Landscape Configuration Dominants the Spatial Variation in Riverine Nitrogen and Phosphorus Levels in an Agricultural Watershed

The effects of natural geomorphologic characteristics and land-use patterns on riverine nitrogen (N) and phosphorus (P) levels are complex and scale-dependent in agricultural watersheds. Quantifying the scale effects of various landscape categories and distinguishing their individual and intersective effects are useful for effective landscape planning to reduce non-point source (NPS) pollution; however, these effects have not been clarified. Based on riverine water quality data from 13 subtropical agricultural catchments with different landscape characteristics in China, this study quantitatively analyzed the influences of the land-use composition, land-use configuration, and geomorphology on riverine N and P levels across multiple scales. Analysis of variance (ANOVA) revealed that riverine total nitrogen (TN), nitrate nitrogen (NO3--N), ammoniacal nitrogen (NH4+-N), total phosphorus (TP), dissolved phosphorus (DP) concentrations presented significant spatial variations (p < 0.05). Redundancy analysis (RDA) showed the overall variation of riverine pollutant concentrations were better explained by landscape metrics at the 100-m buffer scale. Variation partitioning analysis (VPA) indicated that spatial scale differences in the impact of geomorphology on riverine N and P levels was the most apparent among the three landscape categories. The individual influence of geomorphologic characteristics on the variability of riverine N and P levels decreased with increased spatial scales; these geomorphologic factors largely shaped water quality mainly through their interaction with land use. In the studied watershed, the land-use configuration was the greatest driver of the variation in riverine N and P levels, and its influence on riverine N and P levels was less sensitive to spatial scale changes. Thus, NPS pollution could be regulated by promoting intensive farmland management, improving garden plots aggregation along riparian corridors, and decreasing dispersed residential land distribution at the catchment scale. Overall, our results offer practical insights for the management of NPS pollution in watersheds.