Effects of soil aggregates and minerals on microbial necromass carbon are regulated by parent materials

Microbial necromass carbon (MNC) is a key soil organic carbon (SOC) pool. Soil aggregates and minerals play pivotal roles in controlling soil carbon storage. Although soil minerals and aggregates are tightly linked to parent materials, it is uncertain whether the relative importance of aggregates and minerals in governing MNC varies depending on soil parent materials. In this study, we determined the MNC content in forest soils originating from two contrasting soil parent materials, namely karst (carbonate rocks) and non-karst soils (clastic rocks) across a climatic gradient in southwest China. The MNC contents (24.5 ± 0.8 mg g−1 in karst and 24.1 ± 1.8 mg g−1 in non-karst soils) and their respective contribution to SOC (43.7 ± 1.3 % in karst and 41.1 ± 1.4 % in non-karst soils) were similar between the two parent materials. In karst soils, MNC was influenced by aggregates rather than minerals, whereas the opposite was true in non-karst soils. A higher proportion of microaggregates directly increased the MNC content in karst soils, whereas greater mineral contents indirectly increased the MNC through their effects on microbial biomass carbon. The divergent effects of aggregates and minerals on MNC were predominantly attributed to the formation of secondary carbonate crystals in calcium-rich karst soils as well as to the differences in pH between karst and non-karst soils. Additionally, climate (mean annual temperature and precipitation) and vegetation properties (aboveground biomass or richness) indirectly affected the MNC through their effects on aggregates or minerals in karst and non-karst soils. Consequently, the influence of soil aggregates and minerals on microbially derived carbon is mediated by the parent material. Our findings emphasize the importance of considering lithology-dependent drivers in predicting soil carbon accumulation, particularly in relation to the stability of microbial-derived carbon.