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Title: Implications of a Conservation Practices Inventory for Water Quality in an Iowa Watershed

item Tomer, Mark
item HADISH, G
item James, David
item Cole, Kevin

Submitted to: ASA-CSSA-SSSA Annual Meeting Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: 11/16/2006
Publication Date: 11/16/2006
Citation: Tomer, M.D., Hadish, G., James, D.E., Cole, K.L. 2006. Implications of a Conservation Practices Inventory for Water Quality in an Iowa Watershed [abstract]. ASA-CSSA-SSSA Annual Meeting Abstracts. Nov. 12-16, 2006, Indianapolis, IN. 2006 CDROM.

Interpretive Summary:

Technical Abstract: The Iowa River’s South Fork drains 78,000 ha of glacial terrain. Subsurface drainage is extensive and corn and soybeans occupy 85% of the watershed. There are about 100 confined livestock operations in the watershed, most producing swine. Given prevailing practices, we estimate manure from these operations is annually applied to 27% of the watershed, at 200 kg nitrogen (N) ha-1 yr-1 preceding corn, which may lead to soil phosphorus (P) accumulations. An inventory of conservation practices and residue cover at planting was conducted in 2005. A sequence of United States Department of Agricultural-National Agricultural Statistics Service (USDA-NASS) annual crop cover data was used to identify and map crop rotations. We found that 87% of the area in corn in 2004 had >30% residue cover at the next planting, whereas, only 47% of the 2004 soybean acreage had >30% residue. This raises concerns for the common practice of fall-applying manure into soybean residue. Decreases in residue and increases in soil P are likely consequences, which can interact to magnify runoff P losses. Stream discharge and nutrient concentrations are being monitored at stream and tile gauges in the watershed. Concentrations of nitrate-nitrogen (NO3-N) and total P exceed ecological thresholds most of the time (>70%). But the economic value of NO3-N losses, in terms of fertilizer costs, are about 20 times that of P. Producers therefore have economic incentive to decrease N leaching, and may only need to identify management options that do so. Economic incentives to reduce P losses are comparably small. Management alternatives that can reduce losses of both nutrients are needed.