Stable Soil Organic Carbon Pool Research Articles

Accumulation of soil microbial extracellular and cellular residues during forest rewilding: Implications for soil carbon stabilization in older plantations

Soil microbial residues are a critical component of stable soil organic carbon pools in terrestrial ecosystems. Plantations are anticipated to increase soil stable carbon stocks and contribute toward addressing global climate change. How rewilding, as a strategy to restore the multiple ecological functions of plantations, affects soil microbial residues remains to be elucidated. Here, we divided microbial residues into extracellular residues (using extracellular polymeric substances, i.e., EPS, as a proxy) and cellular residues (estimated by amino sugars) and investigated the changes in their accumulation over four decades (6–45 years) of Metasequoia glyptostroboides plantation development. It was found that forest rewilding resulted in a linear accumulation of soil microbial residues and enhanced their contributions to the soil organic carbon. Specifically, EPS-polysaccharide increased by 126.5%, while the total cellular residue carbon, fungal cellular residue carbon, and bacterial cellular residue carbon increased by 73.4%, 77.2%, and 54.3%, respectively. Correspondingly, the contributions of EPS-polysaccharide and total cellular residue carbon to the soil organic carbon increased by 66.1% and 32.1%, respectively. The main drivers behind extracellular and cellular residues differed, with fine root biomass being the main driver of extracellular residues, while soil nitrogen and organic carbon content were the main drivers of cellular residues. Our study demonstrated that through rewilding, microbial extracellular and cellular residues could continuously accumulate in soils and contribute significantly to the sequestration of atmospheric carbon, highlighting the need to incorporate both microbial residues into soil carbon models to address global climate change. The results suggested that plantations should be allowed to grow older rather than being prematurely harvested, as they promote soil carbon sequestration and stabilization, which has important implications for the establishment of stable soil carbon pools in plantations and can be an invaluable asset in the fight against climate change.

Long-term fertilization affects microbial necromass accumulation by regulating nutrient and enzymatic stoichiometry in a calcareous Fluvisol

Microbial necromass accumulation through rhizodeposition plays a critical role in increasing the stable soil organic carbon pool, which is greatly affected by fertilization. However, the legacy effects of long-term fertilization on microbial necromass accumulation remain unclear. Here, we selected a calcareous Fluvisol treated with no fertilizer (Ctrl), P and K fertilizers (PK), N and K fertilizers (NK), and N, P and K fertilizers (NPK) for 34 years and conducted a 12-week microcosm experiment with simulated daily rhizodeposition to disentangle the legacy effects of long-term fertilization on microbial necromass accumulation. We measured soil C, N and P contents and C-, N- and P-acquiring extracellular enzyme activities and analyzed phospholipid fatty acids (PLFAs) and amino sugars to indicate microbial biomass and necromass, respectively. We found that compared with the Ctrl and NK treatments, the PK and NPK treatments significantly decreased the C:N and C:P ratios and the net accumulation of microbial necromass but significantly increased the ratios of C- to N- and C- to P-acquiring enzyme activities and the fungal PLFAs abundance in soils. The net accumulation of microbial necromass was positively correlated with soil nutrient stoichiometry (mainly the C:N and C:P ratios) but negatively correlated with soil enzymatic stoichiometry (mainly the ratios of C- to N- and C- to P-acquiring enzyme activities) and the fungal to bacterial PLFA ratio. Taken together, these results suggest that long-term fertilization has legacy effects on microbial necromass accumulation and that fertilization-induced changes in nutrient and enzymatic stoichiometry regulate microbial necromass accumulation through rhizodeposition in the calcareous Fluvisol.

