Pyrochar versus hydrochar: contrasting modulation of soil bacterial communities under severe nitrate-dominated acid rain stress in a Quercus acutissima plantation.

Nitrate-dominated acid rain (NAR) has become an increasingly prevalent component of contemporary atmospheric deposition and can profoundly alter soil bacterial communities in forest ecosystems. Biochar amendment has been proposed as a potential strategy for modifying soil conditions under acid deposition stress. However, the material-dependent effects of pyrochar (PC) and hydrochar (HC) on bacterial community responses under severe NAR stress remain unclear. Here, a field experiment was conducted in a Quercus acutissima plantation to compare the effects of PC and HC on soil bacterial community structure and predicted functional potential under simulated severe NAR stress. Compared with the NAR-only treatment, PC and HC reduced the relative abundance of Chloroflexi by 41.20 and 38.06%, respectively, and increased Actinobacteriota by 43.30 and 38.52%. LEfSe analysis revealed Chloroflexi as a phylum showing differential enrichment under NAR stress, whereas Acidobacteriota and Proteobacteria were differentially enriched in PC-NAR and HC-NAR treatments, respectively. Predicted phenotypic and functional profiles further indicated different bacterial responses to PC and HC. Exploratory association analysis suggested that PC-associated responses were more closely related to microbial biomass-associated variables, whereas HC-associated responses showed stronger relationships with soil chemical properties and microbial biomass. These findings highlight the material-dependent roles of PC and HC as carbon-based amendments and provide field-based insights into selecting biochar types for managing bacterial community responses in NAR-affected plantation soils.