Nitrogen loss from a mixed land use watershed as influenced by hydrology and seasons

SummaryNon-point nitrogen (N) loss from agriculture is an environmental concern among scientists, decision-makers, and the public. This study investigated NO3–N and total N losses from a mixed land use watershed (39.5 ha) in the Appalachian Valley and Ridge Physiographic Province as influenced by hydrology (flow type, runoff volume, storm sizes, and precipitation amount) and seasons (pre-growing, growing, and post-growing seasons) from 2002 to 2006. Stream discharge was monitored every 5-min and water samples for NO3–N and total N analyses were collected weekly for base flow and for every storm. The majority of NO3–N (about 75%) and total N (about 65%) were exported in base flow, which contributed about 64% of the total flow in an average year and had greater flow-weighted mean NO3–N concentration (5.6 mg L−1) than storm flow (3.4 mg L−1). A substantial proportion of total N was in the form of NO3–N in base flow (58.1%) and small storms with <1-year return period (48.4%), suggesting that base flow and small storm flow were probably dominated by NO3–N-rich lateral subsurface flow. As storm size (runoff volume, flow rate, and return period) increased, the NO3–N concentration decreased following a power relationship. In contrast, total N concentration increased with increasing storm size, which was attributed to surface runoff that flushed NH4–N and dissolved and particulate organic N into the stream. The NO3–N and total N losses from this watershed were greater during the pre-growing (January–April) and post-growing (October–December) seasons, which contributed >73% of the overall NO3–N and total N losses. These two seasons also had greater flow-weighted mean NO3–N (4.8 and 5.5 mg L−1, respectively) and total N (9.8 and 10.1 mg L−1, respectively) concentrations and greater discharge (40% and 32% of total discharge, respectively) than during the growing season. Greater contribution of NO3–N to total N loss in storm flow was also observed during the pre-growing and post-growing seasons than during the growing season, while contribution of NO3–N to total N loss in base flow was consistent across seasons. Management practices to reduce N loss from this watershed should target periods of base flow when NO3–N and total N concentrations are greater (before and after the crop growing season) and target areas along the stream where seeps are present, perhaps including perennial plant species in localized riparian buffers and introducing cover crops on the agricultural land during the fallow season.

► Greater NO3–N concentration and NO3–N and total N loads were observed in base flow and small storms.
► Non-growing season had greater NO3–N and total N concentrations and loads than the growing season.
► Discharge, precipitation, surface and subsurface runoff are important controls of N loss from the study watershed.
► Management practices to reduce N loss from this watershed should target periods of base flow and non-growing season.
► Management practices to reduce N loss should target areas along the stream where surface and subsurface flows are present.