Denitrification and Nitrous Oxide Emissions in Annual Croplands, Perennial Grass Buffers, and Restored Perennial Grasslands

Inclusion of perennial vegetation filter strips (PFSs) in the toeslope of annual cropland watersheds can decrease NO3−–N losses to ground and surface waters. Although PFSs are similar to riparian buffers, the processes responsible for NO3−–N removal from PFSs are not well understood. Our objectives were to (i) determine the importance of denitrification as a sink for NO3−–N loss from PFSs and (ii) evaluate how PFSs alter the biophysical processes that affect the relative importance of N2O and N2 emissions. To address our objectives, we used a coupled field laboratory approach with experimental watersheds that included the following treatments: (i) PFSs covering the bottom 10% of the watershed and an annual corn–soybean crop rotation covering the remaining upslope 90% (PFS); (ii) 100% corn–soybean rotation (CORN); and (iii) 19-yr-old 100% restored native grassland (RNG). In situ N2O flux rates and laboratory N2O/(N2 + N2O) ratios were highest in CORN watersheds followed by PFS and RNG watersheds. In contrast, potentially mineralizable C and denitrification enzyme activity (DEA) were highest in PFS and RNG watersheds and lowest in CORN watersheds. Furthermore, there was a negative correlation between N2O/(N2 + N2O) ratio and DEA. In the laboratory, N2 fluxes were highest in PFS followed by RNG and CORN. These results indicate that PFS watersheds support greater total denitrification while emitting less N2O than croplands. Greater potentially mineralizable C in PFS and RNG suggest C availability is an important factor affecting more complete denitrification. These results suggest PFSs function similar to riparian buffers and have potential to reduce NO3−–N losses from annual croplands by denitrification to N2.