Organic Carbon Components Research Articles

Deciphering the carbon and nitrogen component conversion in humification process mediated by distinct microbial mechanisms in composting from different domestic organic wastes.

This study aimed to compare the process of maturity and humus fraction evolution as well as bacterial community dynamics in composting from different domestic organic wastes (food waste (FW), and vegetable waste (VW)) and decipher the key biotic influencing factors of humic acid formation through correlation analysis and ecological network. The results showed that organic carbon components in FW with high ratio of soluble organic carbon and hemicellulose were more easily to be degraded in composting compared to VW. After 30days of composting, the content of HA-C generated by VW was 35.41%, higher than 29.01% of FW, and the growth rate of HA-C generated was 38.42% and 28.34%, respectively. PARAFAC analysis showed that the structure of HA generated in VW was more complex, and the proportion of humic acid-like components (C3 + C4) was 60.32%, while FW only accounted for 43.86%. However, the evolution growth rate of aromatic components in HA in FW was 26.88% in 30days of compost, which was higher than 15.17% in VW. High-throughput sequencing indicated that Lactobacillus was the initial dominated genera in composting from different domestic wastes. Thermobifida, Thermovum, and Pusillimas as well as Aeribacillus were core bacterial genera that promoted the humification process in FW and VW, respectively. Network analysis showed that there was higher bacterial interacted connection degree and complexity in FW compared to VW. This study was of great significance for optimizing organics conversion and humification efficiency of household waste composting.

Effect of Land Use on the Stability of Soil Organic Carbon in a Karst Region

The composition of soil organic carbon and its stability mechanism are the key to understanding the terrestrial carbon sink capacity. The stability of soil organic carbon in a karst ecosystem greatly affects the soil carbon fixation capacity. In order to understand the impact of human activities on the stability of soil organic carbon in karst areas, the karst valley area of Zhongliang Mountain in Chongqing was selected as an example, and soil samples of four typical land use modes (mixed forest, bamboo forest, grassland, and cultivated land) were collected in layers to analyze the total organic carbon (TOC) and heavy fraction organic carbon (HFOC). The distribution characteristics of light fraction organic carbon (LFOC), labile organic carbon (LOC), and recalcitrant organic carbon (ROC) were analyzed quantitatively by using a structural equation model to provide basic data for soil carbon sink assessment and soil quality protection in karst areas. The results showed that the organic carbon components under different land use patterns in karst areas had obvious surface accumulation, and the content of organic carbon components in the surface layer was 1.2 times that in the bottom layer. Except for LFOC, the content of other organic carbon components was the highest in the mixed forest, followed by that in the bamboo forest and wasteland, with the lowest in cultivated land. Mixed forest ω(TOC) content was the highest, 42.5 g·kg-1, followed by that of bamboo forest (36.6 g·kg-1) and grassland (18.7 g·kg-1), and cultivated land content was the lowest, 13.4 g·kg-1. The soil organic carbon content of cultivated land was 68.5%, 63.5%, and 28.3% lower than that of mixed forest, bamboo forest, and grassland, respectively. Mixed forest had the highest content of ω(HFOC), 21 g·kg-1, followed by those of bamboo forest (20.9 g·kg-1), grassland (18.2 g·kg-1), and cultivated land (13.5 g·kg-1). The mixed forest ω(LOC) content was the highest, 16.3 g·kg-1, followed by those of bamboo forest (14.9 g·kg-1), grassland (11.5 g·kg-1), and cultivated land (5.3 g·kg-1). Mixed forest ω (ROC) content was the highest, 25.7 g·kg-1, followed by those of bamboo forest (21.6 g·kg-1), grassland (15.9 g·kg-1), and cultivated land (10.3 g·kg-1). The bamboo forest land ω(LFOC) content was 15.9 g·kg-1, followed by those of mixed forest (13.9 g·kg-1), grassland (7.3 g·kg-1), and cultivated land (4.9 g·kg-1). The recalcitrant organic carbon index (ROCI) was used to indicate the stability of soil organic carbon. The variation range of ROCI was 33.9%-64.5%, of which the highest was mixed forest (64.5%-66.3%), and the lowest was cultivated land (33.8%-39.6%). The ROCI of mixed forest, bamboo forest, and grassland were 1.8 times, 1.6 times, and 1.4 times that of cultivated land, respectively. Karst area ω (inert organic carbon) content and ROCI showed that human agricultural activities caused the reduction in soil organic carbon content and the destruction of soil physical structure, resulting in the accelerated decomposition and turnover rate of soil organic matter. The most important factor affecting soil stability in karst areas was soil pH. Tillage activities caused soil pH to rise, reduced soil microbial activity, and were not conducive to the accumulation of the inert organic carbon and soil organic carbon pool in the soil.

