Urban grasslands, landscapes dominated by turfgrasses for aesthetic or recreational groundcovers, are rapidly expanding in the United States and globally. These managed ecosystems are often less diverse than the natural or agricultural lands they replace, leading to potential losses in ecosystem functioning. Research in non-urban systems has provided evidence for increases in multiple ecosystem functions associated with greater plant diversity. To test if biodiversity-ecosystem function findings are applicable to urban grasslands, we examined the effect of plant species and genotypic diversity on three ecosystem functions, using grassland assemblages of increasing diversity that were grown within a controlled environment facility. We found positive effects of plant diversity on reduced nitrate leaching and plant productivity. Soil microbial diversity (Mean Shannon Diversity, H’) of bacteria and fungi were also enhanced in multi-species plantings, suggesting that moderate increments in plant diversity influence the composition of soil biota. The results from this study indicate that plant diversity impacts multiple functions that are important in urban ecosystems; therefore, further tests of urban grassland biodiversity should be examined in situ to determine the feasibility of manipulating plant diversity as an explicit landscape design and function trait.

Urban grasslands are turfgrass dominated landscapes of varying functions and uses that are ubiquitous in areas associated with human population growth and urbanization. While these landscapes are perceived to serve a primarily aesthetic function, they provide a multitude of beneficial ecosystem functions that impervious surfaces do not provide. Urban grassland soils have been shown to accumulate carbon (C) and nitrogen (N) for decades, matching native grasslands and eastern hardwood forest soils in terms of C and N densities. The establishment and maintenance of urban grasslands alters many microbially-mediated biogeochemical processes in soils, including soil organic matter (SOM) dynamics. Despite strong evidence of alterations to soil C and N cycling, the impacts of maintaining urban grasslands on soil microbiomes and their functions remain understudied compared to other ecosystems. Typical management practices can directly and indirectly affect edaphic factors in urban grasslands, which in turn, could impact soil processes mediated by microorganisms. We reviewed the existing literature on urban grassland management, focusing on how mowing, fertilization, irrigation, grass species composition, and soil cultivation could impact the composition and function of soil microorganisms. Although sparse, the literature indicates that the techniques used to maintain urban grassland habitats broadly select for copiotrophic microorganisms adapted to higher resource availability. Additionally, the studies indicate that greater soil fertility and plant productivity found in urban grasslands facilitate the accumulation of soil C and N, as well as SOM as compared to other land-use types. However, effects on soil biology and biogeochemistry depend on specific management practices, which are quite variable. Future research on soil C and N dynamics in urban grasslands should focus on the dominant component of this ecosystem-residential lawns-however, much of the existing scientific literature featuring turfgrass systems focus heavily on golf courses, athletic fields, and major tourist parks.

Agricultural over-fertilization may adversely impact plant−microbial interactions affecting crop yield. It is unclear if soil microbiomes respond quickly to changes in fertilizer inputs once conditioned to specific nutrient regimes. We conducted a growth chamber study assessing the compositional and functional resilience of root-associated microbiomes of Medicago sativa to nutrient regime changes, and consequences for plant growth. Plants were grown with a common starting soil microbiome under four nutrient treatments: control (no fertilizer), organic phosphorus (compost added), low inorganic P (low triple superphosphate, TSP) and high inorganic P (high TSP). After several conditioning generations, in which microbiomes from rhizospheres of high biomass plants were transferred forward, microbiome composition was distinct across the four treatments. The resulting microbiomes were then transplanted into each of the nutrient treatments, leading generally to functional changes in hydrolytic enzyme activity and taxonomic convergence with other microbiomes transplanted into the same nutrient regime. However, high inorganic P-conditioned microbiomes were resistant to compositional change. Correspondingly, M. sativa grown with high inorganic P-conditioned microbiomes had lower biomass, fewer nodules, and lower %N than plants grown under the same nutrient regime with other microbiomes. These findings suggest that excessive inorganic P fertilization may change microbiomes such that they negatively affect plant growth.

T urfgrass landscapes have expanded rapidly in the United States in recent decades and will continue to become a dominant vegetation cover in urbanizing ecosystems. Within a 15-yr period (1982-1997), urban land cover expanded in the United States by 50% (Fulton et al., 2001). Although turfgrasses comprise only a portion of developed landscapes, collectively, they are estimated to cover 1.9% of the total terrestrial land area of the United States (Milesi et al., 2005). In fact, turfgrass in the United States covers an area three times larger than any irrigated crop (Milesi et al., 2005). The continuing expansion of developed lands suggests that turf establishment is a consequence of urbanization but it also reveals the potential to develop and manage turfgrass to increase ecosystem services in urban environments. In 2010, the US Census Bureau found that over 80% of the nation lives in urban areas. Urbanization has increased by approximately 1.8% since 2000 (US Census Bureau, 2011). Remote sensing analysis of recently subdivided suburban parcels suggests between 25 and 90% of the landscape is pervious (Cappiella and Brown, 2001). Turfgrasses can be assumed to be the primary land cover of pervious landscapes within urban areas (Milesi et al., 2005). The dominance of turfgrass in developed landscapes is evidenced by a study conducted in an urbanized landscape in Ohio showing that 23% of the land area was covered with turfgrass lawns (Robbins and Birkenholtz, 2003). Similarly, an extensive study of the Chesapeake Bay watershed showed a 61% (3186 km 2) increase in urbanized land from 1990 through to

Summary Climate change‐related soil salinization increases plant stress and decreases productivity. Soil microorganisms are thought to reduce salt stress through multiple mechanisms, so diverse assemblages could improve plant growth under such conditions. Previous studies have shown that microbiome selection can promote desired plant phenotypes, but with high variability. We hypothesized that microbiome selection would be more consistent in saline soils by increasing potential benefits to the plants. In both salt‐amended and untreated soils, we transferred forward Brassica rapa root microbiomes (from high‐biomass or randomly selected pots) across six planting generations while assessing bacterial (16S rRNA) and fungal (ITS) composition in detail. Uniquely, we included an add‐back control (re‐adding initial frozen soil microbiome) as a within‐generation reference for microbiome and plant phenotype selection. We observed inconsistent effects of microbiome selection on plant biomass across generations, but microbial composition consistently diverged from the add‐back control. Although salt amendment strongly impacted microbial composition, it did not increase the predictability of microbiome effects on plant phenotype, but it did increase the rate at which microbiome selection plateaued. These data highlight a disconnect in the trajectories of microbiomes and plant phenotypes during microbiome selection, emphasizing the role of standard controls to explain microbiome selection outcomes.