Background: Fine-scale heterogeneity in soil nutrient availability can enhance plant growth, but its effect across soil depth remains inadequately understood. Previous studies on soil depth gradients suggest that deeper soils are inherently more productive in herbaceous communities, a pattern closely linked to increased species diversity. Nevertheless, little is known about the interactive effects of soil depth heterogeneity and species diversity on plant community productivity.
Methods: We conducted a greenhouse experiment in which plant communities were grown under three soil treatments (homogeneous, large-patch heterogeneous, and small-patch heterogeneous) and two species diversity treatments (low and high). The community comprised twelve common herbaceous species with varying functional traits and root strategies.
Results: Different plant species exhibited varied responses to soil depth heterogeneity. Increased soil depth significantly promoted the growth of legumes and forbs, and enhanced overall community productivity. Elevated species diversity significantly affected community productivity and altered the responses of legumes and forbs to soil depth heterogeneity. Forbs grew more in large-patch heterogeneous and deeper soils within high-diversity communities, while legumes grew more in these soils within low-diversity communities. In high-diversity settings, legumes tended to avoid deep soil layers to alleviate competitive pressure from forbs. Thus, increased species diversity may trigger selection effects within the community, leading to divergent plant growth strategies and modifying the influence of soil depth heterogeneity on plant growth. However, small-patch heterogeneity at reduced scales may represent another form of habitat fragmentation in natural plant communities.
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