Michael Schonauer, 800 Cape Henry, Columbus, MO 43228, N. R. Kitchen, USDA-ARS, Cropping Systems and Water Quality Res. Unit, 269 Ag Engineering Building, Columbia, MO 65211, Robert Lerch, USDA-ARS, 1406 Rollins St., Rm. 265, Columbia, MO 65211-0001, and E. J. Sadler, USDA-ARS CS&WQRU, Univ. of Missouri, Rm 269 Ag. Eng. Bldg.,, Columbia, MO 65203.
Nonpoint
source pollution associated with runoff from agricultural fields continues to
be a problem in many areas of the Midwest U.S. due to excessive fertilizer
application, runoff prone soils, and climatic conditions that produce
large thunderstorms during and after spring planting and fertilizer
application. The objective of our research was to evaluate the influence
of three cropping systems on dissolved nitrogen (N) and phosphorus (P) concentrations
and losses in surface runoff at the field scale. The study was conducted from
1993 through 2001 on three watersheds ranging in size from 7.3 to 34.4 ha located
in north-central Missouri. Soils are primarily Vertic Epiaqualfs and are poorly
drained because of a naturally occurring claypan soil
horizon located 150 to 450 mm below the soil surface. The cropping
systems evaluated were a conservation tillage corn-soybean rotation with high
chemical inputs (CS1), a no-till corn-soybean rotation with medium chemical
inputs (CS2), and an adaptive cropping, tillage, and chemical input system that
included a corn-soybean-wheat rotation (1993 through 1996) and a no-till corn-soybean
rotation with variable rate N application (CS3). The watersheds were instrumented to measure
surface runoff and collect samples for water quality analysis. Mean
annual runoff losses were 275, 256, and 216 mm for CS1, CS2, and CS3,
respectively. Mean annual NO3-N, NH4-N, and PO4-P concentrations
were < 5, 0.7, and 0.3 mg/L for the corn years of the rotation and < 2,
0.1, and 0.2 mg/L for the soybean years.
Annual NO3-N, NH4-N, and PO4-P losses ranged from 0.2 to 15.8, 0.02 to
3.2, and 0.08 to 1.2 kg/ha. No-till did
not decrease surface runoff relative to conservation tillage. Fertilizer
incorporation did not reduce dissolved N concentrations and losses relative to
no-till. Dissolved N concentrations and
losses were much higher from CS3 than CS2, even for soybean cropping.