Effects of Experimental Nitrogen Deposition and Litter Manipulation on Soil Organic Components and Enzyme Activity of Latosol in Tropical Rubber Plantations

It is of great scientific significance in regulating plantation ecosystem restoration to investigate the effects of the nitrogen (N) deposition and litter manipulation on soil organic carbon components and enzyme activities. A micro-plot experiment was conducted with four nitrogen additions[CK (0 kg·hm-2·a-1, calculated by N), LN (50 kg·hm-2·a-1), MN (100 kg·hm-2·a-1), and HN (200 kg·hm-2·a-1)] and two litter treatments[LR (litter removal) and L (litter retained)] for tropical rubber plantations in western Hainan Island. The soil physico-chemical properties, soil organic carbon components, and enzyme activities in 0-10 cm and 10-20 cm depths were analyzed. The results showed that soil pH significantly decreased with elevated N addition and litter removal. The contents of NO3--N and NH4+-N significantly increased with elevated N addition. Moreover, there was a significant interaction between N addition and litter treatment on the contents of NO3--N and NH4+-N (P < 0.05). Compared to that with L, LR reduced SOC and its component contents; particularly, the largest decrease was in LFOC by 29.0%-81.4% in the 0-10 cm depth and 23.5%-58.4% in 10-20 cm, respectively. The contents of SOC and its components presented a trend of increasing first and then decreasing with elevated N addition irrespective of litter treatment, and those contents were significantly higher at LN than those at HN. There was a significant interaction between N addition and litter treatment on SOC, LFOC (0-10 cm), and HFOC contents. Compared with that under L, PPO activity was significantly reduced at LR under CK and LN but was significantly increased at LR under MN and HN, respectively. Variance analysis showed significant interactive effects between N addition and litter treatment on PPO and CBH (0-10 cm) activities, and the soil enzyme activity (BG, PPO, and CBH) responding to N addition was greater than that to the litter treatment. Pearson correlation analysis showed that SOC content was extremely positively correlated with MBC, POC, LFOC, and HFOC contents. To summarize, litter retained combined with low N deposition played an important synergistic role of improving SOC pool and soil enzyme activities for tropical rubber plantation systems.

Effects of mixed broadleaved tree species with pure Pinus massoniana plantation on soil microbial necromass carbon and organic carbon fractions

Mixing native broadleaved tree species is a widely used method for renovating Pinus massoniana plantations. Soil microbial necromass carbon and organic carbon fractions are important parameters for evaluating the impacts of tree species mixing and soil organic carbon (SOC) stability. However, their responses to the mixing and renovation of P. massoniana plantation has not been understood yet. Here, we selected a pure P. massoniana plantation (PP) and a mixed P. massoniana and Castanopsis hystrix plantation, with ages of 16 (MP16) and 38 years (MP38), respectively, as the research objects. We quantified soil physical and chemical properties, microbial necromass carbon content, and organic carbon components at different soil layers to reveal whether and how the introduction of C. hystrix into P. massoniana plantation affected soil microbial necromass carbon and organic carbon components. The results showed that the mixed P. massoniana and C. hystrix plantation significantly reduced fungal necromass carbon content and the ratio of fungal/bacterial necromass carbon in the 0-20 cm and 20-40 cm soil layers. There were no significant differences in microbial necromass carbon contents, bacterial necromass carbon contents, and their contributions to SOC among the different plantations. The contribution of fungal necromass carbon to SOC was higher than that of bacterial necromass carbon in all plantation types. The contribution of soil mineral-associated organic carbon (MAOC) to SOC was higher than that of occluded particulate organic carbon (oPOC) and light-free particulate organic carbon (fPOC) for all plantation types. Mixing the precious broadleaved tree species (i.e., C. hystrix) with coniferous species (P. massoniana) significantly increased MAOC content and the contribution of MAOC, oPOC, and fPOC to SOC in the 0-20 cm and 20-40 cm soil layers. The MAOC of MP38 was significantly higher than that of PP in all soil layers and the MAOC of MP38 stands were significantly higher than MP16 stands in the 20-40 cm, 40-60 cm, and 60-100 cm soil layers, indicating that hybridization enhanced SOC stability and that the SOC of MP38 stands were more stable than MP16 stands. SOC and total nitrogen contents were the main environmental factors driving the changes in soil microbial necromass carbon, while soil total nitrogen and organically complexed Fe-Al oxides were the primary factors affecting organic carbon fraction. Therefore, SOC stability can be enhanced by introducing native broadleaved species, such as C. hystrix, during the management of the P. massoniana plantation.

