Comparing nitrate sink strength in perennial filter strips at toeslopes of cropland watersheds

Integration of perennial filter strips (PFS) into the toeslopes of agricultural watersheds may decrease downstream nitrate (NO) losses. However, long-term NO removal depends on the relative importance of several NO sinks in the PFS. Plant biomass and labile soil organic matter (SOM) are temporary NO sinks, while stable SOM is a long-term, but potentially finite, NO sink. In contrast, denitrification is a permanent NO sink. We investigated the relative importance of these NO sinks in PFS at the toeslope of row crop watersheds in Iowa. Using 25- × 30-cm in situ mesocosms, we added NO to PFS soils and quantified NO-N recovery in plant biomass and SOM after one growing season. Further, we compared NO-N recovery in particulate (relatively labile) and mineral-associated (relatively stable) SOM in mesocosms with and without growing perennial vegetation. To determine the potential importance of denitrification, we compared denitrification enzyme activity in soils from paired watersheds with and without PFS. Transfer of NO-N into labile and stable SOM pools was rapid and initially independent of growing vegetation. However, SOM and plant biomass were both relatively minor NO sinks, accounting for <30% of NO-N inputs. Denitrification enzyme activity data indicated that dissolved organic carbon derived from perennial vegetation increased potential denitrifier activity in PFS soils compared with row crop soils. Together, these results constrain SOM and plant biomass as NO sinks and indicate that denitrification was the most important NO sink in perennial filter strips over one growing season.