Tree mixing mitigates mulching-induced trade-offs in diversity and interactions of soil decomposers and ectomycorrhizal fungi in frequently burned conifer woodlands

Prescribed burning and mechanical mulching are widely used to regulate tree recruitment and maintain open forest structure, but their effects on soil microbial communities, particularly the responses of different microbial functional guilds across varying forest compositions, remain poorly understood. We conducted a factorial field experiment in eastern Texas, USA, across three frequently burned forest types (loblolly pine Pinus taeda -dominated, shortleaf pine P. echinata -dominated, and mixed loblolly-shortleaf pine forests) to assess how mulching influences diversity, composition, and assembly of soil microbial functional guilds. Mulching significantly reduced the alpha diversity of soil decomposers across forest types, while microbial community composition was more strongly shaped by forest type than by management. Network analyses further showed that mulching effects on fungal-bacterial interactions were context dependent: mulching reduced network complexity and robustness in mixed stands, but increased robustness and lowered vulnerability in single-species stands. Mulching also consistently decreased the relative abundance and phylogenetic diversity of ectomycorrhizal fungi (EMF) and weakened EMF associations with free-living microbes. Nevertheless, mixed-conifer forests supported more abundant and diverse EMF communities and fostered more complex free-living microbe-EMF interactions than single-species-dominated forests, regardless of mulching treatment, suggesting that tree mixing buffered mulching-associated impacts on EMF-centered interactions. Overall, our findings reveal context-dependent trade-offs associated with mulching in frequently burned woodlands and demonstrate that overstory composition mediates belowground microbial responses to fuel-reduction practices.