Interactive effects of soil pH and substrate quality on microbial utilization

Both soil pH and substrate quality exert a critical influence on the decomposition of organic substrates and the production of microbial biomass. However, it is elusive the interactive effects of soil pH and substrate quality on the microbial assembly, and subsequently the dynamics of organic substrates. We applied two substrates (maize flour and maize straw) with different substrate qualities to soils of different pHs (from 4.63 to 6.32), sampled from a field trial previously unamended, or amended with wheat biochar (10 t ha(-1)) or organic fertilizer (10 t ha(-1) composted pig manure). The natural abundance C-13 technique was used to determine the net mineralization rate of added C4 maize materials and the assimilation of substrates into microbial biomass C (MBC) during 60 days incubation. At the early stage, soil pH and substrate quality both positively regulated substrate mineralization, while the level of soil C saturation dominated the biomass synthesis efficiency (BSE) compared with soil pH and substrate quality. Soil pH had contradictory relationships with mineralization and BSE at the final stage of mineralization, implying high pH was tightly associated with higher C use efficiency (CUE). Overall, the bacterial community assembly was determined by the interactive effect of soil pH and substrate quality, while soil pH was the key driver to shape fungal community structure, diversity, and composition. Soil pH had a consistent effect on microbial utilization, which, however, were only affected by substrate quality in the early stage.