Effect of Nitrogen Addition on Soil Microorganisms in Leymus secalinus Grassland in Northern Shanxi

This study aimed to investigate the effects of different nitrogen addition levels on soil microbial community structure, diversity and nitrogen cycling functional genes in Leymus secalinus grassland of northern Shanxi, which held the profound significance for understanding nutrient cycling in grassland ecosystems under the context of nitrogen deposition.

This experiment was conducted based on the multi-level nitrogen (N) addition platform established in 2017 at the Youyu Loess Plateau Grassland Ecosystem Research Station, Shanxi Province. Eight treatments were set up: N0, N1, N2, N4, N8, N16, N24, and N32, corresponding to nitrogen application rates of 0, 1, 2, 4, 8, 16, 24, and 32 g N·m^-2·a^-1, respectively. Utilizing metagenomic technology, soil microorganisms were studied under different levels of nitrogen addition.

(1) Nitrogen addition significantly altered the soil bacterial and fungal community structures. It increased the relative abundance of Acidobacteria and Thermodesulfovibrionales in the bacterial community while reducing that of Nitrospirales. In the fungal community, nitrogen addition increased the relative abundance of Ascomycota but decreased that of Mucoromycota and Basidiomycota. (2) Nitrogen addition significantly affected bacterial community structures richness, but had no significant effect on diversity, with no significant impact on the richness and diversity of fungal communities. (3) Microbial differential groups enriched with different levels of nitrogen addition were different. (4) Nitrogen addition significantly increased the relative abundances of nitrification genes amoABC and hao; the relative abundances of denitrification genes nirK and nirS significantly increased under the N4 treatment, while norBC relative abundances significantly increased under N1 and N24 treatments; the relative abundance of dissimilatory nitrate reduction genes nirBD and nrfAH increased significantly under the N16-N32 treatments, while the relative abundance of napAB and narGHI decreased significantly under the N1, N2, N4, N16 and N24 treatments; the relative abundances of nitrogen fixation genes nifDKH and vnfDKGH significantly increased under the N32 treatment (5) With the increase in nitrogen addition levels, soil water content (SWC) and pH decreased significantly, while total carbon (TC) and total nitrogen (TN) showed a trend of first increasing and then decreasing. Similarly, microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) also exhibited a trend of first increasing and then decreasing. These changes in soil factors significantly affected the composition of the bacterial community structure, among which SWC was the most significant factor. (6) The genes involved in the process of dissimilatory nitrate reduction to ammonium were significantly negatively correlated with SWC and pH. The genes involved in the process of assimilatory nitrate reduction to ammonium were significantly positively correlated with MBC and MBN, nitrogen fixation process genes were significantly positively correlated with SWC, nitrification process genes were significantly negatively correlated with soil moisture, and were significantly positively correlated with NO₃^--N.

In summary, nitrogen addition significantly affected the structure of soil microbial communities by influencing soil water content (SWC) and pH, which notably impacted the relative abundances of genes involved in nitrification, dissimilatory nitrate reduction to ammonium, nitrogen fixation, and denitrification processes, thereby altering the progression of soil nitrogen cycling. Soil microorganisms play a pivotal role in nutrient cycling. Further investigating the effects of nitrogen addition on soil microorganisms not only provided a theoretical basis for the rational management of Leymus secalinus grasslands in northern Shanxi but also held significant implications for the restoration of ecosystem functions in these grasslands.