Influence of watershed suburbanization on leaf litter decomposition and microbial activity

Urbanization can result in multiple stressors for freshwater ecosystems including altered flow regimes, higher sediment loads and increased inorganic nutrient supply. The effects of urbanization on ecosystem processes including organic matter decomposition are poorly understood. Relationships between decomposition and nutrient levels are inconsistent, possibly due to interactions among stimulatory and inhibitory factors. We incubated leaf litter from two tree species (maple and oak) that differ in foliar chemistry in four streams that varied in the extent of suburbanization (human population density, concentrations of nitrate and chloride) and assessed decomposition rates and microbial activity. We found both stream and leaf species effects but the response of maple to the gradient of suburbanization was much stronger than oak. Although suburban streams were associated with reduced interspecific variability in decomposition rates, fungal biomass only responded to differences among leaf species. We observed high rates of mineralization suggesting that decomposition was not nutrient limited. Results show that multiple stressors may act antagonistically to create a muted signal of suburbanization, conditions where the effects of leaf species emerge to drive microbial activity. Understanding how decomposition responds to multiple factors associated with urbanization is essential for predicting how detrital-based food webs will respond to the Anthropocene.