Effects of long-term nitrogen addition on the metabolic diversity of microbial carbon sources in subtropical forest soils

Soil microbial functional diversity plays an important role in maintaining the function and stability of the ecosystem. The rapid increase in global nitrogen deposition caused by human activity will cause changes in the structure and function of the soil microbial community. However, we know very little about how elevated N inputs over the long-term will influence microbial carbon source utilization diversity in tropical forests. We investigated the soil microbial community diversity of carbon metabolism and their driving factors based on a 15-year long field experiment monitoring different N levels at the Dinghushan biosphere reserve station using Biolog-Eco microplate technology. The results showed that:(1) compared with the control, carbon metabolic activity(AWCD) significantly decreased by 15.3%, 32.9% and 38.0% under low, medium, and high N treatments, respectively. In addition, the Shannon diversity and richness indexes significantly decreased after N application;(2) The utilization of carbohydrate, carboxylic acids, amino acids, amines, and phenolic acids significantly declined with increasing applications of N. Of these, amines showed the strongest response, and the intensity of their utilization reduced significantly by 80.2% after long-term high N addition.(3) The principal component analysis showed that there were significant differences in the microbial utilization of carbon at different N levels(P=0.001);(4) The variation portioning analysis showed that soil and plant factors explained 90.7% of the variations in microbial carbon source metabolism.(5) The canonical correspondence analysis(CCA) showed that soil pH(P =0.009) was the most important environmental factor influencing variations in the utilization of microbial carbon sources in different N treatments. However, plant richness and litterfall had no significant effect on utilization. Thus, our study suggests that N addition over the long-term significantly alters the diversity of soil microbial carbon source utilization in subtropical forests with soil pH as the main influencing factor. The results of this study provides an important theoretical basis for understanding belowground diversity of subtropical forests under global climatic changes.