with BL, the SOC stocks of SF, EM, and PY increased by 57.7, 38.0, and 26.9%, respectively. Both the LOCF and the NLC stocks were significantly increased by the different restoration strategies relative to the BL, while natural vegetation restoration was more beneficial for C sequestration than afforestation. Compared with BL, in SF, EM, and PY, the CPMI increased by 70.1, 35.8, and 32.6%, respectively. These findings suggest that although afforestation can significantly increase C sequestration, natural regeneration may be a more effective approach in terms of SOC sequestration. Soil C accumulation would be limited by nitrogen and phosphorus during the vegetation restoration in the karst areas. In degraded karst ecosystems, the CPMI could be used as a representative index in evaluating the impacts of vegetation restoration on SOC concentration and soil quality.

Carbon cycle in response to residue management and fertilizer application in a cotton field in arid Northwest China

Understanding the influence of farming practices on carbon (C) cycling is important for maintaining soil quality and mitigating climate change, especially in arid regions where soil infertility, water deficiency, and climate change had significantly influenced on agroecosystem. A field experiment was set up in 2009 to examine the influence of residue management and fertilizer application on the C cycle in a cotton field in the Xinjiang Uygur Autonomous Region of Northwest China. The study included two residue management practices (residue incorporation (S) and residue removal (NS)) and four fertilizer treatments (no fertilizer (CK), organic manure (OM), chemical fertilizer (NPK), chemical fertilizer plus organic manure (NPK+OM)). Soil organic carbon (SOC) and some of its labile fractions, soil CO2 flux, and canopy apparent photosynthesis were measured during the cotton growing seasons in 2015 and 2016. The results showed that SOC, labile SOC fractions, canopy apparent photosynthesis, and soil CO2 emission were significantly greater in S+NPK+OM (residue incorporation+chemical fertilizer) than in the other treatments. Analysis of all data showed that canopy apparent photosynthesis and soil CO2 emission increased as SOC increased. The S+OM (residue incorporation+organic manure) and S+NPK+OM treatments were greater for soil C sequestration, whereas the other treatments resulted in soil C loss. The S+NPK treatment is currently the standard management practice in Xinjiang. The results of this study indicate that S+NPK cannot offset soil C losses due to organic matter decomposition and autotrophic respiration. Residue return combined with NPK fertilizer and organic manure application is the preferred strategy in arid regions for increasing soil C sequestration.

Linkage between soil organic carbon and the utilization of soil microbial carbon under plastic film mulching in a semi-arid agroecosystem in China

ABSTRACTStudying changes in soil organic carbon (SOC) pools and soil microbial C substrate utilization under plastic mulching in different seasons is of great significance for improving soil fertility and sustainable agricultural development. Based on a 2-year plastic film mulching experiment in northeastern China, we investigated the SOC, labile SOC fractions under three treatments: non-mulching (NM), autumn mulching (AM) and spring mulching (SM). The results showed that SOC decreased with soil depth under the AM and SM treatments compared with the NM treatment. The microbial biomass carbon (MBC) and dissolved organic carbon (DOC) under the AM treatment increased significantly in the 0–10 cm soil layer, by 31.2% and 27.2% (p < 0.05), respectively. The AM treatment significantly increased the utilization of amino acids and carbohydrate C sources. Redundancy analysis (RDA) indicated that MBC was the main factor influencing microbial metabolic functional diversity and accounted for the largest variation in the 0–10 cm layer. Pearson’s correlation analysis illustrated that MBC was strongly correlated with the utilization of the microbial C substrate. We suggest that AM may be an effective and sustainable management practice for improving soil quality and maintaining microbial functional diversity in semi-arid agroecosystems in this area.

