Denitrification and a Nitrogen Budget of Created Riparian Wetlands

Riparian wetland creation and restoration have been proposed to mediate nitrate‐nitrogen (NO 3 − –N) pollution from nonpoint agricultural runoff. Denitrification by anaerobic microbial communities in wetland soils is believed to be one of the main sinks for NO 3 − –N as it flows through wetlands. Denitrification rates were quantified using an in situ acetylene inhibition technique at 12 locations in three wetland/riverine sites at the Olentangy River Wetland Research Park, Columbus, Ohio for 1 yr. Sites included two created flow‐through experimental wetlands and one bottomland forest/river‐edge site. Points were spatially distributed at inflows, center, and outflows of the two wetlands to include permanently flooded open water, intermittently flooded transitions, and upland. Annual denitrification rates (median [mean]) were significantly higher ( p < 0.001) in permanently flooded zones of the wetlands (266 [415] μg N 2 O–N m −2 h −1 ) than in shallower transition zones (58 [37.5] μg N 2 O–N m −2 h −1 ). Median wetland transition zone denitrification rates did not differ significantly ( p ≥ 0.05) from riverside or upland sites. Denitrification rates peaked in spring; for the months of April through June, median denitrification rates ranged from 240 to 1010 μg N 2 O–N m −2 h −1 in the permanently flooded zones. A N mass balance analysis showed that surface water flux of N was reduced by 57% as water flowed through the wetland, but only about 3.5% of the N inflow was permanently removed through denitrification. Most N was probably lost through groundwater seepage. Comparison with denitrification rates measured previously in these wetlands suggests that these rates have remained steady over the past 4 to 5 yr.