Coupled control of land use and topography on nitrate-nitrogen dynamics in three adjacent watersheds

To investigate the factors controlling nitrate-nitrogen (NO3−‐N) dynamics during snowmelt season and rainfall events, this study was conducted in three adjacent headwater stream watersheds with coupled land use and topography characteristics in eastern Hokkaido, Japan. The agriculture-dominated watershed (AW) had flat topography in agricultural area, the forest-dominated watershed (FW) was characterized by a steep slope in forest area, and the mixed agriculture-forested watershed (AFW) had flat topography in the agricultural area and steep topography in the forest area. Results showed that the timing of NO3−-N export is different between the forested and steep watershed FW and the agricultural and flat watershed AW. The NO3−-N export peaked before discharge peak with quick subsurface flow during snowmelt and rainfall events in the AW, while after discharge peak with slow subsurface flow in the FW. The difference in the timing of NO3−-N export is attributed to the subsurface flow which is regulated by the coupled characteristics of topography and land use. The fast release of NO3−-N in the FW was attributed to the “flushing mechanism”, which was driven by the rapid response of the subsurface flow due to the macropores in the forest soil and the steep slope. The AW showed a consistent “prolonged flush” of NO3−-N, where NO3−-N concentrations peaked after the peak of discharge, which might be attributed to the slow occurrence of subsurface flow because of the flat slope and the low hydraulic conductivity of the pasture. In the AFW, the NO3−-N concentration peaked before the discharge peak during the snowmelt season but after the discharge peak during the rainfall events, indicating other factor such as the macropores related to the freeze/thaw cycles replaced of the coupled characteristics of topography and land use controlling the timing of NO3−-N export in the mixed watershed.