Nitrogen and phosphorus addition did not affect neutral sugars in plant but decreased them in soil in an alpine grassland on the Tibetan Plateau

As crucial carbon (C) and energy sources for soil microorganisms, neutral sugars are essential component of the labile soil organic carbon (SOC) pool derived from plants and microorganisms and can directly affect the accumulation of SOC. However, the changes in labile SOC components with elevated soil nitrogen (N) and phosphorus (P) inputs remain unclear. Taking advantage of a 10-year field N and P addition experiment in an alpine grassland on the Tibetan Plateau, we analyzed the compositions and concentrations of neutral sugar in plant versus soil, coupled with soil nutrient properties and plant biomass, we assessed factors affecting the preservation of neutral sugars in soil. We found that the concentrations of neutral sugar were lower in plant shoots than those in roots. The concentrations of neutral sugar and the GM/AX ratio (GM: galactose and mannose; AX: arabinose and xylose) in shoots and roots were insensitive to nutrient addition. The concentrations of neutral sugar in the topsoil were significantly lower than those in the subsoil, and both significantly decreased with N alone and P alone addition. The GM/AX ratio was significantly higher in the topsoil than that in the subsoil and both increased with nutrient addition (except for NP addition in the topsoil). Additionally, SOC was negatively correlated with soil neutral sugar concentrations but positively correlated with the GM/AX ratio. This result emphasizes the important role of microbial transformation of neutral sugars in regulating the relative magnitude of the labile SOC pool versus the stable SOC pool, which is consistent with the concept of “microbial carbon pump” (MCP). This study demonstrates differential compositions and concentrations, as well as responses of neutral sugars to N and P addition in plant versus soil, providing new information to understand the effect of nutrient addition on the SOC pool in the plant-soil system.

Climate warming alters the relative importance of plant root and microbial community in regulating the accumulation of soil microbial necromass carbon in a Tibetan alpine meadow.

Climate warming is predicted to considerably affect variations in soil organic carbon (SOC), especially in alpine ecosystems. Microbial necromass carbon (MNC) is an important contributor to stable soil organic carbon pools. However, accumulation and persistence of soil MNC across a gradient of warming are still poorly understood. An 8-year field experiment with four levels of warming was conducted in a Tibetan meadow. We found that low-level (+0-1.5°C) warming mostly enhanced bacterial necromass carbon (BNC), fungal necromass carbon (FNC), and total MNC compared with control treatment across soil layers, while no significant effect was caused between high-level (+1.5-2.5°C) treatments and control treatments. The contributions of both MNC and BNC to soil organic carbon were not significantly affected by warming treatments across depths. Structural equation modeling analysis demonstrated that the effect of plant root traits on MNC persistence strengthened with warming intensity, while the influence of microbial community characteristics waned along strengthened warming. Overall, our study provides novel evidence that the major determinants of MNC production and stabilization may vary with warming magnitude in alpine meadows. This finding is critical for updating our knowledge on soil carbon storage in response to climate warming.

Microbial necromass carbon in estuarine tidal wetlands of China: Influencing factors and environmental implication

Microbial necromass is an important component of the stable soil organic carbon (SOC) pool. However, little is known about the spatial and seasonal patterns of soil microbial necromass and their influencing environmental factors in estuarine tidal wetlands. In the present study, amino sugars (ASs) as biomarkers of microbial necromass were investigated along the estuarine tidal wetlands of China. Microbial necromass carbon (C) contents were in the range of 1.2–6.7 mg g−1 (3.6 ± 2.2 mg g−1, n = 41) and 0.5–4.4 mg g−1 (2.3 ± 1.5 mg g−1, n = 41), which accounted for 17.3–66.5 % (44.8 % ± 16.8 %) and 8.9–45.0 % (31.0 % ± 13.7 %) of the SOC pool in the dry (March to April) and wet (August to September) seasons, respectively. At all sampling sites, fungal necromass C predominated over bacterial necromass C as a component of microbial necromass C. Compared to bacterial necromass C, fungal necromass C showed a stronger connection with ferrous oxides (Fe2+) and total Fe concentrations. Both fungal and bacterial necromass C contents revealed large spatial heterogeneity and declined in the estuarine tidal wetlands with the increase in latitude. Statistical analyses showed that the increases in salinity and pH in the estuarine tidal wetlands suppressed the accumulation of soil microbial necromass C.

