Effects of Long-Term Nitrogen Application on Soil Fungal Diversity, Functional Groups and Assembly Processes in Tea Gardens

This study aimed to investigate the characteristics of soil fungal diversity, functional groups, and community assembly in tea garden soils subjected to varying long-term nitrogen applications. Additionally, it sought to determine the optimal nitrogen fertilizer application rate for tea gardens, thereby providing a scientific foundation for the sustainable and rational use of nitrogen in tea cultivation.

The field experiment (small cement pond) was set up in the base of the Tea Research Institute, Fujian Academy of Agricultural Sciences, (beginning in 2011), and four nitrogen levels were applied: N0 (0), N1 (112.5 kg N·hm^-2), N2 (225 kg N·hm^-2), N3 (450 kg N·hm^-2), and each treatment was repeated four times. ITS high-throughput sequencing was used to analyze the effects of long-term nitrogen applications on the soil fungal diversity, functional groups, and community assembly.

Compared with N0, long-term nitrogen increased the yield of spring and autumn tea by 137.79%-430.20% and 33.43%-67.49%, respectively, but there was no significant difference between N2 and N3 treatments. The soil fungal diversity tended to initially increase and then decrease with increasing nitrogen addition in two seasons. Compared with N0, N3 significantly increased the Ace, Chao1 and Shannon of fungal. Results of non-metric multidimensional scaling analysis (NMDS) and permutational multivariate analysis of variance (PERMANOVA) showed that long-term nitrogen application drastically changed the community structures of soil fungi in tea plantations. The functional prediction with FUNGuild showed that long-term nitrogen application significantly changed the functions of soil fungi during the spring period, but the changes were not significant during the autumn. Compared with the N0 treatment, N2 and N3 treatments decreased the relative abundance of plant pathogens and soil saprotrophs fungi (especially during the spring tea), and N2 treatment increased the relative abundances of two beneficial fungi, namely ectomycorrhizal fungi and arbuscular mycorrhizal fungi. Redundancy analysis (RDA) showed that the soil pH, nitrate nitrogen, ammonium nitrogen and available potassium were the main factors for determining fungal community structure and functional groups. Compared with N0 treatment, the application of nitrogen fertilizer increased the number of edges, average degree, average clustering coefficient, and network density of the fungal network, and so enhanced the stability of the fungal community and improved its resistance to disturbances. In the process of community succession, stochastic processes dominated the construction of the tea garden soil fungal community under long-term nitrogen application, and the deterministic processes were enhanced in the community assembly under the low and medium nitrogen treatments.

From the perspective of yield, community diversity, enrichment of enrichment of beneficial fungi, and network stability, the applying nitrogen fertilizer of 225 kg N·hm^-2 was a reasonable amount for tea plant.