Increased nitrogen loading from watersheds can contribute to coastal eutrophication and harmful algal blooms. Despite significant recent investment in watershed restoration, there is a lack of data on ecosystem function associated with stream restoration and stormwater management projects. We chose to study the effects of local carbon sources on denitrification rates because labile organic carbon is a required ingredient for denitrification and because restoration projects often include tree plantings which can alter allocthonous sources (increase leaf inputs and decrease grass inputs) and eventually decrease autochthonous sources (by shading out algae). We 1) measured in situ denitrification rates at restored sites and at urbanized sites and forested reference sites for comparison and 2) evaluated the effects of local carbon amendments (leaves, algae, grass, baseflow water, and stormflow water) on denitrification potential in laboratory assays.
Results/Conclusions
We measured substantial rates of in situ denitrification in restored streams and stormwater management ponds. Denitrification response to different organic carbon sources varied by site; the restored sites was more similar to forested reference sites than suburban sites. Suburban streams had a significantly larger denitrification response to the grass source than the other sites. We hypothesize that the amounts and bioavailability of organic carbon sources may be driving these variations in denitrification response. Watershed restoration strategies may influence denitrification rates, but more research is necessary to elucidate the effects of hydrology, organic carbon sources, and microbial ecology on regulating nitrogen transformations in coastal watersheds.