Interaction of landscape setting and stream flow seasonality on nitrogen concentrations in a subtropical river, China

The upper Han River, with a drainage area of 95, 200 km², has a dominant vegetated land use of forests and shrubs with a total coverage of 81.3%, and agricultural lands, mostly along the riverine network, account for 14% of the total basin area. In this study, water samples were collected in 42 sites during the time period of 2005–2006 in the river, and nitrate-nitrogen and ammonium-nitrogen were determined in situ. Consequently, multivariate statistics analyses were conducted to investigate the interactive influence of landscape setting and stream flow seasonality on nitrogen concentration in river waters. In the high flow period, nitrogen concentrations (NH4+–N andNO3−–N) in stream water were significantly correlated with forest coverage and bare land at the catchment level, and with respect to the riparian zones of 100, 200, 500, 1000-m width they were only significantly correlated with bare land. In the low flow period, nitrogen concentrations were respectively correlated with vegetation and bare land regardless of varying scale landscape settings, and additionallyNH4+–N was significantly related to agricultural land in riparian zones of 500 and 1000-m width. Nevertheless, nitrogen could be predicted using bare land in the high flow period and in the low flow period vegetation was also a critical variable in particular forNH4+–N. Generally, nitrogen was better predictable by land use in the high flow period rather than low flow period, and land use composition within an entire catchment rather than this in the riparian zone was a better predictor of nitrogen concentrations in rivers. Thus, the interaction between landscape setting and water flow seasonality played an important role in regulating nitrogen concentrations in the subtropical river.