Nitrate Dynamics in Riparian Forests: Microbial Studies

While riparian forests have a demonstrated ability to remove nitrate (NO⁻₃) moving from uplands before it enters streams, there is considerable uncertainty as to the mechanisms of NO⁻₃ removal in these areas. We characterized spatial and temporal variation in denitrification enzyme activity (DEA), microbial biomass C and N content, soil respiration and potential net N mineralization and nitrification in three riparian forest sites consisting of soil catenas containing moderately well, somewhat poorly, poorly, and very poorly drained soils (inceptisols and entisols). These measurements were made in conjunction with studies of NO⁻₃ removal from groundwater during growing and dormant seasons that are reported in a companion paper. Two of the sites were on stratified glacial drift, one with an undeveloped upland and one with an upland with high density unsewered residential development that produced groundwater at the edge of the riparian zone with NO⁻₃-N concentrations between 8 and 12 mg/L The third site was on unstratified glacial drift with an undeveloped upland. Hydric surface (0–15 cm) soils (poorly and very poorly drained) consistently had higher DEA than upland-wetland transition zone (moderately well and somewhat poorly drained) surface soils. Spatial patterns of microbial biomass C and N content were more variable but showed the same general pattern as DEA. Levels of DEA and microbial biomass were consistently low or undetectable at and below the seasonal high water table. Surface soil DEA and microbial biomass were correlated with NO⁻₃ removal from groundwater during the growing season. Low levels of DEA and microbial biomass in the subsurface however, suggested that plant uptake was the dominant groundwater NO⁻₃ sink during the growing season. During the dormant season, water table levels were higher and groundwater-borne NO⁻₃ was able to interact with near surface soil and be removed by denitrification and/or microbial immobilization. Potential net N mineralization was quite variable both within and between sites, while potential net nitrification was very low at most sites. A notable exception was in the transition-zone soils at the stratified drift site with a densely developed upland, which had relatively high rates of net NO⁻₃ production. The high nitrification rates observed at this site may suggest that the long-term buffering potential of this site is limited.