Long-term nitrogen addition changes soil microbial community and litter decomposition rate in a subtropical forest

Large amounts of nitrogen (N) are loaded in tropical and subtropical regions due to anthropogenic activities. However, the long-term effect of this deposition on soil biodiversity and its ecological functions remains poorly understood. In this study, we established a 10-year-long field experiment examining the effect of simulated N deposition on soil bacterial diversity and litter decomposition processes in a subtropical Chinese fir forest. The N levels were 0 (control), 6, 12, and 24 g m−2 yr−1. Soil bacterial diversity was measured by high throughput sequencing analysis, while litter decomposition was investigated by a microcosm experiment with a reciprocal soil and litter design. Our long-term experimental results demonstrated that N addition significantly changed soil pH, NH4+-N, available P and soil moisture; reduced soil bacterial diversity; and changed soil microbial community composition at the phyla and class levels. Meanwhile, the rate of litter decomposition decreased in treatments with high doses of N addition. The decreased soil pH and changes in understory plants were significantly correlated with the altered soil microbial diversity and community composition according to the structure equation model analysis. Our results showed that changes in soil microbial community composition (but not soil microbial diversity) mainly contributed to the decline in litter decomposition in higher doses of N addition. Our findings conclude that global change phenomena such as N deposition would depress both above- and below-ground biodiversity and would strongly affect ecosystem functions and processes in subtropical forests.