Forests have a higher soil C sequestration benefit due to lower C mineralization efficiency: Evidence from the central loess plateau case

Soil carbon (C) mineralization varies with vegetation restoration and plays an essential role in the soil C cycle. However, the response of soil C mineralization to different vegetation restoration types remains unclear. In this study, three vegetation restoration types for the restoration of cropland, namely forestland (Robinia pseudoacacia), shrubland (Caragana korshinskii), and grassland (Artemisia sacrorum and Stipa bungeana), were selected to determine how vegetation restoration types affect soil C mineralization. The vegetation in the assessed areas had been restored 20 years before this study. The results showed that vegetation restoration significantly increased soil CO2 efflux, whereas soil C mineralization efficiency (CME) decreased after cropland conversion. The lowest soil CO2 efflux and CME were observed in forestland, with the highest stable soil organic carbon (SOC) pool. Vegetation restoration significantly increased SOC and its C fraction stocks, and the variation in soil C fractions was regulated by vegetation restoration type. Soil microbial biomass carbon (MBC) and dissolved organic carbon (DOC) effectively reflected the variation in soil C mineralization and stability of the SOC pool, respectively. Soil CO2 efflux was controlled by litter biomass, macroaggregates, particulate organic C, MBC, and DOC, whereas CME was mainly influenced by aboveground biomass, pH, silt and clay content, macroaggregates and DOC. The study suggested that forestland should be selected as the preferred vegetation restoration type in the central Loess Plateau owing to its lower CME with similar C stocks.

Fertilizer quality and labile soil organic matter fractions are vital for organic carbon sequestration in temperate arable soils within a long-term trial in Switzerland

Agricultural management of soils has led to severe losses of soil organic matter (SOM), accompanied by an increased release of CO2 into the atmosphere and a reduction of soil fertility. Especially under the aspect of global warming and the increasing demand for food, there is a need for sustainable management options increasing soil organic carbon (SOC) storage in agricultural soils, but knowledge gaps exist regarding C persistence in, and its transfer between functional SOC pools, within different farming systems. Here we report on impacts of different farming systems on the temporal dynamics of SOM fractions within the DOK long-term trial (Switzerland), from 1982 to 2017. A purely minerally (CONMIN), a minerally and organically (CONFYM), and a purely organically fertilized farming system (BIODYN) were compared with an unfertilized control (NOFERT). We separated archived soils from the Haplic Luvisol (0–20 cm depth) into particulate (POM) and mineral-associated OM (MAOM) fractions, via physical fractionation, and analyzed the chemical composition of selected fractions via solid-state 13C CPMAS-NMR spectroscopy. We demonstrate that under none of the analyzed farming systems, additional SOC was sequestered in the clay-sized MAOM fraction (<6.3 µm) over a period of 36 years. In all fertilized systems, the amount of SOC in this pool did not change, but strongly decreased in NOFERT (-27%). Bulk SOC increased in BIODYN (+13%) and CONFYM (+5%), but decreased in CONMIN (-8%) and NOFERT (-20%). As no additional SOC accumulated in the clay-sized MAOM fraction, this implies that bulk SOC increases were solely stored within labile POM fractions. NMR spectra showed comparable POM chemical compositions between different systems. Differences in fertilizer quality (BIODYN = composted farmyard manure vs CONFYM = stacked farmyard manure + mineral fertilizer) and the omission of pesticides resulted in better conditions for POM stabilization and consequently significantly higher C contents of occluded POM (oPOM) within aggregates, in BIODYN. However, this labile fraction is at high risk of being lost within a few days, as illustrated by the strong annual oPOM-C content fluctuations depending on the timing of soil sampling after harvest. The highest post-harvest oPOM-C losses in BIODYN indicate the higher dynamics compared to CONFYM. It is anticipated that only sustainable fertilization methods with continuous application of solely organic fertilizers in the long-run can maintain SOC in the labile POM fractions at elevated levels, thereby ensuring soil fertility. It also illustrates the need for prevention of major losses by careful management of the labile POM fractions, as this OM could associate with fine mineral particles at a later stage and thus contribute to OC sequestration in the stable SOC pool. Overall, the potential of arable soils to accumulate stable OC for long-term sequestration is questioned.

