if (i) sufficient heat units are available to promote cover crop growth, root exploration, and N uptake during fall and winter; and (ii) mineralization rates of the green-Overwinter N leaching increases the potential for NO 3 contamination of ground water under irrigated desert soils. Our objectives were manured residues in the spring coincide with the timing to (i) identify winter cover crops for recovering N and (ii) determine of cash crop N demand. Effective fall-sown cover crops the N availability following green manure cover crops relative to N should exhibit rapid germination, aggressive and extenuptake by a succeeding potato (Solanum tuberosum L.) crop. A 2-yr sive rooting systems (Sainju et al., 1998), good winter field study was conducted on commercial fields on Quincy loamy hardiness, and early spring regrowth. Cereal and brassand (mixed mesic Xeric Torripsamments). Cover crops were seeded sica crops display these characteristics and are well following sweet corn (Zea mays L.) as fall-incorporated sudangrass suited for winter cover cropping (Wagger and Mengel, (Sorghum bicolor L., 'Sordan'); fall-and spring-incorporated white 1988; Brinsfield and Staver, 1991). These cover crops mustard (Brassica hirta Moench, 'Martegena'); and spring-incorpocan accumulate up to 150 kg N ha Ϫ1 (Hoyt and Mikrated wheat (Triticum aestivum L., 'Stevens'), rapeseed (B. napus kelsen, 1991; Shennan, 1992; Ditsch et al., 1993), with L., 'Jupiter'), and rye (Secale cereale L.). Potato ('Russet Burbank') was planted 3 to 5 wk after spring incorporation of green manures. rooting systems reaching depths of between 80 and 150All cover crops except sudangrass accumulated 112 to 142 kg N ha Ϫ1 cm Sarrantonio, 1992). when planted in August at Plymouth, WA, but N uptake was de-Following green manure incorporation, cover crops creased by Ͼ50% when planted in September at Quincy, WA. Overmay supply 20 to 55% of the recovered N to the subsewintering cover crops lowered soil mineral N at 0 to 180 cm by 155 quent crop (Sims and Slinkard, 1991; Malpassi et al., kg N ha Ϫ1 compared with bare fallow at Plymouth. Fall-incorporated 2000). Subsequent fertilizer requirements can be greatly mustard released greater inorganic N over the winter compared with reduced or eliminated (Griffin et al., 2000) if N release is spring incorporation. Soil NH 4 and NO 3 increased following overwinsynchronized with N demand of the succeeding summer tering cover crops by potato planting, providing timely increases in crop (Stute and Posner, 1995). Poor synchronization N availability to the potato crop. Winter cover crops can improve N or delays in N mineralization of cover crop residue, cycling and reduce the amount of N below the root zone in potatobased rotations.

Microsprinkler irrigation may result in increased efficiency of N and water application to citrus. However, best management practices (BMPs) have not yet been developed for microsprinkler use, particularly on newly established citrus. Experiments were conducted during 1997-98 in central Arizona to evaluate the effects of N rate and fertigation frequency on `Newhall' navel oranges (Citrus sinensis) planted in Mar. 1997. Two experiments were conducted, each with factorial combinations of N rate (0 to 204 g/tree/year) and fertigation frequency (weekly to three times per year). In one experiment, nonlabeled N fertilizer was used, and in the other 15N-labeled fertilizer was used. Trunk diameter, leaf N, and 15N partitioning in the trees were monitored. During 1997, neither trunk diameter nor leaf N were affected by N rate or fertigation frequency. No more than 6% of N applied was found in the trees. During 1998, leaf N in fertilized plots was significantly higher than in nonfertilized plots, but leaf N in all trees remained above the critical N concentration of 25 mg·g-1. During 1998, no more than 25% of the fertilizer N applied was taken up by the trees. Results suggest that N applications are not needed during the first growing season after planting for microsprinkler-irrigated citrus in Arizona. Only low rates of N (≤68 g/tree/yr) may be needed during the second growing season to maintain adequate tree N reserves.

Overwinter N leaching increases the potential for NO3 contamination of ground water under irrigated desert soils. Our objectives were to (i) identify winter cover crops for recovering N and (ii) determine the N availability following green manure cover crops relative to N uptake by a succeeding potato (Solanum tuberosum L.) crop. A 2‐yr field study was conducted on commercial fields on Quincy loamy sand (mixed mesic Xeric Torripsamments). Cover crops were seeded following sweet corn (Zea mays L.) as fall‐incorporated sudangrass (Sorghum bicolor L., ‘Sordan’); fall‐ and spring‐incorporated white mustard (Brassica hirta Moench, ‘Martegena’); and spring‐incorporated wheat (Triticum aestivum L., ‘Stevens’), rapeseed (B. napus L., ‘Jupiter’), and rye (Secale cereale L.). Potato (‘Russet Burbank’) was planted 3 to 5 wk after spring incorporation of green manures. All cover crops except sudangrass accumulated 112 to 142 kg N ha−1 when planted in August at Plymouth, WA, but N uptake was decreased by >50% when planted in September at Quincy, WA. Overwintering cover crops lowered soil mineral N at 0 to 180 cm by 155 kg N ha−1 compared with bare fallow at Plymouth. Fall‐incorporated mustard released greater inorganic N over the winter compared with spring incorporation. Soil NH4 and NO3 increased following overwintering cover crops by potato planting, providing timely increases in N availability to the potato crop. Winter cover crops can improve N cycling and reduce the amount of N below the root zone in potato‐based rotations.

Citrus production in the southwestern U.S. is highly dependent on inputs of irrigation and N fertilizer to achieve optimum fruit yield and quality. Microsprinkler irrigation may allow for substantial increase in efficiency of N and water application. However, best management practices have not yet been developed for microsprinkler use, particularly on newly established citrus trees. Experiments were conducted during 1997–98 in central Arizona to evaluate the effects of various N rates and fertigation frequencies on growth and N partitioning in young `Newhall' navel oranges planted in Apr. 1997. Two experiments were conducted, each with factorial combinations of N rate and fertigation frequency. In one experiment, non-labeled N fertilizer was used and in the other 15N-labeled N fertilizer. Trunk diameter, leaf N, and 15N partitioning in the trees were measured. During 1997, neither trunk diameter or leaf N were affected by N rate or fertigation frequency. No more than 6% of the N applied was taken up by the trees, and about 50% of the fertilizer N taken up was found in the leaves. Trees grew much more rapidly in 1998. Leaf N in fertilized plots was significantly higher than in control plots, but leaf N in all trees remained above the critical level of 2.5%. Despite rapid tree growth during 1998, no more than 25% of the fertilizer N applied was taken up by the trees. About 60% of the fertilizer N taken up was found in the leaves. Results suggest that N applications are not needed during the first growing season after planting for microsprinkler-irrigated citrus trees in the Southwest. Only modest rates (68 to 136 g/tree) will be needed during the second season to maintain adequate tree reserves.