Nitrate removal in urban wetlands: Examining the roles of vegetation, soils, and hydrology in the creation of “hot spots” and “hot moments” of microbial activity

Wetlands can provide desirable ecosystem services such as microbial nitrogen removal. The high level of spatial and temporal heterogeneity of such microbial processes, however, often complicates efforts to manage and restore function to degraded wetlands. Few studies have linked proposed "hot spots" and "hot moments" of nitrogen removal in a wetland landscape to patterns of hydrology, vegetation and soils; such studies are needed to quantify and predict nitrogen removal dynamics. Urban settings pose a particular modeling challenge because soils, vegetation, and hydrology can be even more heterogeneous than in less disturbed sites. We examined factors mediating "hot spots" and "hot moments" of linked nitrification/denitrification at well-defined spatiotemporal scales in an urban wetland ecosystem in New Jersey. We carried out buried-core measurements of nitrogen mineralization and static core, acetylene-based measurements of denitrification over 10-day sequences during three seasons under the same vegetation type (Phragmites australis) but in different soils (fill material, remnant salt marsh soils); this was done to document temporal variability and the role of soil properties in nitrogen cycling. Highly significant (p < 0.005) differences were found in denitrification dynamics between soil types within seasons ("hot spots") and between seasons within the same soil type ("hot moments"). Differences among and within soil types likely represent a hydrologic trade-off between conditions favorable to denitrifying bacteria (suboxic/high moisture), and conditions favorable to nitrifying bacteria, which supply nitrate to denitrifiers, but metabolize under oxic (low moisture) conditions. This trade-off was apparent in the significant (p < 0.05) quadratic relationship between denitrification and soil extractable nitrate within each season, in the low levels of both denitrification and extractable nitrate in the wettest sites, and in the significant linear relationship found between soil moisture and texture. Our results suggest that onsite nitrate removal can be augmented in Phragmites-dominated areas through primarily hydrologic means.