Indirect influence of soil enzymes and their stoichiometry on soil organic carbon response to warming and nitrogen deposition in the Tibetan Plateau alpine meadow.

Despite extensive research on the impact of warming and nitrogen deposition on soil organic carbon components, the response mechanisms of microbial community composition and enzyme activity to soil organic carbon remain poorly understood. This study investigated the effects of warming and nitrogen deposition on soil organic carbon components in the Tibetan Plateau alpine meadow and elucidated the regulatory mechanisms of microbial characteristics, including soil microbial community, enzyme activity, and stoichiometry, on organic carbon components. Results indicated that both warming and nitrogen deposition significantly increased soil organic carbon, readily oxidizable carbon, dissolved organic carbon, and microbial biomass carbon. The interaction between warming and nitrogen deposition influenced soil carbon components, with soil organic carbon, readily oxidizable carbon, and dissolved organic carbon reaching maximum values in the W0N32 treatment, while microbial biomass carbon peaked in the W3N32 treatment. Warming and nitrogen deposition also significantly increased soil Cellobiohydrolase, β-1,4-N-acetylglucosaminidase, leucine aminopeptidase, and alkaline phosphatase. Warming decreased the soil enzyme C: N ratio and C:P ratio but increased the soil enzyme N:P ratio, while nitrogen deposition had the opposite effect. The bacterial Chao1 index and Shannon index increased significantly under warming conditions, particularly in the N32 treatment, whereas there were no significant changes in the fungal Chao1 index and Shannon index with warming and nitrogen addition. Structural equation modeling revealed that soil organic carbon components were directly influenced by the negative impact of warming and the positive impact of nitrogen deposition. Furthermore, warming and nitrogen deposition altered soil bacterial community composition, specifically Gemmatimonadota and Nitrospirota, resulting in a positive impact on soil enzyme activity, particularly soil alkaline phosphatase and β-xylosidase, and enzyme stoichiometry, including N:P and C:P ratios. In summary, changes in soil organic carbon components under warming and nitrogen deposition in the alpine meadows of the Tibetan Plateau primarily depend on the composition of soil bacterial communities, soil enzyme activity, and stoichiometric characteristics.

Effects of thinning on accumulation of soil microbial residue carbon of Picea asperata plantations in sub-alpine region of western Sichuan, China

Microbial residues are an important component of soil organic carbon (SOC). It is unclear how long-term thinning affects the accumulation characteristics of microbial residue carbon (C). We analyzed the differences in soil physicochemical properties, microbial communities, extracellular enzyme activities, and microbial residue C in topsoil (0-10 cm) and subsoil (20-30 cm) in Picea asperata plantation of non-thinned (control, 4950 trees·hm-2) and thinned for 14 years (1160 trees·hm-2) stands, aiming to reveal the regulatory mechanism of thinning on microbial residue C accumulation. The results showed that thinning significantly increased SOC content, total nitrogen content, available phosphorus content, the proportion of particulate organic C, soil water content, C-cycle hydrolase, and acid phosphatase activities, but significantly reduced the proportion of mineral-associated organic C. Thinning significantly affected the content of fungal and microbial residue C, and the contribution of microbial residue C to SOC, and these effects were independent of soil layer. The content of fungal and microbial residue C was 25.0% and 24.5% higher under thinning treatments. However, thinning significantly decreased the contribution of microbial residue C to SOC by 12.3%, indicating an increase in the proportion of plant-derived C in SOC. Stepwise regression analysis showed that total nitrogen and soil water content were key factors influencing fungal and micro-bial residue C accumulation. In summary, thinning promoted microbial residue C sequestration by altering soil pro-perties and changed the composition of SOC sources.

