We conducted a 512-day incubation experiment to study the dynamics of microbial necromass and soil carbon fraction in the ‘litter-soil’ transformation interface soil layer (TIS) during litter decomposition, using a perennial C3 herb, Stipa bungeana, in the loess hills. The results showed that soil microbial necromass was dominated by fungi in the early and middle stages, and by bacteria in the late stage. The contribution of fungal necromass C to mineral-associated organic C (MAOC) was significantly higher (38.7%-75.8%) than that of bacteria (9.2%-22.5%) and 2-3 times more than the contribution rate of bacterial necromass. Soil organic C (SOC) content was decreasing during litter decomposition. The input of plant C resources stimulated microbial utilization of soil C fractions. The continuous decrease in particulate organic C during the early and late stages of decomposition was directly responsible for the decrease in SOC content. In contrast, the fluctuating changes in microbial necromass C and MAOC played an indirect role in the reduction of SOC. The increase in soil microbial necromass C caused by a single exogenous addition of litter did not directly contribute to SOC accumulation.

Microorganisms are essential actors in the biogeochemical cycling of elements within terrestrial ecosystems, with significant influences on soil health, food security, and global climate change. The contribution of microbial anabolism-induced organic compounds is a non-negligible factor in the processes associated with soil carbon (C) storage and organic matter preservation. In recent years, the conceptual framework of soil microbial carbon pump (MCP), with a focus on microbial metabolism and necromass generation process, has gained widespread attention. It primarily describes the processes of soil organic C formation and stabilization driven by the metabolic activities of soil heterotrophic microorganisms, representing an important mechanism and a focal point in current research on terrestrial C sequestration. Here, we reviewed the progress in this field and introduced the soil MCP conceptual framework 2.0, which expands upon the existing MCP model by incorporating autotrophic microbial pathway for C sequestration and integrating the concept of soil mineral C pump. These advancements aimed to enrich and refine our understanding of microbial-mediated terrestrial ecosystem C cycling and sequestration mechanisms. This refined framework would provide theoretical support for achieving China's “dual carbon” goals.

Soil organic carbon (SOC) is the core component of terrestrial carbon (C) sink. Exploring the transformation and stabilization mechanism of SOC is key to understand the function of terrestrial C sink which copes with climate change. The traditional perspective is that plant residues are the initial source of SOC. The new concept of “soil microbial C pump” emphasizes that the synthesized products of soil microbial assimilation are important contributors to the stable SOC. This provides a new insight to the sequestration mechanism of SOC. Due to the complex and variable decomposition process of plant residues and the high heterogeneity of microbial residues, the transformation and stabilization mechanism of plant residues and microbial residues into SOC is still unclear. We reviewed research progress in plant and microbial residues, and introduced the characterization methods of quantification and transformation of plant residues and microbial residues, and also summarized the new findings on the transformation of plant and microbial residues into SOC. We further discussed the contribution and driving factors of microbial and plant-derived C to SOC. Finally, we prospected the future development direction and research focus in this field. This review would provide the scientific reference for the research of soil C sequestration in terrestrial ecosystem.

Microbial necromass carbon (MNC) is an important contributor to soil organic carbon (SOC). Soil carbon storage has increased significantly since the return of farmland to forestland (grassland) on the Loess Plateau. However, the contribution of MNC to SOC accumulation in different vegetation types and the influence factors remain unclear. Herein, we used the biomarker (amino sugar) technique to determine the MNC content and analyzed the influencing factors in 0-5 cm and 5-20 cm soil layers of natural grassland, shrubland (Caragana microphylla), and forestland (Quercus liaodongensis) in the Loess Plateau. The results showed that: 1) the soil pH decreased significantly from grassland to shrubland and then to forestland within the same soil layer. However, the SOC, total nitrogen (TN), microbial biomass carbon (MBC), and microbial biomass nitrogen (MBN) contents showed a reverse trend, with forestland displaying the highest values followed by shrubland and then grassland. The 0-5 cm had significantly higher values than the 5-20 cm depth. 2) The MNC contents varied 0.69-16.41 g·kg^-1 in the two soil horizons of the three vegetation types. There were significant increases in the contents of bacterial necromass carbon (BNC), fungal necromass carbon (FNC), and MNC in the 0-5 cm soil from grassland, shrubland to forestland. The contents of MBC were 1.9 times higher in forestland than in shrubland, and 3.2 times higher in shrubland than in grassland. In the 5-20 cm soil layer, the contents of FNC and MBC were significantly higher in the forestland than in the shrubland and grassland. The FNC content was significantly higher than that of the BNC, ranging from 1.16 to 9.83 times greater than the BNC. 3) The contribution of MNC to SOC was 0.6 and 0.7 times higher in shrubland and forestland than in grassland, respectively, with FNC accounting for 15.2%-42.7%, and BNC accounting for 1.4%-7.4%. 4) pH, TN, MBC, and MBN were important factors that influenced MNC accumulation. In summary, the variation in vegetation type altered soil nutrients, microbial activity, and soil pH, resulting in forestland and shrubland being more beneficial to the formation and accumulation of MNC, which was dominated by fungi, compared to grassland.