Earthworms promote the accumulation of maize root-derived carbon in a black soil of Northeast China, especially in soil from long-term no-till

Recent studies have shown that earthworms regulate dynamics of plant litter and soil organic carbon. However, the effects of earthworms on rhizospheric carbon, which is the main energy source of soil food webs and a vital factor in soil carbon sequestration, remain unclear. Here, a 13CO2-labeling experiment was performed to determine the effects of earthworms on the fate of maize rhizodeposit carbon from black soil (Mollisol) following conventional tillage (CT) and 14 years (2002–2016) of conservation tillage (no-till, NT). A total of 4 treatments was established: NTE (no-till soil with earthworms), CTE (conventional till soil with earthworms), NTC (control, no-till soil without earthworms) and CTC (control, conventional till soil without earthworms). The results indicated that earthworms increased the amounts of root-derived in soils relative to the amounts in the corresponding no-earthworm control treatments. Maize biomass was significantly higher in the NTE treatment than in the CTE treatment (P < 0.05). Compared with the control treatments (NTC and CTC), the presence of earthworms stimulated maize biomass growth. On day 30 after labeling, the amount of rhizodeposit carbon in NT soil was significantly higher than that in CT soil (P < 0.05), and a higher δ13C signature in the soil organic carbon (SOC) was observed in the NTE treatment than in the CTE treatment (P < 0.05). These findings indicate that earthworms in NT soils were more likely due to the higher biomass inputs and were more likely to assimilate rhizodeposit carbon from roots than that in CT soils. We found that neither earthworm presence nor tillage significantly affected the δ13C signatures in the labile SOC fractions (dissolved organic carbon, DOC; microbial biomass carbon, MBC and particulate organic carbon, POC). Our results suggest that the presence of earthworm affects microbes and may increase the retention of rhizodeposit carbon. In the earthworm-worked soils, higher amounts of carbon were mineralized by soil microorganisms in the CT soil than in the NT soil. This finding demonstrates that earthworms in NT soils could promote the accumulation of rhizodeposit carbon. Our work highlights how earthworms in long-term NT soil facilitate photosynthesized carbon accumulation belowground.

Changes in carbon pools and biological activities of a sandy loam soil under medium‐term conservation agriculture and diversified cropping systems

SummaryConservation agriculture (CA) practices such as zero tillage (ZT) and permanent raised beds (PB) accelerate deposition of soil organic matter and augment associated biological properties of soil through enhanced inputs of organic carbon. However, the potential benefit of CA under intensive cereal‐based systems for key soil health indicators (such as carbon pools and biological activities) is only partially known. Therefore, we analysed the effect of three medium‐term tillage practices and four intensive crop rotations on selected soil organic carbon pools and microbial properties. The tillage practices consist of ZT, PB and conventional tillage (CT) in main plots and four crop rotations (MWMb, maize–wheat–mungbean; MCS, maize–chickpea–Sesbania; MMuMb, maize–mustard–mungbean; MMS, maize–maize–Sesbania) in subplots. The experimental design was split‐plot with three replications. After 6 years, we observed a significant positive effect of CA practices on soil organic carbon (SOC) content, labile SOC fractions, soil microbial biomass carbon (MBC) and dehydrogenase activity (DHA). The total organic carbon (TOC) was greatly affected by medium‐term tillage and diversified cropping systems; it was larger for CA and MCS and MWMb systems. The interaction effect between tillage and cropping systems for SOC content was not significant at all soil depths. Significantly larger contributions (8.5–25.5%) of labile SOC pools to TOC at various soil depths were recorded in PB and ZT. There was a significant positive effect of CA practices and diversified crop rotations on MBC and DHA at all the soil depths and sampling times, but the interaction effect between tillage and cropping systems was not significant. Thus, our medium‐term (≥ 5‐years) study showed that the combination of CA (PB and ZT) practices and appropriate choice of rotations (MCS and MWMb) appears to be the most appropriate option for restoration and improvement of the soil health of light‐textured Inceptisols through the accumulation of soil organic matter (SOM) and improvement in soil biological properties.Highlights Effect of conservation agriculture (CA) on soil labile carbon inputs and biological properties. Observed changes in SOC stock and C‐pools at different soil depths after 6 years. Significant effects of tillage and crop rotations observed for labile‐C pools. Adoption of ZT and PB enhanced SOC stock, C‐pools and microbial activity compared to CT.

