Soil microbial necromass: The state-of-the-art, knowledge gaps, and future perspectives

Soil physical and biogeochemical processes are key to regulate the soil microbial necromass (SMN) patterns. Increasing evidence showed that the SMN contributes to the formation of soil organic matters (SOM), and subsequently SOM stabilization and soil nutrient cycling. This study aims to review the state-of-the-art research on SMN and associated impacts on soil ecosystem functions and services, and also to provide future research perspectives on linking SMN with SOM formation, transformation, and stabilization patterns. We conducted a comprehensive bibliometric analysis that targeted 1118 relevant articles from 2001 to 2020 based on the Web of Science database towards deciphering the state-of-the-art research on SMN and the historical development trends. We found that the number of SMN relevant publications has been growing rapidly since 2001, especially during the past three years (from 2017 to 2020). Taking the soil amino sugars (AS) as the biomarker of SMN, enables quantifying the potential linkage of SMN to the origins of SOM. Analysis results revealed that the dynamics of SMN that originated from the variety of microbial biomass effectively contribute to the SOM accumulation and stabilization and thereby, carbon (C) and nitrogen (N) cycling. In addition, the major factors regulating the SMN formation and accumulation, and subsequently nutrient cycling likely include soil physicochemical properties, changing climate conditions (e.g., global warming, N deposition, aridity, and elevated CO2 concentration), soil biota, and agricultural management practices. Nevertheless, deeper analysis employing mechanistic modeling, in situ spectromicroscopic, and molecular biology techniques is urgently needed for systematic quantifications of the SMN dynamics and SOM turnover towards an advanced understanding of soil nutrient cycling, soil C stock and neutrality, soil health and productivity, and links to sustainable agriculture development and ecosystem services.