Water quality in an irrigated sandy soil: ridge tillage in rotated corn and soybean compared with full-width tillage in continuous corn

Abstract

Agrichemical contaminant movement into groundwater under irrigated sandy soils can be sensitive to agricultural management systems. Sometimes changes in water quality caused by current agrichemical use cannot be distinguished from land management changes. Groundwater quality in a surficial aquifer under a Zimmerman fine sand (mixed, frigid, argic, Udipsamments; FAO – Cambric Arenosols) in Minnesota USA was compared as related to two farming systems: ridge tillage in a corn ( Zea mays L.)–soybean ( Glycine max L.) rotation, and conventional full-width tandem disk tillage in continuous corn. Measured from 1991 to 1995 were: grain yields; atrazine, alachlor, metribuzin, and nitrate-N in soil to a depth of 90 cm; and water table concentrations of atrazine, alachlor, metribuzin, and nitrate-N in the surficial aquifer. Changes in groundwater quality were monitored with water table piezometers. Grain yields from corn when rotated with soybeans were greater than with continuous corn. Soil herbicide concentrations (atrazine, alachlor, and metribuzin) were greatest directly after application and decreased during the growing season to less than 50 μg kg −1 in the surface 15 cm by the next spring. Herbicides banded over the row in the ridge tillage systems were detected only in soil samples taken under the crop row, while herbicides were detected under the row and between rows when broadcast applied in the continuous corn system. Alachlor and metribuzin were not detected in the groundwater during this study, but atrazine and associated breakdown products, deethylatrazine and deisopropylatrazine were detected in the groundwater. Deethylatrazine was detected in significant quantities (3 μg l −1) under the continuous corn system but in only small quantities (0.04 μg l −1) under the corn–soybean ridge tillage system. Nitrate-N concentrations at the water table did not increase under either cropping system during the study. Soil concentrations significantly declined after 1992 because mineralization of organic N from a previous alfalfa ( Medicago sativa L.) history had disappated. The corn–soybean ridge tillage system reduced atrazine metabolites entry into groundwater under this sandy soil compared to a full-width tandem disk tillage continuous corn system. Best management practices for N use in this sandy soil under irrigation with both management systems did not allow an increase of nitrate-N in the groundwater.