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USDA-ARS National Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011
*Corresponding author.
ABSTRACT
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.
Received for publication April 2, 1991.
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