Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 30, 1999
Publication Date: N/A
Interpretive Summary: One of the environmental concerns with row crop agriculture is the high concentration of nitrate-nitrogen (NO3-N) in surface and groundwater with continuous corn. Nitrogen fertilizer is applied to corn each year and excess N leaches through the soil to groundwater. Frequently the NO3-N concentrations in the groundwater exceed the maximum contaminant level (MCL) of 10 ppm. In a corn/soybean rotation, a rotation that benefits bot crops, N fertilizer is not applied during the soybean year. If credit is given for the N carried over from the soybean year to the corn year, less N fertilizer needs to be applied to rotated corn than continuous corn. Four large, undisturbed lysimeters (84 sq.ft in area and 8 ft deep) were used to study NO3-N leaching in a corn/soybean rotation. For 6 years the corn in rotation received 200 lb N/acre (a common N rate for continuous corn). For the next 6 years, 2 lysimeters received 175 lb N/acre and 2 lysimeters received 125 lb N/acre during the corn years. Most water and NO3-N leaching occurs during the late winter/early spring when soils are wet and crops are not using water. During the first 6 years, higher NO3-N levels (exceeding the MCL) occurred following corn than after soybeans. During the second 6 years NO3-N concentrations leached were similar following corn and soybeans. The multi-year average NO3-N concentrations in groundwater under a corn/soybean rotation will probably be lower than under continuous corn, especially if an N carry-over from the soybeans is credited and N fertilizer is reduced in the corn year. Nevertheless, average NO3-N levels may not be less than the MCL. This research will assist extension personnel, farm consultants and farmers develop crop plans that will be more environmentally friendly.
Technical Abstract: High rates of N fertilizer in the production of continuous corn (Zea mays L.) have resulted in excessive NO3-N leaching, with concentrations in groundwater frequently exceeding the MCL of 10 mg/L. On a multi-year basis, less N is applied when corn is rotated with a legume such as soybeans [Glycine max (L.) Merr.]. Giving an N credit for the soybeans, less fertilizer N needs to be applied to rotational corn than continuous corn. Four large, undisturbed monolith lysimeters (8.1-m**2 surface area, 2.4 m deep) were used to investigate NO3-N leaching in a corn/soybean rotation in eastern Ohio. Lysimeter soil is a moderately well-drained Keene silt loam on a 6% slope. In the spring of each corn year, 140 kg N/ha as NH4NO3 was surface applied to two lysimeters and 196 kg N/ha to two lysimeters for 6 years. Prior to this treatment, each lysimeter was in a corn/soybean rotation for 6 years, receiving 224 kg N/ha in the spring of the corn year. A rye (Secale cereale L.) winter cover crop was used following soybeans. The highest NO3-N levels and the most transport occurred during the winter/spring, soil moisture recharge period (November through April). NO3-N levels in the percolate from all of the lysimeters were similar to each other and had a 6-yr, flow-weighted average of 9.9 mg/L. NO3-N losses under soybeans were similar to the losses under corn, especially when N fertilizer rates were reduced to allow for the legume credit from the previous year. This reduction resulted in lower N fertilizer inputs in a corn/soybean rotation and subsequently lower NO3-N levels and transports due to leaching during the corn-impacted years. Even so, average NO3-N levels may not be less than the MCL. For a given year, weather can impact percolation and leaching more than the current crop.