Promoting effect of tree mixture on litter quality and microbial diversity governs microbial necromass accrual in previously degraded soils

Abstract

Introducing broadleaf tree species into pine forests has been widely adopted as a strategy to mitigate forest degradation and enhance soil carbon sequestration. However, the microbial-mediated mechanisms underlying the effects of mixed coniferous-broadleaf forests on soil carbon sequestration in degraded forest ecosystems remain inadequately elucidated. This study investigated two previously degraded sites in southern China, presenting vegetation cover since 1981 and 2000, respectively. Within these sites, single Pine (Pinus massoniana) plantations and mixed plantations of pine and broadleaf species (Pine+Schima superba) were established as part of restoration efforts. Amino sugars were quantified as biomarkers of topsoil (0–10 cm) microbial necromass, and microbial community diversity and structure were assessed via high-throughput sequencing techniques (16S rRNA and ITS sequencing) and phospholipid fatty acids analysis. Our results showed that tree mixture increased soil microbial necromass, with the fungal necromass increasing more rapidly than bacterial necromass, suggesting that fungal necromass rapidly responds to tree species mixture effects, but bacterial necromass drives the sustained accumulation of microbial necromass C in the later restoration stages. Furthermore, mixed plantation soils yield higher bacterial α-diversity (Chao1), and ectomycorrhizal fungi abundance, all of which positively influence microbial necromass accumulation. Structural equation modelling results showed that tree mixture strengthens the integrated effects of litter quality and soil nitrogen status on microbial community composition (16S rRNA and ITS sequencing) and functions (FUNGuild) and thus enhancing microbial necromass. Importantly, this ecological restoration effect shifted microbial necromass regulation from dual independent pathways (physicochemical vs. microbial factors) to an integrated soil-microbial feedback loop, with mixed pine-broadleaf plantations enhancing the interaction between soil nutrient availability and microbial functional diversity to promote microbially-derived carbon accrual in soils. These findings demonstrate that introducing tree species with high-quality litter constitutes an effective vegetation restoration strategy, simultaneously accelerating soil organic matter sequestration in degraded pine forest ecosystems.