Processes Controlling Nutrient Loss in Runoff from Winter Applied Dairy Manure.

Agricultural nutrient management is an issue due to nitrogen (NH₄) and phosphorus (P) loss from fields and water quality degradation. Better information is needed on the risk of nutrient loss in runoff from dairy manure applied in winter. We investigated the effect of temperature on nutrient release from manure to water, and of manure quantity and placement within a snowpack on nutrient release to melting snow. Temperature did not consistently affect manure P and NH₄ release during water extraction. Manure P release, but not NH₄ release, was significantly influenced by the water to manure solids extraction ratio. During snowmelt, manure P release was not affected by manure placement in the snowpack, and the rate of P release decreased as application rate increased. Water extraction data can reliably estimate P release from manure during snowmelt, but snowmelt water interaction with more solids manure and subsequent P release is incomplete compared to liquid manures. The rate of manure NH₄ released during snowmelt was the same regardless of application rate. For the semi-solid manure, NH₄ released during snowmelt increased with the depth of snow covering it, mostly likely due to less NH₃ volatilization. For the liquid manure, there was no effect of manure placement within the snowpack on NH₄ released during snowmelt. Water extraction data can also reliably estimate manure NH₄ release during snowmelt, but NH₃ volatilization needs to be considered for liquid manures at all placements in a snowpack and semi-solid manures applied on top of snow.