Examining the contributions of maize shoots, roots, and manure to stable soil organic carbon pools in tropical smallholder farming soils

Continuous inputs of organic matter are vital for sustaining soil organic carbon (SOC) and productivity of soils in smallholder crop-livestock systems. However, the dynamics of the different inputs i.e. maize shoots, roots and manure used are poorly understood. Along with organic inputs, use of mineral fertilizers can alter the nutrient stoichiometry of organic matter inputs and have implications for SOC turnover. This study sought to understand how maize-based inputs and alterations to nutrient stoichiometry contribute to stable SOC pools. We hypothesized that higher quality litter (i.e., manure) contributes more than maize residues to stable SOC pools and that N, P and S additions, designed to balance the stoichiometry of inputs to reflect the stable fine fraction of soil organic matter (C:N:P:S-10,000:833:200:143) results in greater SOC stabilization. We used a 13C natural abundance approach, where the C4 maize residues were incubated for 11 months to trace C stabilization into different SOC pools within a C3 soil. Contrary to our expectations, we observed greater recovery and stabilization of shoot-derived C (2 X more than manure and 1.63 X more than roots) in the mineral-associated organic matter (MAOM) fraction. Mineral N, P and S additions reduced new C recovery in MAOM by 40 % compared to no mineral nutrient’s additions. Our study highlights the importance of residue retention as a strategy to maintain SOC and soil health in smallholder systems, and our results challenge the idea that nutrient additions increase C stabilization of added residues.

Soil microbial metabolism on carbon and nitrogen transformation links the crop-residue contribution to soil organic carbon

The beneficial effect of crop residue amendment on soil organic carbon (SOC) stock and stability depends on the functional response of soil microbial communities. Here we synchronized microbial metagenomic analysis, nuclear magnetic resonance and plant-15N labeling technologies to gain understanding of how microbial metabolic processes affect SOC accumulation in responses to differences in N supply from residues. Residue amendment brought increases in the assemblage of genes involved in C-degradation profiles from labile to recalcitrant C compounds as well as N mineralization. The N mineralization genes were correlated with the C and N accumulation in the particulate and mineral-associated C pools, and plant-derived aliphatic forms of SOC. Thus, the combined C and N metabolic potential of the microbial community transforms residue into persistent organic compounds, thereby increasing C and N sequestration in stable SOC pools. This study emphasizes potential microbially mediated mechanisms by which residue N affects C sequestration in soils.

Examining the Contributions of Maize Shoots, Roots, and Manure to Stable Soil Organic Carbon Pools in Tropical Smallholder Farming Soils

Examining the Contributions of Maize Shoots, Roots, and Manure to Stable Soil Organic Carbon Pools in Tropical Smallholder Farming Soils

Coastal soil texture controls soil organic carbon distribution and storage of mangroves in China

Mangroves are among the most carbon-rich forests in the tropics, but the spatial variation in soil organic carbon (SOC) storage and the mechanisms controlling their stability remain contentious. Here, we proposed a new framework based on distinct coastal soil textures (muddy, sandy, and mud-sand mixed soil) and found that coastal soil texture exhibits regional-scale variations in mangrove soil C:N:P stoichiometry and SOC storage. Using this soil texture classification framework, we identified the important role of mud-sand mixed soil, which functions as a blue carbon hotspot in coastal wetlands. We showed that mangrove SOC storage was underestimated by approximately 37% in mud-sand mixed soil and overestimated by approximately 54% in muddy soil. We further revealed that the chemical composition of SOC and clay mineralogy are responsible for SOC stability by combining physical and chemical characterizations. As a microbial-derived C, glomalin-related soil protein (GRSP) contained higher alkyl C (∼58%) and lower O-alkyl C (∼17%) than mangrove soils, indicating that GRSP contributes to the stable SOC pool by its recalcitrant structure. We further found that the C:N:P stoichiometric signature of GRSP can capture variations in mangrove SOC storage compatible with distinct coastal soil textures. These results highlight the role of GRSP in regulating SOC stabilization mechanisms, potentially attenuating coastal blue carbon-climate feedback. Regional-scale variation in SOC linked to coastal soil textures can provide more robust estimates of the contribution of mangrove SOC to global C dynamics.