Effects of long-term litter removal on soil organic carbon and multiple components in subtropical <i>Cunninghamia lanceolata</i> forest

Effects of long-term litter removal on soil organic carbon and multiple components in subtropical <i>Cunninghamia lanceolata</i> forest

Effect of soil flooding and drying on the metabolic pathways of CO2 fixing microorganisms along an elevation gradient in the Three Gorges Reservoir drawdown area

AbstractThe construction of dams and subsequent water level fluctuations significantly alters the environmental conditions of reservoir ecosystems, affecting the metabolic pathways of CO2 fixing microorganisms (CFMs) and carbon storage in drawdown areas. We investigated the response of soil bacterial communities and carbon fixation pathways to periodic flooding and drying at different elevations by collecting surface soil samples (0–10 cm) in the drawdown area of the Three Gorges Reservoir. The results show that periodic flooding and drying increased the complexity of bacterial co‐occurrence networks. We identified the reductive citrate cycle (rTCA cycle), dicarboxylate‐hydroxybutyrate cycle (DC/4‐HB cycle), 3‐Hydroxypropionate cycle (3‐HP cycle) and reductive pentose phosphate cycle (Calvin cycle) as the main carbon fixation pathways. Notably, as the inundation duration increased, the abundance of the Calvin cycle and its key gene (cbbL) gradually decreased. We also found that soil moisture, pH and the different organic carbon components affected the composition of bacterial communities and the abundance of carbon fixation pathways. These findings elucidate evolutionary trends bacterial communities and the impacts of water level fluctuations on microbial‐mediated carbon cycling processes because of dam construction.

Microbial Necromass, Lignin, and Glycoproteins for Determining and Optimizing Blue Carbon Formation.

Coastal wetlands contribute to the mitigation of climate change through the sequestration of "blue carbon". Microbial necromass, lignin, and glycoproteins (i.e., glomalin-related soil proteins (GRSP)), as important components of soil organic carbon (SOC), are sensitive to environmental change. However, their contributions to blue carbon formation and the underlying factors remain largely unresolved. To address this paucity of knowledge, we investigated their contributions to blue carbon formation along a salinity gradient in coastal marshes. Our results revealed decreasing contributions of microbial necromass and lignin to blue carbon as the salinity increased, while GRSP showed an opposite trend. Using random forest models, we showed that their contributions to SOC were dependent on microbial biomass and resource stoichiometry. In N-limited saline soils, contributions of microbial necromass to SOC decreased due to increased N-acquisition enzyme activity. Decreases in lignin contributions were linked to reduced mineral protection offered by short-range-ordered Fe (FeSRO). Partial least-squares path modeling (PLS-PM) further indicated that GRSP could increase microbial necromass and lignin formation by enhancing mineral protection. Our findings have implications for improving the accumulation of refractory and mineral-bound organic matter in coastal wetlands, considering the current scenario of heightened nutrient discharge and sea-level rise.

Efficient prediction of SOC and aggregate OC components by continuous wavelet transform spectra under different feature selection methods

Soil Organic Carbon (SOC) fixation plays a vital role in reducing greenhouse gas emissions and the formation of soil aggregates is recognized as a crucial process in SOC fixation. In the current study, we characterized spectral information on SOC and aggregate Organic carbon (OC) components by using a rapid and robust method. Soil samples were collected from different layers to obtain the SOC and aggregate OC components to record corresponding spectra. The Continuous Wavelet Transform (CWT) spectra were then combined with various characteristic wavelength selection methods to build Multivariate Linear Regression (MLR), BP neural network (BP), and Random Forest (RF) models. Finally, the optimal combination of characteristic wavelengths based on the best-performing model for each index was determined. The results demonstrated that the model's accuracy, which was built using the set of characteristic wavelengths of the best model for all metrics, was higher than that of the best model for a single metric. This was due to the correlation between SOC and aggregate OC components. The best set of characteristic wavelengths were those selected by the Sequential Backward Selection method with wavelet decomposition scales of 23, 26, and 27, and those selected by the Minimum Redundancy Maximum Relevance method with wavelet decomposition scale 27. The RF model for Free silt and clay particles organic carbon (FSP-OC) showed the highest accuracy among the four SOC prediction models. In addition, it was discovered that as the wavelet decomposition scales become more similar, the extracted characteristic wavelengths also become more similar, resulting in similar model predictions. In summary, the accuracy of the predictive model for SOC and its constituents can be demonstrably improved by reasonable combination of characteristic wavelengths selection method and wavelet decomposition scale.

