The contribution of microbial necromass carbon to soil organic carbon in soil aggregates

Microbial necromass is known to be the main component of soil organic carbon (C) sequestration. However, the contribution of microbial necromass to soil aggregates has not been comprehensively studied because of the heterogeneity of microhabitats in different soil aggregates, especially in different ecosystems on a global scale. To address this gap, we conducted a global-scale meta-analysis to investigate the contribution of microbial necromass to carbon sequestration in aggregates and identify the climatic and soil factors that influence necromass carbon in aggregates. The database of fungal and bacterial necromass carbon was based on the glucosamine and muramic acid contents in microaggregates (< 250 μm, 226 samples), medium macroaggregates (250–2000 μm,132 samples), and macroaggregates (> 2000 μm, 66 samples). On average, microbial necromass carbon in microaggregates, medium macroaggregates, and macroaggregates contributed 48.5 %, 40.9 %, and 42.5 % of aggregate carbon sequestration, respectively. The contribution of fungal necromass carbon was significantly higher than that of bacteria (average 30.9 % vs. 13.1 %, respectively). Meanwhile, the ratio of fungal to bacterial necromass carbon increased with the increasing aggregate size, from 2.68 (< 250 μm) to 3.11 (250–2000 μm) to 3.14 (> 2000 μm) for micro, medium size, and macroaggregates, respectively. In addition, the screening method for aggregates and the type of land use significantly affected the contribution of microbial necromass to aggregate carbon sequestration. Specifically, grassland and dry sieving significantly increased the contribution of microbial necromass carbon to aggregate carbon. Finally, the results from PLS-PM showed that climatic factors regulated fungal and bacterial necromass carbon through soil pH, sand, and nutrients in aggregates, thereby enhancing the contribution of microbial necromass to aggregate carbon sequestration. In addition, fungal and bacterial necromass carbon improved with increasing carbon and nitrogen content of the aggregates. Furthermore, a lower soil pH seemingly stimulates the accumulation of fungal and bacterial necromass in soil aggregates. These findings emphasize that the contribution of microbial necromass carbon to aggregate carbon sequestration, especially fungal necromass carbon, is an important method for improving carbon sequestration in soil aggregates.