Factors controlling sizes and stabilities of subsoil organic carbon pools in tropical volcanic soils

Soil organic carbon (SOC) in the subsoil may not be so resistant to decomposition as previously assumed, while the mechanisms controlling C dynamics in subsoils are not yet known. This study aimed to (1) identify the factors that control SOC pools in subsoil and (2) compare the differences in SOC pools and controlling factors between the subsoil and topsoil. Subsoils (20–40 cm) were sampled along elevational gradients from two volcanic regions with less-disturbed vegetation each from Tanzania (11 sites) and Indonesia (12 sites). The sizes and mean residence times of labile, intermediate, and stable SOC pools were estimated by fractionation and model fitting to CO2 release during long-term incubation. The controlling factors of each SOC pool were determined by accompanying partial correlation and path analyses. In subsoil, the intermediate SOC pool predominantly controlled the SOC stability within decades. Climatic, geochemical, and biotic factors controlled different SOC pools. Temperature negatively affected the sizes of all three pools. The nanocrystalline minerals contents predominantly and positively controlled the sizes of intermediate and stable SOC pools, and the mean residence time of intermediate SOC pool. Biotic and climatic factors (i.e., microbial biomass, available N for microbes, and excess precipitation) controlled the labile SOC pool. Compared with topsoil, stabilized organic matters were more in the intermediate rather than in the stable SOC pool, and the temperature had a more significant effect on the stable SOC pool in subsoil than in topsoil. Available N for microbes partially controlled the labile and intermediate SOC pools in subsoil (more limited available N for microbes), but not in topsoil. Thus, subsoil SOC would be more sensitive to climate change than topsoil SOC. This study helped to understand the SOC stabilization mechanism and emphasized the high climate- and mineral-dependence of SOC in subsoil of tropical volcanic regions.

The influence of tree species on soil organic carbon stability under three temperate forests in the Baihua Mountain Reserve, China

Soil organic carbon (SOC) is an important component of the carbon (C) cycle in forest ecosystems, and is largely influenced by tree species. Leaf litter originated from different vegetations introduce organic materials into soil with different quantities and qualities, and influence the formation and stability of soil C pool. We present a study on the influence of tree species on SOC stability under three temperate forests: pine (Pinus tabuliformis Carr.), larch (Larix principis-rupprechtii Mayr.) and birch (Betula dahurica Pall.) in the Baihua Mountain Reserve. The SOC and permanganate oxidizable C (POXC) concentrations in the 0–5, 5–10, 10–20 and 20–30 cm soil layers were determined. The SOC stability was evaluated by analyzing the chemical composition of SOC and the distribution of organic C in soil particle-size fractions under different tree species. The birch forest had significantly higher SOC and POXC concentrations than pine and larch in all the four soil layers. These differences might be partly related to the altitude, and also caused by the differences in litter characteristics between birch and the two coniferous species. The organic C was enriched in clay-sized fraction in birch forest soil, but showed higher concentrations in coarse sand fraction (2000-250 μm) under pine, suggesting a higher potential of birch forest for C accumulation in stable SOC pools. The chemical composition of SOC was determined by solid-state 13C nuclear magnetic resonance (NMR) spectra and showed that the birch forest soil had higher alkyl C/O-alkyl C, aromatic C/O-alkyl C and hydrophobic C/hydrophilic C ratios than pine and larch forest soils in the top 5 cm soil layer, reflecting a more recalcitrant SOC composition under birch than under pine and larch. We concluded that the birch forest could not only increase SOC concentration in mineral soils but also had a higher potential to accumulate C in stable soil C pools, and thus is more beneficial to SOC stabilization than the pine and larch forests. This study highlights that future strategy of tree species selection needs to consider the potential effects of tree species on the stability in addition to the concentration of SOC.