Effect of land use conversion on the soil aggregate-associated microbial necromass carbon in estuarine wetland of the Pearl River in China

Microbial necromass carbon (MNC), as an important component of the soil organic carbon (SOC) pool in natural environments, has increasingly gained significant attention in recent years. However, the redistribution pattern of microbial necromass in aggregate fractions of estuarine soils induced by human activity still remain unclear. In this study, we assessed the effect of land use conversion on the redistribution and accumulation patterns of aggregate-associated MNC in estuarine wetland soils on the Pearl River estuary of China. Typical soil samples in 0–30 cm profiles under different land uses [Natural Wetland (NW), Farmland (FL), and Construction Land (CL)] were analyzed. The results showed that the NW conversion to FL and then CL contributed to the continuous losses of SOC, however, the aggregate-associated MNC content was higher in FL and lower in CL compared to NW. The MNC contribution to total SOC in aggregate fractions gradually increased from NW (16.41 % − 23.62 %) to CL (23.87 % − 30.98 %) and then FL (34.62 % − 38.83 %) in profiles. Fungal necromass C was the primary contributor to the total MNC in each aggregate fraction. In NW, the MNC was mainly present in large aggregate size, however, the MNC was gradually enriched into smaller size aggregates, especially for the bacterial necromass C, after the conversion from NW to FL or CL. Microbial biomass carbon had a direct and positive effect on the MNC in soil aggregates under different land uses, except for in silt + clay fraction from NW. Moreover, pH and soil moisture content were two key variables affecting the MNC content and accumulation by mediating the microbial biomass of different land uses, while the dominate factors controlling the MNC accumulation varied with size fractions in aggregates. Our results emphasize the influence of land use conversion on the soil aggregate-associated MNC accumulation in estuarine wetland.

Effects of short-term drought, nitrogen application and their interactions on the composition and functional genes of soil microbial communities in alfalfa grassland on the Loess Plateau

Drought and nitrogen addition are important factors influencing soil microorganisms and changes in the soil environment. In the future, droughts will become more frequent, shorter, and more severe. However, little is known about the soil organic carbon components, enzyme activity, and composition, structure, and functional genes of soil microbial communities under short-term drought and nitrogen application conditions. In this study, we used metagenomics sequencing technology to explore the changes in the composition and functional genes of soil microbial communities under short-term drought, nitrogen application, and their interactions in the artificial grasslands of the Loess Plateau. The results indicated that (I) short-term drought, nitrogen application, and their interactions all increased the particulate organic carbon (POC) content. (II) Short-term drought increased the contents of soil microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN), as well as soil enzyme activity. (III) Short-term drought significantly increased the bacterial alpha diversity, whereas the interactions of short-term drought and nitrogen application enhanced the fungal alpha diversity. (IV) The interactions of short-term drought and nitrogen application inhibited Fusarium to prevent plant diseases. (V) Short-term drought enriched the relative abundance of genes related to carbon cycling and amino acid metabolism, while nitrogen application reduced genes related to carbon cycling but enriched genes related to glycan biosynthesis and metabolism. These results clearly showed that, short-term drought altered the composition and functional genes of soil microbial communities. Our research suggests that in the event of frequent short-term droughts in the future, nitrogen addition can be considered to maintain the diversity of soil microbial communities and sustain soil carbon and nitrogen cycling.

Substitution of Chemical Fertilizer with Organic Fertilizer Can Affect Soil Labile Organic Carbon Fractions and Garlic Yield by Mediating Soil Aggregate-Associated Organic Carbon

