Nitrogen and Water Interactions in Subsurface Drip-Irrigated Cauliflower II. Agronomic, Economic, and Environmental Outcomes

BMPs (Arizona Legislature, 1987). Best management practices are designed to maintain or enhance yields Water 3 N rate experiments were conducted on subsurface dripand profitability, and to minimize future additions of N irrigated cauliflower (Brassica olearacea L. var. botrytis L.) during three winter growing seasons in southern Arizona. A range of water to groundwater. The use of subsurface drip irrigation and N rates were selected to permit the calculation of appropriate offers the potential for increased waterand N fertilizerwater 3 N production functions. The objectives were to (i) determine use efficiency and is increasing in the desert Southwest the effects and interactions of irrigation water and N inputs on crop and California. Currently 3600 ha in Arizona and N uptake, residual soil NO3–N, N-use efficiency, and unaccounted 22 300 ha in California are irrigated with subsurface drip fertilizer N, and (ii) evaluate agronomic, economic, and environmental systems (Anonymous, 1994; Anonymous, 1998). Several production criteria during three growing seasons. Spatial analysis was recent studies have illustrated the efficient nature of used to identify overlap of acceptable zones of marketable yield, net subsurface drip irrigation for delivery of water and nutrireturn, and unaccounted fertilizer N within each growing season. ents (Pier and Doerge, 1995b; Thompson and Doerge, Acceptable yields and net return were defined as $95% of the maximum predicted response within the range of the treatments; acceptable 1996b). unaccounted fertilizer N was defined as #40 kg ha21. Net returns and Evaluation of any crop production system should adaboveground plant biomass N were significantly affected (P , 0.01) dress agronomic, economic, and environmental outby N rate and in 2 yr by irrigation. There were also significant irrigation comes. Drip irrigation allows great flexibility in both treatment 3 N rate interactions for net returns and biomass N. Residwater and N management. Water and N are the two ual soil NO3–N concentrations increased with N rate and decreased inputs to irrigated cropping systems that have the most with soil water tension (SWT). Average amounts of residual soil impact on agronomic, economic, and environmental NO3–N (0–0.9 m) for the highest N rate during the three seasons were outcomes (Letey et al., 1977). These three criteria have 317, 296, and 180 kg ha21 for the low, medium, and high irrigation only recently been evaluated simultaneously for driptreatments, respectively. Unaccounted fertilizer N was significantly affected (P , 0.05) by irrigation treatment, N rate, and irrigation irrigated crops. The interactive effects of water and N treatment 3 N rate interactions each year. Overlap of acceptable zones management on yields are reported for drip-irrigated of marketable yields, net returns, and unaccounted N was achieved in corn (Zea mays L.) (Phene and Beale, 1976; Yanuka et one of the three years. The single combination of SWT and N rate al., 1982), tomato (Lycopersicum esculentum L.) (Barthat came closest to producing optimal or near-optimal agronomic, Yosef and Sagiv, 1982a, 1982b), celery (Apium graveoeconomic, and environmental outcomes in all three years was 10 to lens L.) (Feigin et al., 1982), watermelon (Citrillus lana12 kPa and 350 to 400 kg N ha21. tus [Thumb.] Matsu and Nakai) (Pier and Doerge, 1995b), leaf lettuce (Lactuca sativa L.) (Thompson and Doerge, 1996a), romaine lettuce (Thompson and C over the impacts of agricultural pracDoerge, 1995a), collard (Brassica oleracea L. var. acephtices on the environment is increasing. These conala DC., p.p.), mustard (Brassica juncea [L.] Czerniak), cerns include the leaching of nitrate from crop producand spinach (Spinacea oleracea L.) (Thompson and tion areas into aquifers. Nitrate contamination of Doerge, 1995b). aquifers is especially pronounced in the irrigated SouthThere is a general lack of information regarding the west. The percentage of wells testing above the federal effects of N and water management for drip-irrigated drinking water standard of 10 mg NO3–N L21 in Arizona, cauliflower production. Therefore, additional research California, and Texas ranges from 9.4 to 13.9%. In conis needed to examine the agronomic, economic, and trast, an average of 6.4% of all wells sampled in the environmental response of this crop to N and water U.S. were above 10 mg L21 (Fedkiw, 1991). inputs under subsurface drip irrigation. We used the The approach to minimizing groundwater pollution methods of Pier and Doerge (1995a) and Thompson and with nitrate in Arizona involves the use of best manageDoerge (1996b) to simultaneously evaluate marketable ment practices (BMPs) (Doerge et al., 1991). These yield, net economic return, and unaccounted fertilizer include attention to rate, timing, and placement of N N for subsurface drip-irrigated cauliflower. fertilizers and irrigation water and utilization of approThe objectives of this study were to (i) determine priate tillage practices. Growers who apply N fertilizers the effects and interactions of irrigation water and N are mandated to demonstrate compliance with these inputs on crop N uptake, residual soil NO3–N, N-use efficiency, and unaccounted fertilizer N in subsurface T.L. Thompson, Dep. of Soil, Water, and Environmental Science, Univ. of Arizona, 429 Shantz Building, Room 38, Tucson, AZ 85721; drip-irrigated cauliflower and (ii) use spatial analysis T.A. Doerge, Pioneer Hi-Bred International, Inc., P.O. Box 1150, techniques to simultaneously evaluate agronomic, ecoJohnston, IA 50131; R.E. Godin, Western Farm Service, 24730 Ave. 13, Madera, CA 93637. Received 21 Aug. 1998. *Corresponding author (thompson@ag.arizona.edu). Abbreviations: ANUE, apparent N-use efficiency; BMP, best management practices; SWT, soil water tension. Published in Soil Sci. Soc. Am. J. 64:412–418 (2000).