Contribution of soil microbial necromass to SOC stocks during vegetation recovery in a subtropical karst ecosystem

Carbon sequestration is a key soil function, and an increase in soil organic carbon (SOC) is an indicator of ecosystem recovery because it underpins other ecosystem services by acting as a substrate for the soil microbial community. The soil microbial community constitutes the active pool of SOC, and its necromass (microbial residue carbon, MRC) contributes strongly to the stable SOC pool. Therefore, we propose that the potential for restoration of degraded karst ecosystems lies in the abundance of soil microbial community and the persistence of its necromass, and may be measured by changes in its contribution to the active and stable SOC pools during recovery. We investigated changes in SOC stocks using an established space-for-time chronosequence along a perturbation gradient in the subtropical karst ecosystem: sloping cropland < abandoned cropland < shrubland < secondary forest < primary forest. Microbial biomarkers were extracted from soil profiles from surface to bedrock and used to measure the contributions of the soil microbial community composition (using phospholipid fatty acids, PLFAs) and MRC (using amino sugars) to SOC stocks at each recovery stage. The results showed that the SOC stocks ranged from 10.53 to 31.77 kg m−2 and increased with recovery stage, with total MRC accounting for 17–28% of SOC. Increasing PLFAs and MRC abundances were positively correlated with improved soil structure (decreased bulk density) and organic carbon, nitrogen and phosphorus nutrient. Bacterial MRC contributes more to SOC stocks than fungal residue carbon during vegetation recovery. The PLFA analysis indicated that Gram positive bacteria were the largest microbial group and were relatively more abundant in deeper soils, and biomarkers for saprophytic and ectomycorrhizal fungi were more abundant in soils under woody vegetation. In conclusion, this study suggests that the soil microbial community in karst soils have the potential to adapt to changing soil conditions and contribute substantially to building SOC stocks after abandonment of agriculture in degraded karst landscapes.

Soil organic carbon pools controlled by climate and geochemistry in tropical volcanic regions

Understanding the factors that control the storage of soil organic carbon (SOC) is an urgent priority for mitigating global climate problems. The objective of this study was to determine the factors controlling SOC pools with differing stabilities. Surface soil samples were collected along an elevation gradient from four volcanic regions of Tanzania (two regions) and Indonesia (two regions) under largely-undisturbed vegetation (24 sites in total). A three-pool kinetic model was fitted to accumulative CO2 release curve produced over 343-day incubation to determine the sizes of the labile and intermediate SOC pools (CL and CI, respectively) and their mean residence times (1/KL and 1/KI, respectively), where the size of the stable SOC pool (CS) was measured as non-hydrolyzable carbon. Correlation and path analyses were performed using the results of soil fractionation and model fitting with climatic and geochemical properties.The intermediate pool comprised 50% of total SOC, was responsible for 58% of total accumulative CO2 release, and controlled total SOC stability. The content of nanocrystalline minerals (Alo + 1/2Feo: 5.5–110 g kg−1) was strongly correlated with CI and CS, suggesting that organo-mineral complexes is the essential factor that controls CI and CS rather than soil texture or pH. Temperature (12–26 °C) was weakly correlated with CI, CS, and strongly with CL, which was closely related to microbial biomass carbon. The low temperature at the high elevation sites retards the decomposition of the whole SOC. The significant correlations of excess precipitation with 1/KL and 1/KI represent the effect of moisture on the potential stabilities of the labile and intermediate SOC pools.Climatic factors primarily affect relatively labile SOC pools, whereas geochemical factors influence more stable pools and control total SOC. The findings have important implications for understanding the SOC stabilization mechanisms, which is an essential process of the carbon cycle, in tropical volcanic soils.

Metagenomic and 14 C tracing evidence for autotrophic microbial CO2 fixation in paddy soils.

Autotrophic carbon dioxide (CO2 ) fixation by microbes is ubiquitous in the environment and potentially contributes to the soil organic carbon (SOC) pool. However, the multiple autotrophic pathways of microbial carbon assimilation and fixation in paddy soils remain poorly characterized. In this study, we combine metagenomic analysis with 14 C-labelling to investigate all known autotrophic pathways and CO2 assimilation mechanisms in five typical paddy soils from southern China. Marker genes of six autotrophic pathways are detected in all soil samples, which are dominated by the cbbL genes (67%-82%) coding the ribulose-bisphosphate carboxylase large chain in the Calvin cycle. These marker genes are associated with a broad range of phototrophic and chemotrophic genera. Significant amounts of 14 C-CO2 are assimilated into SOC (74.3-175.8 mg 14 C kg-1 ) and microbial biomass (5.2-24.1 mg 14 C kg-1 ) after 45 days incubation, where more than 70% of 14 C-SOC was concentrated in the relatively stable humin fractions. These results show that paddy soil microbes contain the genetic potential for autotrophic carbon fixation spreading over broad taxonomic ranges, and can incorporate atmospheric carbon into organic components, which ultimately contribute to the stable SOC pool.

