Seven-year N and P inputs regulate soil microbial communities via bottom-up effects on carbon and nutrient supply and top-down effects on protist relative abundance

Information about the interactions between microbial carbon (C), nitrogen (N), and phosphorus (P) mineralization and nutrient inputs will improve our understanding of the effects of global change on soil biogeochemical cycling and ecosystem function. In this study, we explored how N, P, and NP inputs, when applied at low rates (50 kg ha-1 year-1) for seven years, affected the microbial community in an acidic, strongly weathered, red clay soil in a subtropical conifer plantation. N and P inputs regulate microbial communities holistically through influencing soil resources (bottom-up), such as the C and nutrient supply, and predators, such as protists (top-down). We observed that N, P, and NP inputs caused changes in the protist community, which then affected bacterial diversity rather than fungal diversity. The increases in the protist diversity that resulted from the P and NP inputs were associated with increases in the relative abundance of Calditrichaeota, which is involved in C-mineralization, and decreases in the relative abundance of Marinimicrobia, which is involved in N-mineralization. The N and NP inputs caused the soil to become more acidic, exacerbated the microbial C limitation, and reduced the overall microbial functional diversity to carry out C, N, and P mineralisation. We conclude that the nutrient supply directly impacted the C-, N-, and P-mineralising microbial communities (bottom-up), and directly and indirectly affected soil protists (top-down).