Response of Fungal and Bacterial Communities in Fine Root Litter with Different Diameters to Nitrogen Application

The microbiota formed by the combination of root litter and microorganisms exhibits various biogeochemical processes, including nutrient release and gas fluxes. However, the mechanism by which nitrogen (N) deposition changes the microbial community of root litter with different diameters remains unclear. We used high-throughput sequencing to compare the microbial community of root litter of Pinus tabulaeformis treated with N application (0–9 g N m−2 y−1) and with three diameter classes as follows: D1, < 0.5 mm; D2, 0.5 − 1.0 mm; D3, 1.0 − 2.0 mm. (1) The Chao and Shannon indexes of the fungal community significantly increased in D1 and decreased in roots of other sizes after N application (P < 0.05). No significant difference was found in bacterial characteristics among N treatments and root diameters. (2) The relative abundance of Ascomycota increased then decreased with N applications, whereas that of Basidiomycota showed the opposite trend. (3) On the genus level, the relative abundance of three fungal taxonomic populations and six bacterial taxonomic populations had parabolic response to N application and reached the maximum values at 3 or 6 g N m−2 y−1. The relative abundances of Penicillium and Candidatus increased, while Paraphoma and Rhodoplanes decreased with root diameters. (4) Carbon, lignin, and cellulose contents had driving effect on Firmicutes, Chloroflexi, and Geomyces abundances, while having little correlation with fungal populations. Functional populations Verrucomicrobia, Geomyces, and Proteobacteria had negative driving effects on enzyme activities of galactosidase, cellobiase, and alkaline phosphatase. These results occurred due to the nutrient quality of substances and bring the changes in enzymatic hydrolysis processes of root litter. N application had varied effects on richness of the fungal community of root litter with different diameters, and bacterial community has a stable structure and plays an important role in macromolecular decomposition.