Grass cultivation alters soil organic carbon fractions in a subtropical orchard of southern China

Understory management is evidently important for improving the ecological and economic effects of orchards, and grass maintenance in orchards is a potentially preferable floor management measure relative to clean tillage. It is widely accepted that grass maintenance increases soil organic carbon (SOC) content, but whether different SOC fractions respond consistently is less understood. To clarify the potential effects of grass maintenance on SOC fractions, three grass species, including a leguminous species (Stylosanthes guianensis cv. Reyan) and two gramineous species (Paspalum notatum Alain ex Flugge. and Pennisetum americanum x P. purpureum), were planted under young Litchi trees in a subtropical orchard of southern China for eight months using clean tillage as a control. The labile and non-labile SOC fractions and microbial C mineralization were investigated. Relative to the control, grass cultivation significantly increased or trended to increase the SOC content and this was mainly due to increases in the labile SOC fractions. The increased C lability and substrate supply support higher soil microbial biomass C, consequently accelerating the soil C mineralization process and resulting in a greater biologically mineralizable C pool. However, grass cultivation significantly decreased the non-readily oxidizable OC content, although it was relatively more chemically recalcitrant. Our results suggest that grass cultivation may favorably accelerate nutrient cycling in orchards due to higher labile C substrate availability and soil microbial biomass and activity. Nevertheless, grass cultivation could decrease SOC stabilization as indicated by the increased SOC lability in the grass-planting systems.

Evaluating organic carbon fractions, temperature sensitivity and artificial neural network modeling of CO2 efflux in soils: Impact of land use change in subtropical India (Meghalaya)

The present study investigates the changes in land use pattern from fallow land (control) to fruit orchards on soil organic carbon (SOC) dynamics especially, organic carbon stocks, fractions, and CO2 efflux. The SOC fractions such as total organic carbon (TOC), particulate organic carbon (POC), readily oxidizable carbon (ROC), non-labile carbon (CNL), microbial biomass carbon (MBC), carbon management index (CMI) and their sensitivity to land use change were the focus of this study. The organic carbon fractions showed increasing trend (19.4–46%) under fruit orchards compared to control. Furthermore, the orchard with P. persica recorded the highest mean SOC stocks (59.4 Mg ha−1) and TOC (3.03 g 100 g−1) while orchard with C. reticulata showed the highest ROC (6.11 g kg−1) and MBC (401 mg kg−1). Fruit orchards were characterized by high Cmic:CTOC (microbial quotient; qMIC) and POC: TOC ratios, indicating an increase in substrate availability. Furthermore, the CMI was highest under C. reticulata (226.4) followed by P. guajava (173.3), whereas P. communis showed relatively lower value (160.5). While, upper soil layer (0–15 cm) exhibited higher organic carbon stock and fractions viz., 35.1 % and 58.3%, respectively compared to the subsurface soil (15–75 cm) in all orchards. The labile SOC fractions (POC, ROC, and MBC) showed a positively significant correlation with TOC and CO2 efflux indicating that these labile fractions are the sensitive indicators of soil quality changes and improvements. Soil CO2 efflux exhibited a pronounced variation corresponding to land use change with values ranging from 22.80 µg C g−1 h−1 (fallow land) to 27.39 µg C g−1 h−1 (P. guajava). With temperature increment from 25 to 35 °C, the soil CO2 efflux increased from 55% (P. persica) to 88% (fallow) with an average increase of 72%. However, the microbial metabolic quotient (qCO2), also called as specific respiratory activity (SRA), was relatively lower (40.9 µg CO2 mg MBC−1) in orchards compared to fallow land (50.4 µg CO2 mg MBC−1). A multi-layered Artificial Neural Network model (ANN) was also developed, and the experimental CO2 efflux values obtained from different land uses are in good agreement with the predicted CO2 efflux. We conclude that, labile SOC fractions are highly sensitive to land use change and could be effectively used as the sensitive indicators along with CMI for land use change under mid-altitude subtropical ecosystems. In addition, fruit orchards could store more carbon thus could be a potential option to mitigate global warming.

Changes in soil organic carbon fractions in response to different tillage practices under a wheat‐maize double cropping system

