Environmental preferences of soil microbial attributes for long-term nitrogen and acid addition: From phylotype to community

The impact of human-induced nitrogen (N) enrichment on microbial diversity has been extensively studied, with two main hypotheses proposed: soil N availability and soil acidification. However, the specific roles of these two hypotheses and their environmental preferences on soil bacterial and fungal communities are not fully understood. By conducting two independent experiments (a 16-year N and a 6-year acid addition) in a temperate semi-arid grassland, we tested the responses of soil microbial attributes (e.g., richness and relative abundance) at various levels (community, phylum/class, and phylotype) to N and acid addition. At the community level, our results showed that both N and acid addition had a negative effect on the richness of the whole, dominant, and rare bacterial communities; N enrichment only decreased the richness of the dominant fungal community, while acid addition decreased the richness of the whole, dominant, and rare fungal communities. By categorizing the microbial attributes into nine environmental preferences based on their responses to N and acid addition, we found that most bacterial phyla were associated with low N availability and high pH preferences, while most fungal classes had other environmental preferences. Most dominant bacterial phylotypes were linked to low N availability and high pH preferences, while most dominant fungal phylotypes were associated with other environmental preferences and high pH preferences. Conversely, most rare bacterial and fungal phylotypes were linked to other environmental preferences. Our experiments revealed that the decline in bacterial richness caused by N enrichment was predominantly due to their sensitivity to soil acidification, while fungal richness remained largely unaltered. By pinpointing distinct microbial attributes at different levels in response to N and acid addition, our findings could potentially forecast how soil microorganisms will react to future global N deposition.