Increasing soil configurational heterogeneity promotes plant community evenness through equalizing differences in competitive ability.

Compared to homogeneous soils, soil heterogeneity is thought to promote plant species diversity through niche differentiation. The number of patch types within the heterogeneous soil (i.e. the difference in soil configurational heterogeneity) may also play a key role in regulating plant diversity. However, most empirical studies examining heterogeneity-diversity relationships involved only two contrasting types of patches. Moreover, the shape of heterogeneity-diversity relationships may also be changed by background soil fertility. To test how soil heterogeneity and number of patch types within the heterogeneous soil influence plant community evenness and their potential dependence on background soil fertility, we constructed plant communities consisting of four plant species in low- and high-nutrient soils, and manipulated the soils in heterogeneous configurations consisting of two or four types of soil patches and in a homogeneous condition where these soil patches were homogenized. Neither evenness of the plant community nor the difference in competitive ability between plants within the community was significantly different between the homogeneous soil and the heterogeneous soils, suggesting that soil heterogeneity overall had no effect on community evenness. However, evenness was higher and the difference in competitive ability between plants was lower in the heterogeneous soils with four types of soil patches than in the heterogeneous soils with two types of soil patches and also in the low-nutrient soils than in the high-nutrient soils. These results suggest that lowering soil fertility and increasing soil configurational heterogeneity can promote plant community evenness through reducing the difference in competitive ability between plant species within the community.