Denitrification in a Restored Riparian Forest Wetland

Abstract Groundwater nitrate moving from upland areas toward streams can be removed by denitrification in mature riparian forests, but denitrification in restored riparian forests has not been quantified. We determined denitrification rates in a restored riparian wetland below a liquid manure application site. A riparian forest buffer consisting of hardwoods along the stream and pines above the hardwoods was established according to USDA specifications. Denitrification was measured monthly using the acetylene inhibition technique on intact soil cores for 2 mo before manure application began and for 24 mo after manure application. Groundwater movement of NO 3 − ‐N and total Kjeldahl N were estimated biweekly. Average annual denitrification rate was 68 kg N 2 O‐N ha −1 yr −1 . Denitrification was significantly higher in a grassed area than in either of the forested areas. Denitrification did not differ significantly between the hardwood and pine areas. Denitrification was greater than a conservative estimate of groundwater input of total N. Denitrification rates were higher in April and May 1992 and 1993, after manure application to the upland began, compared with April and May 1991, before manure application began. These results indicate that a riparian wetland, which has not undergone hydrologic modifications, can have denitrification rates comparable to mature riparian forests. Higher denitrification rates in an adjacent grassed wetland and lack of differences in denitrification in hardwood and pine zones indicates that the high denitrification rates were due to factors other than the reforestation itself. Compared with groundwater inputs of N, denitrification was an important sink for N moving from the upland management system.