Lithological Controls on Soil Aggregates and Minerals Regulate Microbial Carbon Use Efficiency and Necromass Stability

Microbial carbon (C) use efficiency (CUE) drives soil C formation, while physical-chemical protection stabilizes subsequent microbial necromass, both shaped by soil aggregates and minerals. Soils inherit many properties from the parent material, yet the influence of lithology and associated soil geochemistry on microbial CUE and necromass stabilization remains unknow. Here, we quantified microbial CUE in well-aggregated bulk soils and crushed aggregates, as well as microbial necromass in bulk soils and the mineral-associated organic matter fraction, originating from carbonate-containing (karst) and carbonate-free (clastic rock, nonkarst) parent materials along a broad climatic gradient. We found that aggregate crushing significantly increased microbial CUE in both karst and nonkarst soils. Additionally, compared to nonkarst soils, calcium-rich karst soils increased macroaggregate stability and decreased the ratio of oligotrophic to copiotrophic microbial taxa, leading to a reduction in microbial CUE. Moreover, microbial CUE was negatively associated with iron (hydr)oxides in karst soils, attributed to the greater abundance of iron (hydr)oxides and higher soil pH. Despite the negative effects of soil aggregation and minerals on microbial CUE, particularly in karst soils, these soils concurrently showed greater microbial necromass stability through organo-mineral associations compared to nonkarst soils. Consequently, (i) bedrock lithology mediates the effects of aggregates and minerals on microbial CUE and necromass stability; and (ii) balancing minerals' dual roles in diminishing microbial CUE and enhancing microbial necromass stability is vital for optimizing soil C preservation.