Fertilizers were applied in slow-release form, or as ammonium nitrate, to grazed watersheds in Ohio. Authors measured nitrogen (N) concentrations in surface runoff and sub-surface flow to determine the impacts of these types of fertilizers on water quality. The average N concentration of surface runoff was low; increasing with increasing precipitation, but did not increase with fertilizer application. The average N concentration of sub-surface flow was not affected by fertilizer application for 3 years after treatments began, however, N levels in sub-surface flow exceeded the pollution threshold of 10 mg/ L after 7 years in pastures treated with ammonium nitrate, and after 10 years in pastures treated with slow-release fertilizer. These results suggest that slow-release fertilizers only delay the increase in groundwater pollution that is inevitable with the annual rates of fertilizer application used in this study. The delay in groundwater response to fertilizer treatments emphasizes the need for long term studies and monitoring when groundwater response is measured.
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