Nitrate Removal By Perennial Filter Strips In The Toeslope Of Cropland Watersheds.

Nitrate losses from corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] rotation cropping systems contaminates ground and surface waters. Inclusion of perennial filter strips in the toeslope of cropland watersheds is a promising strategy for reducing NO₃^--N concentration. However, the processes responsible for decreases in groundwater NO₃^--N concentrations are not well understood; perennial filter strips function somewhat differently from more conventional ‘riparian buffers’. Our objectives were to determine the rates of N₂O and N₂ emission from perennial filter strips in croplands, and to evaluate various factors that are associated with emissions of these two distinct N gases. We predicted that potential denitrification is greater in perennial filter strips than cropland, and compared with cropland, perennial filter strips support more complete denitrification of NO₃^--N to N₂. To test these predictions, nitrogen cycling rates were determined using a variety of approaches from  three replicated watershed treatments: (1) perennial filter strips covering the bottom 10% of the watershed and maize covering the remaining 90% of the watershed (PFS), (2) 100% maize (Corn), (3) 100% restored native grassland (RNG). In situ N₂O fluxes, and laboratory-based measurements of the N₂O/(N₂+N₂O) ratio were significantly higher in Corn, followed by PFS, and RNG. However, the opposite trends were observed for in situ CO₂ fluxes and denitrification enzyme activity (DEA)(i.e. RNG>PFS>Corn). Furthermore, there was a significant negative correlation between N₂O/(N₂+N₂O) ratio and DEA. ¹⁵N₂ fluxes measured by ¹⁵N gas flux technique and N₂ fluxes measured by acetylene inhibition technique were highest in PFS followed by RNG and corn. These results indicate that compared to cropland, PFS has the ability to support greater complete denitrification of NO₃^--N to N₂.