| Summary: | Study region: Fall Creek Watershed in central New York, USA. Study focus: Dairy manure is commonly applied to NY, USA agricultural fields as both a crop nutrient source as well as a means of waste disposal. Managing excess manure places an economic burden on small farm operations due the prohibitive cost of existing practices and regional dominance of saturation-excess hydrology. Through a SWAT modeling exercise we evaluate the efficacy of dairy manure application following the topographic wetness index (TI) as a means of reducing non-point source agricultural nutrient runoff. Next, we examine the efficacy of amending dairy manure with chemical N as a means of reducing the rate of soil TDP accumulation. New hydrological insights: We observed that application of manure to drier pastures results in less TDP and NOX surface losses, but an undesirable increase soil TDP accumulation. Further, pastures receiving dairy manure are typically N limited during summer months, limiting plant P uptake. Manure N amendment reduced TDP accumulation and increases crop yield, but slightly increased NOX surface losses. Spreading dairy manure based on the TI concept represents a feasible path towards reduction of agricultural non-point nutrient runoff, although management strategies need to consider ways to also reduce the long-term accumulation of soil P, which could have consequences in the future that are difficult to mitigate. Keywords: Topographic wetness index, Dairy manure, Agricultural runoff, Hydrologic modeling, SWAT, New York USA
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