NITROGEN AND WATER INTERACTIONS IN SUBSURFACE TRICKLE-IRRIGATED LEAF LETTUCE .2. AGRONOMIC, ECONOMIC, AND ENVIRONMENTAL OUTCOMES

Evaluation of trickle-irrigated crop production systems should address agronomic, economic, and environmental outcomes. The objectives of th is research were to: (i) determine plant N uptake, residual soil inorg anic N, unutilized fertilizer N, and unaccounted fertilizer N for subs urface trickle irrigated leaf lettuce (Lactuca saliva L. cv. Waldmann' s Green), and (ii) use spatial analysis techniques to simultaneously e valuate agronomic, economic, and environmental production criteria for leaf lettuce within one growing season. Field experiments were conduc ted using buried trickle irrigation during three winter growing season s in southern Arizona. Deficient to excessive N (35-300 kg ha(-1)) and target soil water tension (SWT) treatments (12.0-4.0 kPa) were applie d in factorial combinations each year. Fertilizer N recovery was deter mined by the difference method. Spatial analysis of response surfaces was used to determine overlap of zones with acceptable values for mark etable yield, net economic return, and unaccounted fertilizer N for le af lettuce during 1992-1993. Maximum unutilized fertilizer N was 216 k g ha(-1) for leaf lettuce, and maximum unaccounted fertilizer N was 14 9 kg ha(-1). Unutilized fertilizer N and unaccounted fertilizer N incr eased sharply when adequate N and water rates were exceeded. Spatial a nalysis of response surfaces for 1992-1993 showed a small region bound ed by 6.6 to 7.3 kPa SWT and 238 to 252 kg N ha(-1) that would have re sulted in >95% of maximum predicted marketable yield and net return wh ile limiting NO3-N concentrations in drainage water to less than or eq ual to 10 mg L(-1).