Consistent effects of nitrogen addition on soil microbial communities across three successional stages in tropical forest ecosystems

Global nitrogen (N) deposition has been having broad impacts on soil microorganisms. It remains unclear how soil microbial biomass and community structure respond to elevated N deposition during forest succession, especially in tropical forest ecosystems. In this study, we conducted a field manipulative experiment of long-term N addition (14 years) at three levels (CK, LN, MN, which stand for 0, 50 kg N ha−1 yr−1 and 100 kg N ha−1 yr−1) along a 3-step succession sequence of tropical forests (early stage: a pine conifer forest, CF; middle stage: a mixed pine conifer and broad-leaved forest, MF; and late stage: a monsoon evergreen broad-leaved forest, BF), and investigated the responses of microbial composition, biomass and community structure using fumigation-extraction and phospholipid fatty acid (PLFA) methods. We found that: 1) Microbial biomass (microbial biomass carbon and nitrogen and PLFA contents) remained stable under long-term N addition, even though N addition significantly increased soil N nutrients (P < 0.05); it increased significantly with forest succession (P < 0.05), and was dominated by soil water content, which was not affected by N treatment; 2) microbial community structure was significantly changed by N addition, but not by forest succession. N addition, especially MN, significantly increased the ratio of cyclopropyl to its precursor (cyc/pre) in the MF and BF (P < 0.05), and cyc/pre of the BF showed the highest correlations with ammonium-N, nitrate-N and electrical conductivity (>0.85, P < 0.05). Consistently, the pure N treatment effect could explain 71.13% of the variance of cyc/pre; and 3) N treatment contributed more variance to the fungal community than to the bacterial community. Our results indicated that soil microbial community structure was more sensitive to N addition than community biomass under long-term N addition, and the structure would be more severely altered under N addition during forest succession or with the increase in N input.