This study aimed to explore the impact paths on soil organic carbon and crop yield of completely or partially substituting chemical N fertilizer with organic fertilizers. A four-year field experiment was conducted and included four treatments: (i) N0, no N fertilization application; (ii) NF, only synthetic N fertilizer application; (iii) 1/2OF, organic fertilizer substituted for 100% of the synthetic N fertilizer, with the total N application amount being equivalent to half that of NF; and (iv) 1/3OF + 2/3NF, organic fertilizer substituted for 1/3 of the synthetic N fertilizer with the total N application amount from organic and synthetic fertilizer being equivalent to that of NF. Soil total organic carbon (TOC), labile organic-carbon fractions (microbial biomass carbon (MBC), dissolved organic carbon (DOC), particulate organic carbon (POC), and easily oxidized organic carbon (EOC)), the carbon pool management index (CPMI), soil aggregated distribution, and water-stable aggregate-associated organic carbon were determined. Structural equation modeling (SEM) was used to clarify the impact paths of TOC and garlic yield changes under different N fertilizer treatments. Results showed that compared with N0 and NF, 1/2OF and 1/3OF + 2/3NF significantly increased TOC contents by 14.1–20.6%. Soil MBC, DOC, and EOC under 1/2OF were significantly higher than under N0, whereas the 1/3OF + 2/3NF treatment had significantly greater POC. The CPMI was improved by organic fertilizer treatment, with 1/2OF treatment being significantly higher than N0 and NF. The proportion of soil aggregate mass with particle sizes >2 mm was significantly greater under N0, while 1/3OF + 2/3NF significantly increased the proportion of particle sizes of 0.5–2 mm. Soil water-stable aggregate-associated organic carbon showed a trend of first increasing and then decreasing, with the largest particle sizes being 1–2 mm. Moreover, organic fertilizer significantly increased soil water-stable aggregate organic carbon compared with N0 and NF. Similarly, the garlic yield increased with organic fertilizer treatment, while 1/3OF + 2/3NF significantly increased the yield by 37.2% and 15.3%, respectively, compared with N0 and NF. Furthermore, SEM analysis indicated that fertilizer regimes could directly affect TOC and labile organic carbon components by affecting aggregate-associated organic carbon. In particular, aggregates with particle sizes of 0.5–2 mm played an important role, indirectly affecting garlic yield and CPMI. These results indicate that organic fertilizer application has the potential to improve soil organic-carbon content and garlic yield; moreover, fully applying organic fertilizer can reduce N fertilizer input while still maintaining an increase in soil organic carbon and crop yield in the short term. However, caution is still needed regarding of the type and quantity of organic fertilizer added in different cropping systems, and with different soil textures.

Geochemical Characteristics and Geological Significance of Mud Shale of the Second Member, Permian Lucaogou Formation, Jimsar Sag, Junggar Basin, NW China

Abstract ––The Lucaogou Formation in Jimsar sag is an important shale oil-producing layer in the Junggar basin. In this paper, core data, organic carbon, rock pyrolysis, biomarker compounds, and organic matter group component separation were used to study the sedimentary environment, parent material type, organic maturity and oil-bearing property of mud shale of the Second member of the Lucaogou Formation. The results showed that the sedimentary environment is a semi-oxidation and semi-reduction environment. The parent material is mainly aquatic with minor input of higher plants. The average TOC is 7.43%, the average value of hydrocarbon generation potential (S1+S2) is 50.54 mg/g, and the average value of Tmax is 446 °C, showing high maturity of the mud shale. The chloroform bitumen “A” features high saturated hydrocarbon content (34.10% on average), low aromatic hydrocarbon content (15.39% on average), and high saturated/aromatic ratio (2.54). The carbon number of saturated hydrocarbons is distributed between n-C13 and n-C33. Their peak are mainly n-C17 and n-C23, which are biased toward medium-low molecular weight alkanes. The average of the ΣC21–/ΣC22+ is 1.19, indicating that the alkane has advantage of short chain. The mud shale in the Second member of the Lucaogou Formation has a large sedimentary thickness, high organic matter content, and high oil generation potential. It is an important replacement target for shale oil in Jimsar sag and has important exploration significance in the future.

Long‐term fertilization regulates dissimilatory nitrate reduction processes by altering paddy soil organic carbon components

AbstractTo enhance rice yields, synthetic nitrogen (N) fertilizers are often applied in rice paddy fields. Under waterlogged conditions, denitrification, anaerobic ammonium oxidation (anammox) and dissimilatory nitrate reduction to ammonium (DNRA) are the main N transformation processes. However, the effects of long‐term fertilization on the dissimilatory nitrate reduction processes remain largely unclear. Here, we investigated rates of denitrification, anammox and DNRA and the abundance of bacterial and nitrate reduction‐related genes (16S rRNA, narG, nirK, nirS, nosZ and hzsB) in rice paddy fields under different fertilization regimes [i.e., non‐fertilization (CK), NPK fertilization (NPK) and NPK fertilization plus straw returning (NPKS)]. The rates of denitrification, anammox and DNRA in the CK treatment were 46.18, 0.35 and 1.29 nmoL N g−1 h−1, respectively, which were significantly (p < .05) increased by 7.32%–27.78%, 2.45–4.12 folds and 1.01–13.23 folds by NPK and NPKS treatments. This was largely consistent with the increased bacterial and nitrate reduction‐related gene abundances in the NPK and NPKS treatments. In addition, NPKS also significantly increased the relative contribution of DNRA to the total nitrate reduction relative to NPK and CK treatments, indicating that NPKS was more beneficial to N retention in the paddy field. Changes in rates of dissimilatory nitrate reduction processes in response to NPK and NPKS treatments were correlated to variations of soil total N, ammonium contents, pH, soil organic carbon (SOC) contents and nitrate reduction‐related genes, among which SOC contents had the highest explanation. Specifically, fulvic‐like compounds, alkyl C and methoxyl and nitrogen‐alkyl C were the key components determining denitrification, anammox and DNRA rates, respectively. Overall, our study suggests that long‐term NPK and NPKS fertilization stimulate rates of dissimilatory nitrate reduction processes mainly by altering soil C components and NPKS is more effective in maintaining soil fertility in the paddy field.