Allocation and turnover of rhizodeposited carbon in different soil microbial groups

Recent advances in soil organic carbon (SOC) formation indicate that labile plant C is disproportionately important for stable SOC pool because it is utilized more efficiently by soil microbes. Rhizodeposited C, accounting for 5–20% of photosynthates, are highly bioavailable and readily metabolized substrates for soil microbes. However, it remains poorly understood how different microbial functional groups contribute to transforming rhizodeposited C to SOC. Here, we synthesized 23 studies performing 13CO2 pulse labelling of plants in conjunction with stable isotope probing of microbial phospholipid fatty acids, and explored the fate of rhizodeposited C in different soil microbial functional groups. Among microbial groups, fungi (25.3%) and gram-negative bacteria (GN; 23.5%) took up most of the rhizodeposition-derived C, while the relative abundance of fungi (13.7%) was less than a half of GN (32.7%). These results suggest that fungi have a higher capability of acquiring rhizodeposition-derived C, which may be due to their hyphal growth form allowing them to obtain rhizodeposited C more effectively. The mean turnover rates of rhizodeposited C in microbial groups ranged from 0.04 to 0.13 day−1. We did not detect significant differences in the turnover rates of rhizodeposited C among microbial groups. Based on the distribution and turnover rate of rhizodeposited C in different microbial groups, we further revealed that GN (31.2%) and fungi (25.8%) produced the most microbial residues via rhizodeposition-derived C. Our results provided quantitative information about the roles of different microbial functional groups in competing for and processing rhizodeposition-derived C, which could help refine parameter estimation in modeling microbial transformation of plant C into SOC under global changes.

Transformation and distribution of straw-derived carbon in soil and the effects on soil organic carbon pool: A review

Farmland soil organic carbon (SOC) pool is a crucial component of global carbon cycle. Due to the widely-implemented straw returning, crop straws have become the primary exogenous carbon source for agricultural soils. The conversion and distribution of straw-derived carbon in soil directly affect the composition and contents of SOC, with further influence on soil nutrient cycling. Based on recent studies, this review investigated the factors impacting the transformation and distribution of straw-carbon; introduced the microbial composition that contributes to the assimilation of carbon from straw; and summarized the effects of straw-carbon on the composition, content, and turnover of SOC. Additionally, we proposed the future research regarding the effects of abiotic factors on the bio-transformation of straw-carbon; the interaction between biotic and abiotic factors during the straw carbon transformation processes; the coupling of carbon and nitrogen from straws into the soil carbon and nitrogen cycles; and the effective control over the transformation of straw-carbon that enters the active or stable soil organic carbon pool. The purpose was to reveal variation characteristics of SOC during straw returning, and provide theoretical basis and technical support for the efficient fertilization and carbon sequestration of straw returning.

A Long-Term Impacts of Planted Fodder Grasses on Soil Quality and Organic Carbon Fraction Pools

