Catchment land use effects on fluxes and concentrations of organic and inorganic nitrogen in streams
Résumé
We present annual downstream fluxes and spatial variation in concentrations of dissolved inorganic nitrogen (NH4+ and NO3-) and dissolved organic nitrogen (DON) in two adjacent Scottish catchments with contrasting land use (agricultural grassland vs. semi-natural moorland). Inter- and intra-catchment variation in N species and the relation to spatial differences in agricultural land use were studied by determining catchment N input through agricultural activities at the field scale and atmospheric inputs at a 25 m grid resolution. The average agricultural N input of 52 kg N ha(-1) yr(-1) to the grassland catchment was more than 4 times higher than the input of 12 kg N ha(-1) yr(-1) to the moorland catchment, supplemented by 123 and 8.2 kg N ha(-1) yr(-1) through atmospheric deposition, respectively. The grassland catchment was associated with an annual downstream total dissolved nitrogen (TDN) flux of 14.4 kg N ha(-1) yr(-1), which was 66% higher than the flux of 8.7 kg ha(-1) yr(-1) from the moorland catchment. This difference was largely due to the NO3- flux being one order of magnitude higher in the grassland catchment. Dissolved organic N fluxes were similar for the two catchments (7.0 kg ha(-1) yr(-1)) with DON contributing 49% to the TDN flux in the grassland compared with 81% in the moorland catchment. The results highlight the importance of diffuse agricultural N inputs to stream NO3- concentrations and the importance of quantifying all the major aquatic N species for developing a better understanding of N transformations and transport in the atmosphere-soil-water system.