Field-scale watershed evaluations on deep-loess soils: III. Rainfall and fertilizer N use efficiencies

Water is often the most limiting crop production factor but it also can cau se nutrient runoff and leaching losses. The objectives of this report were to (1) compare water we efficiency for the production of continuous corn (Z ea mays L.) on long-term conventional-till and ridge-till watersheds in wes tern Iowa, and (2) relate water use to fertilizer N use efficiency. Informa tion on crop yield, rainfall baseflow, runoff; and soil water content was c ollected over a period of 23 yr. Potential evaporation was calculated from microclimate data. Using these data, water we efficiency was computed. Nitr ogen use efficiency was estimated using crop removal and fertilizer applica tion data. There were no significant differences in seasonal rainfall or es timated water loss between the two watersheds, but water use efficiency for ridge-tillage (22.8 kg ha(-1) mm(-1) yr(-1) or 517 lb ac(-1) in(-1) yr(-1) ) was significantly greater than for conventional tillage (20.3 kg ha(-1) m m(-1) yr(-1) or 460 lb ac(-1) in(-1) yr(-1)). The estimated N removal by cr op harvest accounted for 48 and 59% of the applied N. Ridge-till reduced ru noff, increased infiltration, and reduced water loss due to greater residue ; therefore, greater water use efficiency was observed. The ridge-till fiel d had less residual NO3-N in the root zone for subsequent leaching, but hig her N concentrations in the baseflow since average N fertilization rates ex ceeded average crop removal by 68 kg ha(-1) yr(-1). This interaction demons trates the importance of balancing both N and water we efficiencies to achi eve a sustainable production system.