Nitrogen Loss From a Mixed Land Use Watershed as Influenced by Hydrology and Growing Seasons.

Non-point nitrogen 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) 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 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 and small storms with <1-year return period (accounting for 70% and 48%, respectively), 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 concentrations increased with increasing storm size, which was attributed to surface runoff that flushed NH4-N and dissolved and particulate organic N into the stream. Nitrate and total N losses from this watershed were greater during the pre-growing (Jan.-Apr.) and post-growing (Oct.-Dec.) seasons, which contributed >73% of the overall NO3-N and total N losses. These two seasons also had greater 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 the growing season. Greater contribution of NO3-N to total N loss was also observed during the pre-growing and post-growing seasons, which was attributed to lateral subsurface flow with greater NO3-N concentration. Management practices to reduce N loss from this watershed should target periods of base flow when NO3-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 crop during the fallow season.