Protecting and increasing soil carbon (C) storage is crucial for food security and climate change mitigation. While plant inputs were initially considered the primary control, a shift in understanding recognizes that microbial biomass is a critical precursor to soil C stabilization. This paradigm change highlights a knowledge gap concerning the fate of microbial necromass (microbial residues) after microbial death. Microbial necromass can be recycled as a substrate for further growth or stabilized through adsorption to mineral surfaces and protection within soil aggregates. Given that necromass constitutes 50-80% of stable soil organic C, understanding its processing is key to soil C storage. This study proposes that the efficiency of necromass recycling is a critical, previously unassessed determinant of soil C storage. Quantifying necromass recycling efficiency will provide a more complete picture of a soil's capacity to stabilize C. Land use intensity (LUI) is a significant global change driver. Higher soil C losses are observed under intensive management in temperate grasslands. High LUI may promote low microbial CUE (carbon use efficiency), or conversely, high microbial CUE may offset C losses. Soil microbial communities and activities are strongly shaped by climate and LUI. This study hypothesized that high LUI selects for rapid-growth microbes, leading to higher CUE, whereas low LUI sites would contain slower-growing microbes capable of recycling complex substrates like necromass. This study aimed to (1) quantify microbial necromass recycling efficiency in response to LUI, (2) characterize environmental and microbial controls on necromass recycling efficiency, and (3) identify microbes associated with high necromass recycling efficiency.

The literature review extensively cites previous research establishing microbial necromass as a major component of stable soil organic matter. Studies emphasize the importance of microbial processes in soil carbon stabilization, including the microbial efficiency-matrix stabilization (MEMS) framework which links plant litter decomposition with soil organic matter stabilization. Existing research also highlights the influence of land use intensity on soil carbon cycling and microbial communities, with intensive management often associated with increased soil carbon loss. The literature reviewed also touches upon the concept of microbial carbon use efficiency (CUE) and its role in soil carbon dynamics.

The study used 27 sites across nine UK agricultural grassland farms, selected to span gradients in precipitation, temperature, soil pH, and soil C. Three levels of LUI were identified at each farm, resulting in a total of 27 sites. LUI scores were calculated from mowing, tillage, nitrogen addition, and grazing averaged over the previous 10 years. Microbial CUE was assessed in laboratory incubations using two substrates: 13C-glucose (a proxy for root exudates) and 13C-labeled *Escherichia coli* necromass. Stable isotope probing of microbial phospholipids and 16S and ITS sequencing were used to identify microbial communities involved in glucose and necromass uptake. Soil properties (pH, bulk density, total C) were measured. Statistical analyses included ANCOVA, Pearson correlations, variance inflation factor analysis, stepwise linear regression, variance partitioning, redundancy analysis, cluster analysis, and indicator species analysis to determine relationships between LUI, environmental factors, microbial communities, and CUE.

The study found that soil microbial necromass recycling efficiency was not significantly affected by historical LUI. However, it showed a negative correlation with historical precipitation (R²adj = 0.53). Microbial CUE on both glucose and necromass increased with microbial growth rate on the respective substrate. Fifteen bacterial and fungal indicator taxa were identified as having strong predictive power for necromass recycling efficiency. Microbial growth rates were approximately ten times higher on glucose than necromass, and the glucose:necromass growth rate ratio was much higher in high MAP (mean annual precipitation) soils. High MAP environments constrained both necromass-CUE and microbial growth rate on necromass. Soils with low biomass and high growth constraints (high MAP) showed less efficient necromass utilization. Stable isotope probing revealed substrate-specific microbial substrate use, with necromass-C incorporation showing a pairing between high MAP sites and branched chain methyl fatty acids typically associated with actinobacteria. Indicator species analysis identified nine bacterial and six fungal indicator taxa strongly associated with high necromass-CUE. Actinobacteria and Sphingomonadales showed the strongest correlation with high necromass-CUE and low MAP.

The findings indicate that microbial necromass recycling efficiency is influenced primarily by environmental factors, especially historical precipitation, rather than LUI. High microbial growth on necromass and high necromass-CUE are found in low-precipitation environments. The identified indicator taxa provide valuable insights into the microbial communities driving necromass recycling. The lack of LUI effect on CUE suggests that individual management interventions may have interacting effects, potentially masking overall LUI influence. Future SOM decomposition models should incorporate necromass as a climate-sensitive microbial substrate to improve predictions of soil C sequestration.

This study demonstrates the importance of microbial necromass recycling in soil carbon stabilization and highlights the strong influence of environmental factors, particularly precipitation, and specific microbial communities on this process. The identification of indicator taxa offers potential for monitoring and predicting soil carbon sequestration. Future research should investigate other stages of the necromass stabilization process, including biomass turnover to necromass production, necromass allocation, and necromass stability.

The use of lab-grown *E. coli* necromass as a proxy for diverse soil microbial necromass may limit the generalizability of some findings. Sample handling prior to incubation might have reduced the sensitivity of microbial functional responses to historical LUI. The composite nature of the LUI index may mask the individual effects of specific land management practices. The study focuses on UK grasslands and findings may not directly translate to other ecosystems.