Different fractions of soil organic carbon (SOC) act as a major source and sink for atmospheric carbon (C). Thus, understanding SOC dynamics and its allocation into various resistance organic C pools especially in grassland ecosystem may have a greater role in environmental stability. A perennial grass block with three fodder species namely hybrid Napier (Pennisetum glaucum × P. purpureum), Congo signal (Brachieria rosenesis) and combo Napier (P. purpureum) were established at ICAR Research Complex for NEH Region, Tripura centre in 2003 to study the impact of grasses on SOC dynamics and its distribution into different organic C pools in comparison to a natural mixed-grass soil [grasses, those were grown naturally without any management and might be an admixture of non-legume and legume species suitable for this region for example bermuda grass (Cynodon dactylon), buffalo grass (Bouteloua dactyloides), crab grass (Digitaria sanguinalis), green fingers (Lotrochota birotulata), para grass (Brachiaria mutica), rhodes grass (Chloris gayana), crown grass (Paspalum scrobiculatum), Mimosa (Mimosa diplotricha), Euphorbia hirta, Cassia tora, Vicia hirsuta, V. sativa, Melilotus indica etc.]. Water holding capacity (WHC) and field capacity (FC) of different soils under three fodder species of hybrid Napier, Congo signal and combo Napier ranged between 40.65–59% and 28.62–46.84%, respectively. The soils under natural mixed grasses had the highest WHC. Bulk density was the lowest in soils under hybrid Napier. Available nitrogen and potassium were the highest in soils under hybrid Napier, however, available phosphorus was the highest in soils under Congo signal. Total pool of Walkley and Black carbon (WBC) and total carbon (TC) in top 1 m soil profile ranged from 59.7 to 86.3 Mg/ha and 74.8 to 112.6 Mg/ha, respectively, across the treatments. However, TC pools in soils under mixed grasses were 50.4, 22.1 and 28.6% higher than those in soil under hybrid Napier (74.8 Mg/ha), Congo signal (92.2 Mg/ha) and combo Napier (87.5 Mg/ha), respectively. The most important aspect of the study is that about 62.9–65.5% WBC pool and 64.4–66.8% TC pools lied below 30 cm depth across the grasses. Although soils under combo Napier had higher AC pools by 26.4, 28.2 and 57.1% over those of soils under hybrid Napier (49.4 Mg/ha), Congo signal (48.7 Mg/ha) and mixed natural grasses (62.4 Mg/ha), respectively. The PC pools were higher in soils under mixed natural grasses by 99.5, 16.4 and 6.2% over the soils under three fodder species, that is, hybrid Napier (25.4 Mg/ha), Congo signal (43.4 Mg/ha) and combo Napier (47.3 Mg/ha), respectively. Thus, study suggests that the restoration of low carbon degraded land through establishment of mixed grasses is a feasible option in northeastern Indian Himalayan region as grasses have high potential for allocation of SOC into passive carbon (PC) pool (recalcitrant pool). However, among the perennial grasses, combo Napier maintained more of PC pool in soil, thus, it has potential to increase the stable SOC pools in degraded lands and mitigate the changing climate.

Microbial formation of stable soil carbon is more efficient from belowground than aboveground input

The relative contributions of aboveground versus belowground plant carbon inputs to the stable soil organic carbon pool are the subject of much debate—with direct implications for how the carbon cycle is modelled and managed. The belowground rhizosphere pathway (that is, carbon exiting the living root) is theorized to form stable soil carbon more efficiently than the aboveground pathway. However, while several mechanisms have been invoked to explain this efficiency, few have been empirically tested or quantified. Here, we use soil microcosms with standardized carbon inputs to investigate three posited mechanisms that differentiate aboveground from belowground input pathways of dissolved organic carbon—through the microbial biomass—to the mineral-stabilized soil organic carbon pool: (1) the physical distance travelled, (2) the microbial abundance in the region in which a carbon compound enters (that is, rhizosphere versus bulk soil) and (3) the frequency and volume of carbon delivery (that is, infrequent ‘pulse’ versus frequent ‘drip’). We demonstrate that through the microbial formation pathway, belowground inputs form mineral-stabilized soil carbon more efficiently than aboveground inputs, partly due to the greater efficiency of formation by the rhizosphere microbial community relative to the bulk soil community. However, we show that because the bulk soil has greater capacity to form mineral-stabilized soil carbon due to its greater overall volume, the relative contributions of aboveground versus belowground carbon inputs depend strongly on the ratio of rhizosphere to bulk soil. Belowground carbon inputs form stable soil carbon more efficiently through microbial formation than carbon addition aboveground, according to soil microcosm experiments that quantitatively compare soil carbon formation efficiencies from different mechanistic pathways.