Implications of land-use legacy on soil microbiomes of urban grasslands

The expansion of urban areas in response to human population growth is occurring at a rapid pace. Urban areas encroach surrounding landscapes, creating a mosaic of patchy and dynamic development. Across the United States and in many parts of the world, turfgrass ecosystems are a primary groundcover in urban areas. When studied at larger scales, turf lawns coalesce into urban grasslands, a distinct vegetation type that can be studied, characterized, and compared to more well-studied ecosystems. To date, studies of urban grasslands have documented a general increase in soil carbon (C) and nitrogen (N) associated with managed turf, but fewer studies have focused on the microbial mediation of soil nutrient dynamics in these novel urban ecosystems. We sought to 1) search the literature for common turfgrass management practices that have been demonstrated to affect urban grassland soil microbiomes; 2) compare the microbiome composition, activity, and microbial biomass in a chronosequences of lawns established on either formerly agricultural- or forest-history soils to reference agricultural and forest history reference sites; and 3) elucidate the effect of litter quality on the lawn chronosequences to determine if there was an interaction with land-use history or the soil microbiome as might be suggested by current frameworks integrating litter quality, microbial physiology, and soil organic matter formation (e.g. MEMS and MIMICS). Cultural management practices, particularly mowing, are required to maintain quality turf and can alter nutrient cycling in the rhizosphere and at the soil surface depending on how clippings or fall leaf-litter is managed. We found significant shifts in soil microbiome composition when lawns were established on forest-history soils, but not agricultural-history soils. Compositional shifts were related to changing edaphic conditions, particularly pH, %N and %C as lawns aged. We found that short-term additions of litter did not alter the soil microbiome composition, but did affect soil microbial biomass and soil C and N. Relatively more-labile grass clippings stimulated soil C and N accumulation and lower-quality leaf litter increased soil microbial biomass. These observations agree with current soil organic matter models that explicitly consider microbial physiology. Overall, we found land-use history to be a highly significant, but relatively poor predictor of lawn soil microbiome composition. However, the nature of the patterns in lawns were influenced by the previous land-uses, and therefore may still be informative for understanding microbial composition and function at landscape scales. Soil physiochemical conditions, especially pH, were much stronger predictors of lawn microbiomes. Future land-use change studies would benefit from considerations of edaphic conditions rather than generalizations about vegetative cover. The continued study of urban grasslands and urban ecosystems is needed to advance the knowledge of these novel ecosystems and provide the basis for improving their sustainable management