Nitrogen deposition in the middle-aged and mature coniferous forest: Impacts on soil microbial community structure and function

The impact of nitrogen (N) deposition on soil microbial community structure and function has been a focal point of research; however, the influence of forest age and N form on the microbial response to N deposition has yet not to be fully understood. To address this gap, metagenomic sequencing was utilized to explore the responses of soil microbial community structure and functional genes to five years of N addition in middle-aged and mature coniferous forests in southwest China. Adopting a randomized block design, the experiment included two N forms ((NH4)(2)SO4 and KNO3) across four levels of addition (0, 10, 20 and 40 kg N ha(-1) yr(-1)). Our findings reveal that the composition of the microbial community structure and functional genes involved in the carbon (C), N, phosphorus (P), and sulphur (S) cycling varied significantly between middle-aged and mature forest (P < 0.001). At the phylum level, the soil microbial community in the mature forest were dominated by Proteobacteria (30.4-38.8 %), Acidobacteriota (8.5-24.1 %), and Chloroflexota (8.1-21.5 %), and the soil microbial community in the middle-aged forest showed a greater presence of Acidobacteriota (28.9-34.0 %) and Actinobacteriota (13.3-19.2 %) with a lower proportion of Chloroflexota (0.4-1.2 %). In both middle-aged and mature forests, functional gene abundance, and the composition of microbial community and functional genes were unaffected by N addition rate (P = 0.793) and N form (P = 0.725). The taxonomic composition of soil microbial community in the mature forest showed a significant positive correlation with soil pH, soil organic carbon (SOC) and total nitrogen (TN) (P < 0.01), while the microbial community composition in the middle-aged forests was not significant correlated with soil pH, SOC and TN (P > 0.05). These findings underscore that N addition in the middle-aged and mature coniferous forest does not significantly impact the soil microbial community structure and function across different N forms and N addition rate, suggesting that soil microorganisms of the forest ecosystem exhibit strong resilience to increased N supply.