Wetlands store one third of global soil organic carbon (SOC) and are strongly affected by artificial drainage. The impact of drainage-induced water-table decline on carbon cycling in different wetlands, particularly microbial transformation processes, remains unclear. To address this knowledge gap, we collected soil samples from two typical wetlands of China (a nutrient-poor bog located in Dajiuhu and a nutrient-rich fen in Hongyuan) and conducted an incubation experiment with the addition of ¹³C-labeled glucose to analyze the effects of short- and long-term drainage on SOC decomposition, extracellular enzyme activity, microbial carbon use efficiency (CUE), and microbial carbon accumulation efficiency (CAE). The results showed that both short- and long-term drainage significantly increased SOC decomposition rates in both wetlands (from 1.47 μg C·g^-1·h^-1 in submerged soils to 2.47 μg C·g^-1·h^-1 in drained soils), microbial biomass carbon derived from glucose (from 0.21 mg C·g^-1 to 1.00 mg C·g^-1) and CAE (from 0.29 to 0.73), but did not alter CUE (ranging from 0.34 to 0.86). Long-term drainage increased α-glucosidase activity in the Dajiuhu wetland and decreased β-glucosidase and phenol oxidase activities in the Hongyuan wetland. In conclusion, drainage enhanced the ‘microbial carbon pump' and its efficiency in wetlands mainly via increasing microbial intracellular metabolism (including respiration), but also acce-lerated SOC decomposition.

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.

Clarifying the accumulation pattern of soil microbial residue carbon and its contribution to soil organic carbon (SOC) across stand age is helpful to understand the mechanism underlying soil carbon cycling. In this study, we analyzed the differences of amino sugar content, physicochemical properties and microbial composition in surface soil (0-10 cm) in young (6 a), middle-aged (13 a), near-mature (29 a), mature (38 a) and over-mature (57 a) Pinus massoniana plantations of subtropical China, quantified the microbial residue carbon content and its contribution to SOC, and discussed the mechanism. The results showed that SOC, total nitrogen, amorphous iron oxide and leucine aminopeptidase contents in the middle-aged plantation were significantly lower than those in the mature plantation. Soil pH and fungal/bacteria in young plantation were significantly higher than those in other age groups. Across the stand age gradient, the ranges of microbial, fungal and bacterial residue carbon were 7.52-14.63, 4.03-8.00 and 3.48-6.63 g·kg^-1, respectively. The contents of all the residue carbon were significantly higher in the mature plantation than that of the middle-aged plantation, which were positively affected by soil total nitrogen content. The contribution of microbial, fungal, and bacterial residue carbon to SOC was 59.7%-72.3%, 33.4%-45.6%, and 24.3%-30.8%, respectively. The contribution of fungal residue carbon to SOC in young plantation was significantly higher than that in other age groups, and the contribution of bacterial residue carbon to SOC in middle-aged plantation was significantly higher than that in young and near-mature plantations, both of which were affected by soil inorganic nitrogen. Fungal residue carbon content was 1.2-1.7 times as that of bacterial residue carbon content, and dominated for the accumulation of microbial residue carbon. Results of the partial least squares model showed that stand age, soil environmental factors (such as leucine aminopeptidase, amorphous iron oxide, pH, and total nitrogen), bacterial residue carbon, fungal residue carbon and the contribution of bacterial residue carbon to SOC had total effects on the contribution of fungal residue carbon to SOC (-0.37, -1.16, 0.90, 1.09, and 0.83, respectively). In conclusion, stand age promoted the accumulation of microbial residue carbon but did not increase its contribution to SOC.

