Groundwater quality is perceived to be vulnerable under irrigated sandy soil because of increased agrichemical loading. Flushes of NO3-N and deteriorated groundwater quality may also result from accelerated mineralization when converting from dryland to irrigated production. Groundwater quality under a corn (Zea mays L.)-soybean [Glycine max (L.) Merr.] cropping system with banded herbicide (atrazine, alachlor, metribuzin) in ridge-till compared to continuous corn with broadcast applied herbicide (atrazine, alachlor) in a mulch-till cropping system was studied from 1991 to 1995 in southeastern ND, USA. Nitrogen application rate and method differed between the ridge-till and mulch-till systems. Residual soil NO3-N in the upper 90 cm of soil, at the end of the growing season, was similar in ridge-till and mulch-till corn and ca. 32% less in ridge-till soybean. Low-to-moderate levels (20-59 kg ha(-1)) of residual NO3-N in both cropping systems during the fall-to-late-spring period indicate a similar potential for deep percolation of NO3-N, independently of the cropping system. Nitrate-N concentration in the upper 30 cm of groundwater increased after irrigation was initiated in 1991, and then decreased in both cropping systems to within 4 mg l(-1) of the initial concentration. The source of this rise in NO3-N concentration was most likely from increased mineralization of soil N with the advent of irrigation, above-normal precipitation, and residual NO3-N from previous dryland agriculture. Residual herbicide was retained shallow enough in the soil so that cultivation in the mulch-till corn moved enough herbicide from the furrow to the row area, for the row area to contain ca. 76% of the total residual atrazine and 84% of the residual alachlor 11 weeks after application. Metribuzin was seldom detected below the 15 cm soil depth. Alachlor and metribuzin were not detected in groundwater, and atrazine was detected at <0.41 mu g l(-1) in four out of 130 samples in the upper 30 cm of groundwater. Under an irrigated corn and soybean rotation on a sand plain, groundwater quality was not impaired by herbicides and NO3-N concentrations were only temporarily elevated and finally returned to pre-irrigation levels. (C) 1998 Published by Elsevier Science B.V. All rights reserved.
