[Effects of Long-term Nitrogen Application on Soil Bacterial Community Structure， Functional Groups， and Assembly...

Nitrogen fertilizer application is an essential aspect of tea plantation management to improve the yield and quality of tea， and the nitrogen fertilizer constitutes the major artificial factor affecting the soil bacterial community structure. The field experiment （small cement pond） was set up in the base of the Tea Research Institute of Fujian Province （beginning in 2011）， and four nitrogen levels were applied： N0 （0 kg·km^-2）， N1 （112.5 kg·hm^-2）， N2 （225 kg·hm^-2）， and N3 （450 kg·hm^-2， all measured by N）， and each treatment was repeated four times. We used 16S rRNA high-throughput sequencing to analyze the effects of long-term nitrogen applications on the soil bacterial diversity， functional groups， and community assembly. Further， the Pearson correlation analysis and redundancy analysis （RDA） were also used to examine the soil property factors that drive the community structure. The results showed that： ① The soil bacterial diversity followed a unimodal trend with increasing N concentration. Compared with that in the N0 treatment， the soil bacterial diversity index （Ace， Chao1， and Shannon） of N2 and N3 treatments significantly decreased by 6.4%-18.74%， 5.93%-19.72%， and 3.99%-6.61% in the spring sampling period， and the soil bacterial diversity index of N2 and N3 treatments significantly decreased by 10.94%-5.95%， 11.02%-25.30%， and 3.61%-8.62% in the autumn sampling period. ② The results of principal coordinates analysis （PCoA） and permutational multivariate analysis of variance （PERMANOVA） showed that long-term nitrogen application drastically changed the community structures of soil bacteria in tea plantations. Compared with N0， N2 and N3 significantly increased the relative abundance of Firmicutes in two seasons， but N3 significantly decreased the relative abundance of Acidobacteria and Myxococcota， and the most important factors affecting the differences in bacterial communities were ammonium nitrogen， acid phosphatase， and nitrate nitrogen. ③ The functional prediction with FAPROTAX showed that chemoheterotrophy （33.01%）， aerobic chemoheterotrophy （32.34%）， and cellulolysis （11.76%） were the main ecological functions in tea garden soils. Long-term nitrogen application significantly changed the ecological functions of bacteria in tea garden soils， and the relative abundances of several functional genes associated with N cycling were reduced by medium-high nitrogen treatments （N2 and N3）. Redundancy analysis （RDA） showed that the ammonium nitrogen， available potassium， and total nitrogen were the most important factors to explain the differences in bacterial ecological functions. ④ Network analysis revealed that the N1 treatment increased the number of edges， average degree， average clustering coefficient， and network density of bacterial networks， and the N2 and N3 treatments significantly decreased the number of edges， average degree， and network density of bacterial networks compared with those in the N0 treatment， indicating that the of soil bacterial networks were reduced by the middle and high nitrogen treatments. In the process of community succession， stochastic processes dominated the construction of the tea garden soil bacterial community under long-term nitrogen application， and the deterministic processes were enhanced in the community assembly under high nitrogen treatments. ⑤ Correlation analysis showed that Actinobacteria， Firmicutes， and WPS-2 were significantly positively correlated with tea yield and free amino acid and significantly negatively correlated with most of the catechin components， which may be one of the main microbial groups affecting the change in tea quality. Overall， long-term nitrogen application changed bacterial community structure and metabolic function of the tea garden soil， and the medium and low nitrogen treatments were beneficial for maintaining the bacterial diversity and structural stability of the tea garden soil.