Carbon and nitrogen fractions are more important than bacterial composition for carbon and nitrogen mineralization considering parent material and fertilization

Microorganisms and organic carbon (C) pool composition are vital in regulating soil organic matter mineralization. However, our understanding of how parent materials and fertilizers affect this process is still limited. In this two-year study, the changes of cumulative soil organic C and nitrogen (N) mineralization and C and N pools (total, labile, and recalcitrant), C- and N-cycling enzymes, bacterial composition, and C- and N-cycling gene and their interactions were analyzed for soils developed on loess and river alluvium. These soils were under rice-rape seed rotation, and no fertilization (Control), NPK (NPK), and NPK plus organic manure (NPKM) were applied; treatments were arranged into a completely randomized design. Soil organic C mineralization was influenced by soil parent material and fertilization, whereas no interactive effects were found for soil N mineralization. The river alluvium promoted 2.24 times higher C and 1.27 times higher N mineralization than loess; fertilization intensified C and N mineralization in 1.70 and 1.22 times only in loess. Soil C and N fractions were influenced by the parent material, with 1.39 and 1.10 times higher effects found in soils formed on river alluvium. NPKM increased the content of total organic C in 1.31 times, labile C in 1.24 times, recalcitrant C in 1.34 times, total N in 1.21 times, and recalcitrant N in 1.17 times in loess soil. Similarly, in river alluvium soil, the increases were 1.41、1.21、1.53、1.23 and 1.28 times. Fertilization with NPKM increased C- and N-cycling enzyme activities in both soils. Parent material and fertilization shifted bacterial composition by changing pH, subsequently influencing N-cycling genes. However, only parent material affected C-cycling genes. Thus, C and N fractions play a direct role in the mineralization process of soil organic C, while bacteria indirectly influenced the C mineralization processes by promoting the enzyme activities.