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The dynamic process of forest succession is important for terrestrial ecosystems where soil organic matter (SOM) primarily controls biogeochemical cycles and is driven by soil microorganisms. Carbon (C) incorporation into SOM occurs along two different microbial metabolic pathways: ex vivo mostly extracellular enzymatic modification and in vivo turnover within the cells, highlighting the importance of microbial catabolism and anabolism respectively during soil C transformation and sequestration. However, identifying the dual C pathways and understanding the microbially-mediated SOM transformation during forest succession remains elusive, particularly with respect to interactions in the rhizosphere. Here, we used a series of biomarkers, including amino sugars, phospholipid fatty acids, and neutral sugars, to explore how forest succession and rhizosphere dynamics affect microbial contribution to soil organic carbon (SOC) in three successional stages of a subtropical forest ecosystem. We observed generally consistent increases in amino sugar and SOC with forest age in both bulk and rhizosphere soil, demonstrating that microorganisms facilitate SOC accumulation through the in vivo pathway along the successional gradient. The ratio of amino sugar-C to SOC (AS-C/SOC) exhibited a greater increase in bulk than in rhizosphere soil along the successional gradient, indicating the microbial in vivo pathway was relatively more important for SOC accumulation in bulk than in rhizosphere soil. Given there was a significant decrease in the ratios of hexose-to-pentose sugars [(galactose + mannose)/(arabinose + xylose), indicating the contribution of sugars derived from microbes and plants to labile soil C], we hold the opinion that relatively limited microbial C processing occurred in the rhizosphere along the forest successional gradient. Therefore, the microbial ex vivo pathway where microorganisms decomposed (structurally modified) fine root litter that was incorporated into SOC was relatively more important in rhizosphere than in bulk soil for SOC variation during forest succession. Our findings emphasize the role of microbes transforming organic matter in both bulk and rhizosphere soil in a subtropical forest successional gradient and help to enhance our understanding of the function and ecosystem services of these environments.

Soil invertebrates, especially larger macroinvertebrates, move soil, fragment organic matter and change resource accessibility for soil microorganisms. Macroinvertebrates also affect the formation and turnover of aggregates, which are important controls of soil organic matter dynamics because they physically protect organic matter from degradation and influence many belowground processes ranging from microbial activity to nutrient sorption and water flow. We still lack a complete understanding of how different soil invertebrate functional groups affect the incorporation of root derived carbon and nitrogen into belowground pools. We assessed the effects of macroinvertebrates on soil aggregate abundance and composition and quantified the fate of organic matter derived from roots using a two-year macroinvertebrate exclusion-based field mesocosm study coupled with stable isotope labeling in a turfgrass lawn ecosystem. We hypothesized that macroinvertebrates change soil aggregates dynamics by increasing macroaggregates and decreasing microaggregates and enhancing the incorporation of root-derived organic matter into macroaggregates. We found that within the turfgrass root zone, macroinvertebrates increased the proportion of macroaggregates and decreased free microaggregates. In addition, macroinvertebrates increased carbon and nitrogen incorporation into macroaggregates, microaggregates and coarse particulate organic matter, detected via isotopic enrichment of those soil fractions. We found that macroinvertebrates affect the fate of recently fixed root- and rhizodeposit-derived organic matter in a turfgrass lawn ecosystem, and particularly its incorporation into soil aggregates, similar to findings in forest and agricultural systems.

In the United States, there is a growing need for turfgrass management practices that protect community and environmental health. The proportion of the developed landscape in the United States covered by turfgrass is significant and, at present, covers at least 1.9% of the total land area and comprises 60% in parts of the country. As urbanization progresses, there is a critical need to re-examine turf management practices that reduce reliance on pesticide and fertilizer inputs while contributing additional beneficial ecosystem services. In this review, we discuss the functional role of turfgrass in urban ecosystems. We identify key urban ecosystem processes associated with turfgrass and evaluate the potential to integrate biodiversity into their design and management. Specifically, we summarize research on the Biodiversity and Ecosystem Function theory that shows enhanced C storage, N retention, and weed suppression in natural and managed ecosystems, which are traits that are relevant to turfgrass systems. Enhancing biodiversity in turfgrass systems could increase ecosystem services in urban landscapes and should be considered a component of sustainable management practices.

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.