Hot moments and hot spots of nutrient losses from a mixed land use watershed

SummaryNon-point nitrogen (N) and phosphorus (P) pollution from agriculture has increasingly received more public attention. In this study, NO3–N, dissolved P (DP) and particulate P (PP) concentrations and loads were investigated for four sub-basins (labeled 1–4 going up the watershed) within a mixed land use watershed (39.5 ha) in the Appalachian Valley and Ridge Physiographic Province. The hot moments of NO3–N concentration and load occurred in base flow and during the non-growing season. Great and temporally variable DP and PP concentrations were observed in storm flow. The hot moments of DP concentration and load were in storm flow from May to December and from September to Nov, respectively, while the hot moments of PP concentration and load were in storm flow from January to June. The NO3–N, DP, and PP loads were compared for all four sub-basins on a loss per length of stream reach basis to determine the hot spots and their corresponding losses. The hot spots and hot moments of NO3–N loads were in Sub-basins 1 and 4 during the non-growing season base flow period and Sub-basin 2 during the post-growing season base flow period (>110 g m−1 mo−1). The hot spots of DP loads were also in Sub-basins 1 and 4, but during the growing and post-growing season storm flow period (>1.4 g m−1 mo−1). In contrast, the hot spots and hot moments of PP load were in Sub-basin 3 during the pre-growing and growing season storm flow, as much as 13.4 and 14.1 g m−1 mo−1, respectively. Controlling factors of nutrient export were discussed in this study, including season, hydrology (base flow, storm flow, surface and subsurface runoff), and land use. Although different hot moments and hot spots within the watershed were identified for NO3–N, DP, and PP losses, the implementation of a couple of management practices (cover crops and no-till) might be sufficient to effectively reduce nutrient losses from this and similar Valley and Ridge watersheds.

► The NO3–N export was greater in base flow and hot moments of NO3–N export were during the non-growing seasons.
► The dissolved P export was greater in storm flow and hot moments of it were during the growing and post-growing seasons.
► The particulate P export was greater in storm flow and hot moments of it were during the pre- and early growing seasons.
► Sub-basins 1 and 4 were the hot spots of NO3–N and dissolved P exports.
► Sub-basin 3 was the hot spot of particulate P export.