Combined effects of species diversity and soil depth heterogeneity on plant functional groups and community productivity

Fine-scale heterogeneity in soil nutrient availability can enhance plant growth, yet its effect across soil depth remains inadequately understood. Communities with increased species diversity showed varying growth strategies among different plant functional groups to occupy soil niches. However, the interactive effects of soil depth heterogeneity and species diversity on plant functional groups and community productivity remain poorly known. This study aimed to examine whether plant functional groups respond differently to soil depth heterogeneity, whether such heterogeneity modifies interspecific relationships and community productivity, and whether higher species diversity alters plant growth strategies under vertical resource variation. A greenhouse experiment was conducted using twelve common herbaceous species with diverse functional traits and root strategies grown under one homogeneous and two heterogeneous treatments that varied in patch size, crossed with two species diversity levels. Plant species differed in responses to soil depth heterogeneity. Increased soil depth promoted the growth of legumes and forbs and enhanced community productivity. In high-diversity mixtures, forbs accumulated greater aboveground biomass in deeper heterogeneous soils, whereas grasses showed no significant response to soil depth or diversity. Legumes displayed a flexible growth pattern, reducing allocation to deeper layers, which may limit competitive pressure. Higher species diversity strengthened the selection effect in forbs, increasing their proportional contribution and intensifying interspecific competition under soil depth heterogeneity. This study provides experimental evidence for soil–plant interactions driving species coexistence and community productivity. Decreased-scale heterogeneity may also function as a reduced-scale form of habitat fragmentation. Long-term field experiments and multi-dimensional niche analyses are needed to better understand community dynamics across gradients of biotic and abiotic factors.