Gap size modulates soil-microbe-metabolite interactions across stand development stages in Cunninghamia lanceolata

With the advancement of molecular techniques, understanding soil–microbe interactions within forest gaps has become a frontier in forest ecology. In this study, Light Detection and Ranging (LiDAR) and Unmanned Aerial Vehicle (UAV) photogrammetry were combined to precisely quantify gap sizes in Cunninghamia lanceolata plantations of different developmental stages in subtropical China. We systematically examined how gap size affects soil physicochemical properties and microbial communities in middle-aged and mature stands, highlighting the pivotal role of stand development in mediating gap effects. In middle-aged stands, soil total carbon, nitrogen, potassium, and organic matter displayed a "V"-shaped response to increasing gap size, while in mature stands, these indicators increased linearly. Although microbial α-diversity remained stable, β-diversity and community structure were significantly shaped by both gap size and stand age (Analysis of similarities, ANOSIM, R = 0.425, p = 0.019). Linear discriminant analysis effect size (LEfSe) identified gap-specific microbial biomarkers (e.g., Limosilactobacillus reuteri in large gaps of mature stands; Bacteroides caecimuris in small gaps). Microbial networks in mature stands were more complex, with keystone taxa such as Prevotellamassilia potentially driving nutrient cycling. Functional prediction revealed enrichment in pyrimidine metabolism and ABC transporter pathways (e.g., ko00240, ko02010), while CAZy analysis emphasized glycoside hydrolases and glycosyltransferases in carbohydrate metabolism. Metabolomic profiling uncovered gap-sensitive metabolites (e.g., flavonoids, amino acid derivatives) and enriched pathways including flavonoid and lysine biosynthesis (p < 0.01). Correlation analysis linked Prevotella spp. positively with epicatechin/catechin and negatively with miltirone, indicating gap-driven "soil–microbe–metabolite" interactions through environmental filtering. These findings demonstrate that gap size regulates belowground ecological processes by modulating soil conditions, microbial networks, and metabolic functions, offering mechanistic insight into close-to-nature management of C. lanceolata plantations.