Shrubification and snow removal have an antagonistic effect on the accumulation of rhizosphere organic carbon components in peatlands

A warming climate will cause shrubification and less winter snow cover in peatlands, which are an important global carbon sink. Both shrubification and snowpack reduction affect organic carbon sequestration, yet their effects on the dynamics of microbial necromass carbon (MNC) and plant necromass carbon (PNC) in rhizosphere soil are indeterminate. This investigation focuses on the effects of shrubification and snow cover changes on organic carbon components and their mechanisms in rhizosphere soil. For this purpose, a field in situ experiment on snow cover removal and natural snow cover was conducted in shrub and herb peatlands, respectively, in winter, and rhizosphere soil samples were collected in the following growing season. The MNC biomarker amino sugar and the PNC biomarker lignin phenol were quantified, and in addition to the general soil physical, chemical, and biological parameters, root exudates were measured based on untargeted metabolomics. The findings showed that MNC contributes more to SOC in spring, with the majority of it derived from fungal necromass carbon, and the contribution of PNC to SOC gradually increased in later seasons, with a consequent decrease in rhizosphere SOC stability. Shrubification decreased the MNC content by 15.80% and the PNC content by 7.68%, whereas snow removal increased the MNC content by 8.24% and 2.58% and the PNC content by 11.31% and 17.39% in shrub and herb peatlands, respectively. The changes in root exudates may be an important factor regulating the accumulation of different SOC components. Specifically, sugars and amino acids had a positive priming effect on MNC and PNC, whereas organic acids had a negative priming effect. The mechanisms by which root exudates mediated MNC and PNC accumulation differed depending on the season. This research will be useful in forecasting future changes in carbon sink function and providing a theoretical foundation for increasing carbon sink measures.

Effects of Continuous Manure Application on the Microbial Community and Labile Organic Carbon Fractions

The application of organic materials contributes to the sustainable development of agriculture. Increased manure inputs have a fundamental effect on the composition and dynamics of soil organic carbon (SOC). In this study, we conducted a 10-year field experiment in Changchun, Jilin, Northeast China, to investigate the effects of manure addition on soil organic carbon components and soil microorganisms. Specifically, we established four treatments: (i) chemical fertilizer or no addition of manure (CK), (ii) pig manure with chemical fertilizer (ZF), (iii) cow manure with chemical fertilizer (NF), and (iv) chicken manure with chemical fertilizer (JF). The results showed that the JF treatment significantly increased the soil organic carbon (SOC), dissolved organic carbon (DOC), and readily oxidized organic carbon (ROC) content by 20.36%, 105.9%, and 61.32%, respectively, relative to CK. The microbial biomass carbon (MBC) content in JF, ZF, and NF treatments were significantly higher than that of CK, which increased by 107.24%, 116.45%, and 96.71%, respectively. The particulate organic carbon (POC) content in NF and JF treatments differed significantly, increasing by 25.61% and 19.01%, respectively, relative to CK. Redundancy analysis showed that continuous manure application had a positive effect on soil microbial community diversity and abundance, which was favorable for the accumulation of soil carbon. We also found that soil fungi were more sensitive than bacteria to changes in soil carbon composition following manure application. In conclusion, adding different organic materials can better support biodiversity conservation and realize ecosystem services of surface carbon storage and soil conservation. Our results reveal the importance of microbial fixation in soil carbon dynamics according to the different distribution of active organic carbon pools, which will help enhance our understanding of the carbon cycle.

