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.