No abstract

Soil conservation tillage systems, including ridge-tillage, often reduce surface water contamination by pesticides because soil erosion and surface runoff are reduced. However, the effects on losses through subsurface drainage tile are somewhat uncertain. Our field study quantified the effects of four tillage practices in continuous corn (Zea mays L.) and corn-soybean [Glycine max (L.) Merr] rotations on herbicide and nitrate N losses in tile drainage water. Fertilizer and pesticide application methods were uniform for ridge, moldboard, chisel, and no-till systems. Pesticide and nitrate N leaching losses were significantly affected by crop rotation. Tillage practice had little influence on nitrate N and pesticide losses to the subsurface drainage water within a corn-soybean rotation. However, ridge-till and no-till resulted in larger losses of atrazine than the moldboard plow and chisel based systems under continuous corn. Tillage system did not affect the timings of peak tile flow occurrences, although peak tile flow volume was affected by tillage, presumably because each system bad its own macropore system related to preservation or annual destruction of biopores by tillage. Corn yields were significantly higher under corn-soybean rotation than with continuous-corn for all tillage practices. These results indicate that continuous corn production is not an environmentally sustainable practice for this area because it resulted in higher nitrate N leaching losses to groundwater, received higher Napplications, and resulted in lower corn yields than the corn-soybean rotation. The results also reinforce the need for studies on chemical placement, rate, and timing for various tillage practices to reduce tile drainage losses of agricultural chemicals.

An appropriate combination of tillage and nitrogen management practices will be necessary to develop sustainable farming practices. A six-year (1993)(1994)(1995)(1996)(1997)(1998) field study was conducted on subsurface-drained Clyde-Kenyon-Floyd soils to quantify the impact of two tillage systems (chisel plow vs. no tillage) and two N fertilizer management practices (preplant single application vs. late-spring soil test based application) on nitrate-nitrogen (NO3-N) leaching loss with subsurface drain discharge from corn (Zea mays L.) soybean (Glycine max L.) rotation plots. Preplant injected urea ammonium nitrate solution (UAN) fertilizer was applied at the rate of 110 kg ha-1 to chisel plow and no-till corn plots, while the late-spring N application rate averaged 179 and 156 kg ha-1 for the no-till and chisel plow corn plots, respectively. Data on subsurface drainage flow volume, NO3-N concentrations in subsurface drainage water, NO3-N loss with subsurface drainage flow, and crop yield were collected and analyzed using a randomized complete block design. Differences in subsurface drainage flow volume due to annual variations in rainfall significantly (P = 0.05) affected the NO3-N loss with subsurface drainage flows. High correlation (R2 = 0.89) between annual subsurface drainage flow volume and the annual NO3-N leaching loss with subsurface drainage water was observed. The flow-weighted average annual NO3-N concentrations varied from a low of 6.8 mg L-1 in 1994 to a high of 13.9 mg L-1 in 1996. Results of this study indicated that NO3-N losses from the chisel plow plots were 16% (16 vs. 19 kg-N ha-1) lower in comparison with no-till plots, while corn grain yield was 11% higher in the chisel plow plots (8.3 vs. 7.5 Mg ha-1). Late-spring N application applied as a sidedress resulted in 25% lower NO3-N leaching losses with subsurface drainage water in comparison with preplant single N application and also significantly (P = 0.5) higher corn grain yield by 13% (8.4 vs. 7.4 Mg ha-1). These results clearly demonstrate that chisel plow tillage with late-spring soil test based N application for corn after soybean can be a sustainable farming practice for the northeast part of Iowa.

Spatial variability of surface hydraulic properties and the extrinsic (e.g., traffic, cropping, etc.) and intrinsic (e.g., soil type, pore size distribution, etc.) factors associated with these properties are important for infiltration and runoff processes in agricultural fields. Disc infiltrometers measured infiltration at 296 sites arranged on two parallel transects. To examine and differentiate the factors contributing to spatial structure under different field conditions these measurements were made in the corn rows, no‐track interrows, and wheel track interrows of the field using four different soil water tensions Ψ (0, 30, 60, and 150 mm). Unsaturated hydraulic conductivity (K) and saturated hydraulic conductivity (Ks) were maximum in the corn rows and minimum in wheel track interrows, with no‐track interrows intermediate. Exponents (α parameters) of Ks and K relationships (K = Ks exp−αΨ) for corn rows and no‐track interrows were not significantly different from each other but were significantly different from a for the wheel track interrows at P = 0.01 level. Spatial variability of K and Ks values showed some pseudoproportional effect in nugget variance for all three field conditions. No‐track interrows clearly showed an inverse trend for semivariogram of K with changing tension (Ψ) values, whereas differences were found for corn rows and wheel traffic interrows. The spatial structure of a for all three field conditions were mostly white noise. Under corn rows, in addition to random variation, a small five‐row periodic variation at the P = 0.20 level, matching the five‐row traffic configuration, was discovered. The spatial structure of α was influenced by soil type for the no‐track interrows. Spatial structure was absent in wheel track interrows, indicating the destruction of pore structure due to compaction.