Alternative Farming System Effects on Profile Nitrogen Concentrations on Two Iowa Farms

There is a general lack of quantitative data to assess alternate farming systems. Our objectives were to quantify long-term (20-yr) effects of conventional and alternative farming practices on soil NH₄-N, NO₃-N, and total-N concentrations. Contiguous soil map units of Clarion loam (fine-loamy, mixed, mesic Typic Hapludoll), Nicollet loam (fine-loamy, mixed, mesic Aquic Hapludoll), Canisteo silty clay loam (fine-loamy, mixed [calcareous], mesic Typic Haplaquoll), and Webster silty clay loam (fine-loamy, mixed, mesic Typic Haplaquoll) were identified and used to compare long-term farming system effects. Sixty-four soil profile cores averaging 4.5 m in depth were collected from a 16-ha conventional field and from a 16-ha alternative field in April 1989 following a year in which total precipitation was ≈30% below normal (588 vs. 848 mm). In autumn 1990, 32 cores averaging 3.0 m in depth were collected from each of two other 16-ha conventional and alternative fields just north of those sampled in 1989. Farming practice significantly affected total-N, NH₄-N, and NO₃-N concentrations measured at the various sampling depths, but differences were small and inconsistent. Corn (Zea mays L.) grain yields averaged 9093 kg ha⁻¹ in the conventional field and 8977 kg ha⁻¹ in the alternative field in 1989. Comparing conventional and alternative fields, total aerial N accumulation 6 wk after planting averaged 203 and 123 mg plant⁻¹, ear leaf N concentration averaged 26.6 and 20.6 g kg⁻¹, and total N removed by corn grain averaged 91.6 and 85.7 kg ha⁻¹. All measured differences were generally within the 95% confidence interval, and neither system was consistently better. We conclude that well-managed conventional and alternative farming systems do not differ in their risk for groundwater N pollution in central Iowa.