Agrichemical movement in the root-zone of claypan soils: ridge- and mulch-tillage systems compared

Abstract

Climate, poor internal drainage in claypan soils, and cultural factors make it difficult to control agrichemical movement in the southern fringe of the Corn Belt, USA. Ridge- and mulch-tillage systems were evaluated for root-zone water quality and grain production on a claypan soil. In northcentral Missouri the Mexico silt lam (fine, montmorillonitic, mesic Udollic Ochraqualfs; FAO-Mollic Planosols) has a pronounced B t horizon. Zero-tension pan lysimeters at 91 cm were used to monitor nitrate–nitrogen (NO 3–N) and herbicide concentrations in three farming systems: (1) a high chemical input mulch-tillage in a corn ( Zea mays L.)–soybean [ Glycine max (L.) Merr.] rotation (MTH); (2) a reduced chemical input with mulch-tillage in a corn–soybean–wheat ( Triticum aestivum L.) rotation (MTR); (3) a reduced chemical input with ridge-tillage in a corn–soybean rotation (RTR). Farming system treatments were replicated three times with lysimeters placed in two replications. By band-applying herbicide over rows, herbicide mass applied to the MTR and RTR system was 50% less than applied to MTH, where herbicides were broadcast. Only marginal benefits to root-zone water quality could be attributed to this reduced herbicide input. Leachate water flow volume and chemical concentration intercepted by the pans were influenced more by intrinsic, site-specific soil hydrology and year-to-year variation of rainfall intensity and timing than by the management. Root-zone leachate in both MTH and RTR farming systems in 1992 were below USEPA maximum contaminant levels (MCL) of 10 mg l −1 for NO 3–N and 3 μg l −1 for atrazine used for drinking water. Rainfall after herbicide application was more frequent and intense in 1993, but the extent of leaching was site-specific with significant variation among plots within all treatments. For example, maximum NO 3-N and atrazine concentrations in leachate were 42 mg l −1 and 100 μg l −1, respectively, in one MTH plot, and in second plot they were below 7 mg l −1 and 2 μg l −1, respectively. High concentration variability also occurred between RTR plots. Nitrate–N and atrazine were consistently below their MCL on MTR plots. While herbicide movement and loss may be slightly reduced with banding, this management will have low adoption on claypan soils because of the yield-loss risks associated with the small time window allowed for field operations. The same risk is associated with split N applications.