Nonpoint N Runoff from Agricultural Watersheds into the Great Lakes

Eleven agricultural watersheds were continuously monitored for discharge and intensively sampled for runoff N, 1975-77, as part of the IJC Pollution from Land Use Activities Reference Group (PLUARG), Task Group C (Canadian). The watersheds, located in southern Ontario, were sampled between 30 to more than 500 times for NH₄-N, N0₃+N0₂-N, and total Kjeldahl N (TKN).The predominant chemical form of runoff N was N0₃-N with flow weighted concentration means on sampled days ranging from 0.57 to 5.62 mg/L. In contrast, TKN means ranged from 0.64 mg/L to 2.37 mg/L while average soluble NH₄-N concentrations varied from 0.03 mg/L to 0.60 mg/L. High runoff N0₃-N concentrations occurred from watersheds with extensive areas of tile drainage, row crops (especially com), and high kg/ha fertilizer N application rates. Elevated stream TKN concentrations were associated with watersheds with more impermeable soils.Stream N0₃-N loadings ranged from 2.1 ± 0.2 to 39.0 ± 7.6 kg N0₃-N/watershed ha. Significant N fertility losses in excess of 30 kg N0₃-N ha occurred from some watersheds, while other watersheds with extensive areas of hay and pasture and unimproved land gained more N0₃-N in precipitation than was lost as runoff. TKN loads averaged 32% and 25% of total N runoff for the 11 watersheds in 1975 and 1976 respectively. Efforts to reduce Ontario watershed N runoff should concentrate first on soluble N and therefore on improved efficiency of N fertilizer use on the extensive areas of tile drained corn in the lower Great Lakes basin. The effectiveness of standard soil erosion control methods, including grassed waterways and contour planting, should be investigated for reduction of TKN runoff.