A decadal-scale nutrient loading study in a coastal wetland: Impacts on soil microbial processes

Fertilizer use in agricultural lands and treated wastewater disposal have increased bioavailable nitrogen (N) and phosphorus (P) loading to wetlands and aquatic systems with potential implications for nutrient cycling and soil microbial processes. Recently, conflicting results of N loading impacts on microbial-mediated organic matter decomposition in coastal wetland soils have been reported. However, to date, the majority of studies have investigated short-term nutrient loading effects while few have reported long-term integrated impacts. Here, we present results from an 11-year soil nutrient loading study in an oligohaline coastal wetland treated with 0, 50, 200, or 1200 kg N ha−1 yr−1 in combination with 0 or 131 kg P ha−1 yr−1. We measured soil (0-10 cm) physiochemical characteristics, soil microbial biomass N, and measures of biogeochemical cycling of N, including potentially mineralizable nitrogen and denitrification rates. Our results show that soil total P increased with P additions, but no significant differences in measures of soil microbial biomass or activity occurred with N, P, or N × P loading. We conclude that long-term N loading at rates equivalent to Mississippi River diversions had no persistent lasting effects on total N or examined soil microbial measures.