Microbial necromass, an important and stable source of soil organic carbon (SOC), is an important index to evaluate the contribution of microorganisms to SOC transformation and accumulation. It is not clear about the accumulation of microbial necromass in deep soil layer and its contribution to SOC during the restoration process of Caragana korshinskii forests. Combined with the biomarker method, we investigated the carbon contents of bacte-rial, fungal, and microbial necromass in the soil profiles (0-100 cm) of C. korshinskii forests in 16, 28, and 38 years of restoration, with natural grassland as control. We further examined the contribution of microbial necromass to soil organic carbon. The results showed that: 1) Along the soil profile (0-100 cm), the contents of fungal necromass carbon (FNC), bacterial necromass carbon (BNC), and microbial necromass carbon (MNC) significantly decreased with increasing soil depth in natural grassland and C. korshinskii forests. Except for the significant decrease in FNC/SOC, BNC/SOC, and MNC/SOC in the soil of C. korshinskii forests in 38 years of restoration, FNC/SOC and MNC/SOC generally showed an increasing trend followed by a decreasing trend in other plots, while BNC/SOC gradually decreased. 2) With the increases of restoration years, the contents of FNC, BNC, and MNC significantly decreased in C. korshinskii forests. FNC/SOC and MNC/SOC showed an overall increasing trend followed by a decreasing trend, while BNC/SOC gradually decreased. 3) The average contribution of microbial necromass carbon to SOC was highest in C. korshinskii forests in 28 years of restoration (35.0%), followed by C. korshinskii forests in 16 years of restoration (33.5%), natural grassland (31.0%), and C. korshinskii forests in 38 years of restoration (28.6%). In conclusion, when the restoration years of C. korshinskii forests are 16, the contents of microbial necromass carbon and their contributions to SOC are higher compared to natural grassland, which are beneficial for SOC sequestration.

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.

Microbial necromass carbon (MNC) is a crucial source for stable soil carbon pool, and understanding its response to carbon inputs from both aboveground (litter) and belowground (roots) in subtropical forest soils is essential for assessing soil carbon stocks in global ecosystems. In a Castanopsis carlesii plantation at the Sanming Forest Ecosystem National Observation and Research Station in Fujian Province, we conducted an experiment with five treatments, including root removal (NR), aboveground litter removal (NL), no litter input (removals of both aboveground litter and roots, NI), double aboveground litter addition (DL), and control (CK). After seven years, we collected soil samples in the 0-10 cm soil layer to examine changes in MNC content and its contribution to soil organic carbon (SOC). Results showed that NR treatment reduced MNC, bacterial necromass carbon (BNC), and fungal necromass carbon (FNC) by 15.9%, 20.2%, and 14.5%, respectively, while other treatments did not induce significant changes. The NR, NL, NI, and DL treatments did not affect the contributions of BNC, FNC, and MNC to SOC. Correlation and path analyses revealed that litter and root carbon input treatments could alter the MNC content directly or indirectly through changing soil available substrates and microbial community structure. Our results suggested that roots exert a stronger influence on the maintenance of MNC than aboveground carbon source in the mid-subtropical plantations.

To investigate the contribution of microbial necromass carbon (MNC) to soil organic carbon (SOC) and its influencing factors under precipitation changes in grassland, we conducted a precipitation experiment with seven different precipitation levels in the Bothriochloa ischaemum restoration area in the loess hilly region. We analyzed the contents and characteristics of fungal necromass carbon (FNC), bacterial necromass carbon (BNC), and MNC in different fractions of SOC under different treatments, including natural precipitation (CK), and increased and decreased 20%, 40%, 60% of natural precipitation (I₂₀, I₄₀, I₆₀, D₂₀, D₄₀, D₆₀) . The results showed that 1) MNC content in mineral organic carbon (MAOC) ranged from 1.62 g·kg^-1 to 2.17 g·kg^-1, which was higher than that in particulate organic carbon (POC) ranging from 0.69 g·kg^-1 to 1.31 g·kg^-1. The former was approximately 1.4 to 2.8 times as that of the latter. 2) FNC and MNC exhibited similar changes in both MAOC and POC fractions. BNC content in MAOC was approximately 1-3.1 times as that of FNC. FNC content in POC was generally higher than BNC except for I₄₀ and I₆₀ where BNC exceeded FNC. 3) Overall, both increases and decreases in precipitation resulted in elevated MNC/MAOC and BNC/MAOC ratios, but decreased MNC/POC and FNC/POC ratios. The MNC/MAOC ratios in I₆₀ and D₆₀ were 33.2% and 18.1% higher than CK, respectively. The BNC/MAOC ratios in D₆₀, I₄₀ and I₆₀ were 28.0%, 23.0% and 19.1% higher than those in CK, respectively. Except for D₆₀, the FNC/POC and MNC/POC ratios were significantly lower than CK under other treatments. In terms of POC fractions, the MNC/POC ratios of D₄₀, D₂₀, I₂₀, I₄₀ and I₆₀ were 28.4%, 23.3%, 28.8%, 23.3% and 32.2% lower than that of CK, respectively. The FNC/POC ratio of D₄₀, D₂₀, I₂₀, I₄₀ and I₆₀ was found to be lower by 23.3%, 16.1%, 21.0%, 27.0% and 31.0% compared to that of CK, respectively. 4) NH₄⁺-N and pH were the primary factors influencing the content of MNC in different carbon fractions under varying precipitation conditions. In summary, alterations in precipitation (either increase or decrease) enhanced the contribution of BNC-dominated MNC to MAOC, but reduced the contribution of FNC-dominated MNC to POC. This study was of significance for understanding the distribution of microbial necromass across different organic carbon fractions under precipitation alterations.