Thursday, November 16, 2006 - 9:15 AM

Watershed Scale Nutrient Mass Balances in Southern Minnesota.

David Mulla1, Adam Birr2, and Luke Stuewe2. (1) Univ of Minnesota, Dept of Soil, Water & Climate, 1991 Upper Buford, St. Paul, MN 55108, (2) Minnesota Dept of Agriculture, 2300 Silver Creek Rd, Rochester, MN 55906

An N and P mass-balance was calculated within the 4,330 ha Huelskamp Creek watershed in southern Minnesota by surveying farmers that managed 88% of the land area.  N and P outputs from the watershed were monitored in surface water.  The nutrient inputs included inorganic fertilizer (N and P), imported feed (N and P), legume fixation (N), atmospheric deposition (N and P), purchased animals (N and P), and planted seeds (N).  The outputs included crop removal (N and P), crop senescence (N), animals sold (N and P), tile drainage (N and P), denitrification (N), fertilizer volatilization (N), manure storage losses (N), groundwater leaching (N), surface runoff losses (N and P), and milk sold (N and P).  Manure (N and P) and a portion of harvested crops used for animal feed (N and P) were considered internally cycled between the crop and animal agriculture occurring within the watershed.  Net mineralization of N from soil and buildup of soil P were estimated as residual terms in the mass-balances.  Annual N inputs (314.3 Mg) were smaller than N outputs (497.3 Mg), while annual P budget inputs (83.0 Mg) exceeded P outputs (56.8 Mg), indicating a net depletion of N through mineralization (183 Mg yr-1) and net accumulation of soil P (26.2 Mg yr-1).  N lost to surface water (34.8 Mg) and groundwater (16.5 Mg) account for 11% of all N losses, while atmospheric losses of N account for another 31% (159.2 Mg) of all N losses.  Animal feed imported into the watershed accounts for 64% (116.9 Mg) of the N and 79% (57.2 Mg) of the P inputs to the animal side of the balance.  The estimated animal N and P use efficiencies were 42% and 34%, respectively.