Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 6, 2001
Publication Date: July 10, 2001
Citation: Jaynes, D.B., Colvin, T.S., Karlen, D.L., Cambardella, C.A., Meek, D.W. 2001. Nitrate loss in subsurface drainage as affected by N-fertilizer rate. Journal of Environmental Quality. 30:1305-1314.
Interpretive Summary: The use of nitrogen fertilizer in conjunction with corn production has been criticized as a leading source of nitrate contamination of surface waters within the Mississippi watershed. In the Midwest, much of the nitrate entering surface waters is from tile drainage of poorly drained soils. Little quantitative data is available linking nitrogen fertilizer rates applied to corn with nitrate concentrations in tile drainage water. In this study, we quantified the yield, nitrogen budget, and nitrate loss in a producer's field where three different nitrogen fertilizer rates were used - the farmer's typical rate, and 2/3 and 1/3 of this rate. We found that the economically optimum rate was close to the 2/3 rate, but at this rate, the nitrate concentration in tile drainage was above the maximum contaminant level (MCL) set by the USEPA for drinking water. Even at the lower fertilizer rate, nitrate concentrations frequently exceeded the MCL. At all but the highest fertilizer rate used, more nitrogen was removed from the field in grain and tile drainage, than was replenished by fertilizer and other sources. This indicates that the lower fertilizer rates are not sustainable because they are mining nitrogen (and thus fertility) from these very productive soils. This study has immediate implications to regulators looking for solutions to the hypoxia problem in the Mississippi gulf and to farmers who may be required to limit nitrogen inputs to their fields in the future.
The relationships between N-fertilizer rate, yield, and nitrate (NO3) leaching need to be quantified for a range of soil and climate conditions to develop soil and crop management practices that are economically and environmentally sustainable. From 1996 through 1999, we measured yield and NO3 loss from a subsurface drained field at three N-fertilizer rates; a low (L) rate of 67 kg/ha in 1996 and 57 kg/ha in 1998, a medium (M) rate of 135 kg/ha in 1996 and 114 kg/ha in 1998, and a high (H) rate of 202 kg/ha in 1996 and 172 kg/ha in 1998. The 22-ha field was located in central Iowa and had high organic matter soils in the Ottosen - Kossuth association. Corn and soybean were grown in rotation with N-fertilizer applied in the spring to corn only. At the L N-fertilizer rate, NO3-N concentrations in the drainage water exceeded the 10 mg/L maximum contaminant level (MCL) established by the USEPA for drinking water only during the years that corn was grown. At the M and H N-fertilizer rates, NO3-N concentrations exceeded the MCL in all years, regardless of crop grown. Differences in drainage NO3 concentrations between N rate treatments diminished during the years soybean was grown. For all years, the mass loss from the H treatment (48 kg-N/ha) was significantly higher than the mass losses from the M (35 kg-N/ha) and L (29 kg-N/ha) treatments, which were not significantly different. The economically optimum N-fertilizer rate for corn was between 67 and 135 kg/ha in 1996 and 114 and 172 kg/ha in 1998. Economically optimum grain yield cannot be sustained within this field under the current management scheme without producing drainage waters that exceed the MCL for NO3-N.