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Title: FIELD-SCALE WATERSHED EVALUATIONS ON DEEP LOESS SOILS: III. RAINFALL AND FERTILIZER N USE EFFICIENCIES

Author
item Logsdon, Sally
item Karlen, Douglas
item Prueger, John
item Kramer, Larry

Submitted to: Journal of Soil and Water Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/15/1999
Publication Date: N/A
Citation: Logsdon, S.D., Karlen, D.L., Prueger, J.H., Kramer, L.A. 1999. Field-scale watershed evaluations on deep loess soils: III. Rainfall and fertilizer N use efficiencies. Journal of Soil and Water Conservation. 54:711-716.

Interpretive Summary: Water is needed for crop growth, but too much water causes nitrogen (N) to runoff or to be washed through the soil to groundwater. Efficient water use requires reduced runoff, increased infiltration, and greater soil water retention. Our objectives were to compare water use efficiency for continuous corn (Zea mays L.) production on long-term conventional-till and ridge-till watersheds in western Iowa, and relate water use to fertilizer N use efficiency. We collected 23 y of crop yield, rain, baseflow, runoff, and soil water content. Using these data, we estimated N use efficiency from crop removal and fertilizer application data. We showed that using ridge-till management allowed the crops to use water and N more efficiently than conventional-till management. More complete N use left less N in the soil under ridge-till, but more N fertilizer had been added than the crop used. Because the water moved faster through the ridge-till soil than the conventionally-tilled soil, the excess N was washed more rapidly to the groundwater. Farmers will use this information to plan management strategies to sustain crops during dry weather, and regulators and legislators may incorporate it into their plans to reduce offsite contamination of waterways.

Technical Abstract: Water is often the most limiting crop production factor, but it also causes nutrient runoff and leaching losses. Efficient water use requires reduced runoff, increased infiltration, and greater soil water retention. Our objectives were to (1) compare water use efficiency for continuous corn (Zea mays L.) production on long-term conventional-till and ridge-till watersheds in western Iowa, and (2) relate water use to fertilizer N use efficiency. We collected 23 y of crop yield, rain, baseflow, runoff, and soil water content. Using these data we estimated actual evaporation and computed water use efficiency. We estimated N use efficiency from crop removal and fertilizer application data. There were no significant differences in seasonal rainfall or actual evaporation between the two watersheds, but water use efficiency for ridge-till (22.8 kg/ha/mm/yr) was significantly greater than for conventional till (20.3 kg/ha/mm/yr). The estimated N removal by crop harvest accounted for 48 and 59% of the applied N. Ridge-till reduced runoff, increased infiltration, and reduced evaporation; therefore, we observed greater water use efficiency and fertilizer N use efficiency. Better N use left less residual N in the root zone for subsequent leaching, but average N fertilization rates exceeded average crop removal by 68 kg/ha/y in the ridge-till watershed. Combined with increased infiltration associated with ridge-till, this resulted in higher N concentrations in the baseflow. This interaction demonstrates the importance of balancing both N and water use efficiencies to achieve a sustainable production system.