Accumulation of microbial necromass carbon and its contribution to soil organic carbon in artificial grasslands of various vegetation types

Microbial necromass carbon (C) is a crucial soil organic carbon (SOC) component. In the context of alpine grassland degradation on the Qinghai-Tibet Plateau, the establishment of artificial grasslands is an effective restoration method; however, the accumulation of microbial necromass C and its contribution to SOC in these ecosystems, especially for the different plant species composition, remain unclear. We collected surface soil (0-10 cm) from artificial grasslands of four different types in 2022, including annual unicast Triticale and annual grass-legume mixed artificial grasslands sown last time in 2022, and perennial Elymus nutans and perennial Poa pratensis artificial grasslands sown in 2019. By measuring soil moisture and pH value, contents of amino sugars, and microbial biomass (MB) characteristics, we aimed to investigate the variations in microbial necromass C and its contribution to SOC and identify the factors influencing these processes. The content of microbial necromass C followed the order: perennial Elymus nutans > perennial Poa pratensis > annual grass-legume mixed > annual unicast Triticale. This was mainly because belowground biomass indirectly affected microbial necromass C by altering soil properties. The ratio of MB C/N and soil moisture were identified as the primary factors influencing the contribution of microbial necromass C to SOC. The contribution of microbial necromass C to SOC was more favorable under perennial grasslands with a low MBC/MBN ratio and high SWC than under annual grasslands. Thus, from the perspective of microbial necromass accumulation, perennial grasslands were the most suitable vegetation type for sustainable soil restoration.

Straw and straw biochar differently affect fractions of soil organic carbon and microorganisms in farmland soil under different water regimes

Incorporating amendments can change soil organic carbon (SOC) components and biological properties, but the comprehensive utilization of straw and straw biochar in paddy soil remains poorly understood. We studied the changes in the pH, SOC content, labile organic C fractions, and soil microbial communities under alternating wetting and drying (AWD) and continuous flooding (CF). The results showed that the addition of straw and straw biochar increased SOC content, and the relative changes in microbial biomass carbon (MBC), dissolved organic carbon (DOC), and easily oxidizable carbon (EOC) were more sensitive than those in SOC, but soil pH was higher under AWD management than under CF management. Proteobacteria, Bacteroidetes, and Acidobacteria were the three most abundant bacterial phyla, and the three major abundant fungal phyla were Phragmoplastophyta, norank_Eukaryta, and Apicomplexa. A redundancy discriminant analysis (RDA) found that both straw and straw biochar were conducive to bacterial growth, and their combination was more beneficial for fungal growth and development. Aggregated boosted tree (ABT) and Structural equation modeling (SEM) showed that the soil MBC and EOC were the most important factors determining bacterial abundance and diversity, while pH, EOC, and MBC were the main factors influencing the abundance and diversity of fungi. This study explored the changes in SOC fractions and bacterial and fungal communities and provided a theoretical basis for an in-depth understanding of the influence of labile organic C pools on bacterial and fungal communities.

Characteristics and factors influencing soil organic carbon composition by vegetation type in spoil heaps.

The variation of organic carbon content in spoil heaps is closely related to improving soil structure, maintaining soil fertility, and regulating soil carbon cycling balance. Analyzing the soil organic carbon content and related driving factors during the natural vegetation restoration process of spoil heaps is of great significance for promoting the accumulation of soil organic carbon in the spoil heaps. we selected spoil heaps with the same number of years of restoration to research the variations in soil organic carbon components under different vegetation types (grassland: GL, shrubland: SL, secondary forest: SF) and compared the results with those on bare land (BL). Our results showed that vegetation type and soil depth significantly affect the content of soil organic carbon components. There was no difference in soil organic carbon components between SF and SL, but both were considerably superior to GL and BL (p<0.05), and the particulate organic carbon (POC) and light fraction organic carbon (LFOC) contents of SL were the highest. A significant positive linear correlation existed between SOC and active organic carbon components. Pearson's correlation and redundancy analysis showed that the available potassium (AK) and total nitrogen (TN) contents and gravel content (GC) in the BL soil significantly impacted soil organic carbon. When vegetation is present, TN, total phosphorus (TP), and Fine root biomass (FRB) significantly affect soil organic carbon. Structural equation modelling (SEM) shows that AK and soil moisture content (SMC) directly affect the organic carbon composition content of BL, When there is vegetation cover, fine root biomass (FRB) had the largest total effect in the SEM. Soil bulk density (BD) has a negative impact on soil organic carbon, especially in the presence of vegetation. These findings suggest that vegetation restoration can significantly increase soil organic carbon content, FRB, AK, and TN play important roles in enhancing soil organic carbon. Supplementation with nitrogen and potassium should be considered in the bare land stage, and shrubs nitrogen-fixing functions and well-developed roots are more beneficial for the accumulation of soil organic carbon.