PNNL

Abstract

The details of how soil microorganisms contribute to the stable soil organic carbon pool are a pressing knowledge gap with direct implications for soil health and climate mitigation. Microbial necromass has emerged as a dominant pathway to form stable soil carbon, however the quantification of necromass in soils has largely been limited to aminosugar biomarkers. The abundance and chemical composition of other persistent microbial residues remain unresolved, particularly in respect to how these pools may respond to soil characteristics and management practices. Here we use yearlong soil incubation experiments with an isotopic tracer to investigate the relative importance of plants and soil in controlling the molecular composition and persistence of microbial residues. Under our incubation conditions, soils had a stronger impact on microbial carbon accumulation than plant selection. We demonstrate that microbial residues are composed of diverse pools including metabolites, proteins, lipids, and mineral-associated organic carbon. Our results suggest that the management of soil metabolism may be an important means of accumulating soil carbon, especially in silt and clay-rich soils. We provide new chemical characterization of the abundant and persistent accrual of microbial residues in organic solvent extractable organo-mineral phases. The abundance of persistent microbial residues was correlated with microbial biomass abundance. Our work reveals a possible role of cell wall morphology in enhancing microbial carbon persistence; while gram-negative bacteria accounted for the greatest contribution to microbially-derived carbon, residues from gram-positive Actinobacteria and Firmicutes showed greater durability. Together these results offer a quantitative assessment of the relative importance of diverse molecular classes for generating durable soil carbon.