A legacy of tillage can increase soil uniformity in former agricultural sites. Within plant communities, niche-based species sorting may occur among distinct soil patches (microsites), increasing coexistence and diversity, and the interfaces between microsites (microedges) may also provide unique microsites for increased plant diversity. Therefore, soil homogenization may limit plant diversity in former cropland being restored to grassland. We conducted a field experiment to address the question of whether heterogeneous soils and microedges have increased diversity compared to homogeneous soils. To examine substrate homogenization, we compared plots with added sand or woodchip patches to plots where sand or woodchips were added and mixed, and to examine microtopographic homogenization, we compared plots with pits and mounds to level, tilled soil. We arranged the treatment plots within 28 randomized blocks in a former agricultural field, and seeded each plot equally with a mixture of prairie species. Over three growing seasons we assessed treatment effects on percent canopy cover and density at the species level.
Results/Conclusions
A combination of null, negative and positive effects of soil heterogeneity on species diversity occurred. Homogenization significantly increased diversity in the woodchip treatment and decreased diversity in flat topsoil plots relative to topographically heterogeneous plots. In both cases the least productive treatments had the greatest plant diversity. The observed relationships varied over time and were sensitive to drought and the spatial scale over which diversity was quantified. Sand and topographic soil homogenization decreased diversity, but during the second year, when a drought occurred, the effect was suppressed in the sand treatment and magnified in the topography treatment for plots with pits. Several plant species appeared to have an affinity toward microedges, which suggests microedges may act as unique microsites and small scale ecological transition zones. Overall, the different soil treatments led to alternate successional trajectories and increased the variability in plant communities across the field. Adding contrasting soil patches at the 0.5 m spatial scale may therefore facilitate increased diversity and establishment of specific plant species in grassland restoration.