AbstractTo understand the turnover of soil organic carbon (SOC) and the improvement of soil quality in response to tillage practices, it is important to identify changes in labile SOC fractions, for example, permanganate oxidizable organic carbon (POxC) and particulate organic carbon (POC). Five tillage treatments were initially undertaken in a winter wheat (Triticum aestivum L., mid‐October to early‐June)–summer maize (Zea mays L., mid‐June to early‐October) system in the North China Plain in 2008 with changes being examined in 2012–2013. These treatments included plough tillage with residue removed, plough tillage with residue incorporation, no tillage with residue mulching, subsoiling with residue incorporation, and rotary tillage (tillage with a rotary tiller) with residue incorporation for the winter wheat season; summer maize was only managed with the NTM treatment. The greatest POxC and POC concentrations at the 0–5‐cm depth were observed under RTR and NTM treatments (p &lt; .05), respectively. Both STR and RTR recorded larger POxC and POC concentrations at the 5–10‐cm depth (p &lt; .05). Both POxC and POC concentrations for STR treatment were significantly higher than those under RTR, NTM, and PT0 treatments in the 20–50‐cm soil profile. The POC concentrations in each soil layer of 0–30‐cm showed a significant response to residue amount, temperature, and precipitation; and POxC concentrations did not record similar responses. Therefore, subsoiling with residue incorporation could be a potential tillage practice to manage labile SOC pool in top soil (0–50‐cm) in the North China Plain region.

Effect of Wetland Reclamation on Soil Organic Carbon Stability in Peat Mire Soil Around Xingkai Lake in Northeast China

Content and density of soil organic carbon (SOC) and labile and stable SOC fractions in peat mire soil in wetland, soybean field and rice paddy field reclaimed from the wetland around Xingkai Lake in Northeast China were studied. Studies were designed to investigate the impact of reclamation of wetland for soybean and rice farming on stability of SOC. After reclamation, SOC content and density in the top 0–30 cm soil layer decreased, and SOC content and density in soybean field were higher than that in paddy field. Content and density of labile SOC fractions also decreased, and density of labile SOC fractions and their ratios with SOC in soybean field were lower than that observed in paddy field. In the 0–30 cm soil layer, densities of labile SOC fractions, namely, dissolved organic carbon (DOC), microbial biomass carbon (MBC), readily oxidized carbon (ROC) and readily mineralized carbon (RMC), in both soybean field and paddy field were all found to be lower than those in wetland by 34.00% and 13.83%, 51.74% and 35.13%, 62.24% and 59.00%, and 64.24% and 17.86%, respectively. After reclamation, SOC density of micro-aggregates (< 0.25 mm) as a stable SOC fraction and its ratio with SOC in 0–5, 5–10, 10–20 and 20–30 cm soil layers increased. SOC density of micro-aggregates in the 0–30 cm soil layer in soybean field was 50.83% higher than that in paddy field. Due to reclamation, SOC density and labile SOC fraction density decreased, but after reclamation, most SOC was stored in a more complex and stable form. Soybean farming is more friendly for sustainable SOC residence in the soils than rice farming.

Accelerated Soil erosion as a source of atmospheric CO2

Soil erosion, physical transport of soil over the landscape by alluvial and aeolian processes as source of energy, has a strong impact on the global carbon cycle (GCC). Being a light fraction (bulk density of 0.6–0.8 Mg/m3) and concentrated in vicinity of soil surface, soil organic carbon (SOC) is preferentially removed by water and wind erosion. The process of erosion and the attendant transport of SOC are accelerated by conversion of natural to agroecosystems. Whereas the human-induced acceleration of soil erosion has depleted the SOC stock of agroecosystems, the fate of SOC transported over the landscape and that deposited in depressional sites is not understood. While a fraction of SOC transported to and buried under aquatic ecosystems (e.g., flood plains, lakes, ocean) may be protected because of limited microbial activity, labile fractions of SOC being transported over the landscape enroute to the depositional site are vulnerable to decomposition. Depending on the site-specific conditions with regards to the hydrothermal regimes and the degree of aeration, the decomposition may lead to emission of CO2 under aerobic environments, CH4 under anaerobic conditions, and N2O under both situations. The process of soil erosion, especially that by water, is a 4-stage process: (i) detachment, (ii) splash, (iii) transport and redistribution, and (iv) deposition. Breakdown of aggregates, during the first three stages, exposes the hitherto encapsulated SOC to microbial processes and exacerbates its vulnerability to decomposition. Thus, the fate of SOC subject to erosion must be assessed for all landscape positions and integrated over the watershed. Lack of credible data regarding the fate of SOC at different erosional stages is a major cause of uncertainties. Thus, well-planned research at a watershed-level is needed to assess the impacts of erosional processes on decomposition of SOC, gaseous emission, and the soil/ecosystem C budget for diverse soils and management systems in global biomes/ecoregions. The data on global C budget is incomplete without consideration of the impact of erosion on SOC and the attendant gaseous emissions.