Soil Homogenization Modifies Productivity, Nitrogen Retention and Decomposition in Restored Grassland

At the local (within site) scale, soil heterogeneity can influence ecosystem function. However, in former agricultural systems, soil heterogeneity can be low as a legacy of tillage (that is, via the process of soil homogenization). We investigated the relationship between soil homogenization and three ecosystem functional response variables—aboveground productivity, nitrogen retention (assessed via 15N tracer addition) and plant litter decomposition (assessed using litter bags)—during the first 2 years following a tallgrass prairie restoration in a former agricultural field. We compared plots with substrate heterogeneity (topsoil patches enriched with sand or woodchips) with homogeneous plots that contained the same overall ratios of the component materials. We also compared plots with topographic heterogeneity (pits and mounds) with flat plots. The ecosystem response variables varied significantly with soil homogenization, but the effect depended on the type of heterogeneity (sand vs. woodchips vs. microtopography), which was consistent with these sources of heterogeneity having variable effects on soil water and nutrient availability. Soil homogenization reduced aboveground productivity by 50% for the woodchip treatment, but it had the opposite effect for the topography treatment. Homogenization also reduced plant 15N retention for the woodchip treatment by 50%, but the rate of litter decomposition increased by 8%. Overall, variation in nitrogen retention and decomposition were associated with the treatment effects on aboveground productivity and the relative abundances of forbs and grasses. The latter results suggest that the influence of soil homogenization on ecosystem function may occur indirectly as a result of its effects on plant community assembly. Moreover, forb productivity and N retention along woodchip and mound edges differed from the means of the adjacent patches, which revealed that complementarity along microsite edges can contribute to soil heterogeneity effects on ecosystem function.