The Salinas Valley along the central coast of California produces more than 100,000 ha of cool-season vegetables per year. The mild Mediterranean climate allows an extended growing season, and production of two to three crops per year is the norm. With little precipitation during the main production season (March–November), frequent irrigation is required. Fields are typically fallow during the winter when most of the annual precipitation occurs. This intensive production system has led to serious environmental problems. Nitrate contamination of groundwater has become widespread; it has been estimated that NO3-N loading to groundwater exceeds 100 kg·ha−1·yr−1 (Rosenstock et al., 2014). Additionally, excessive extraction of groundwater for irrigation has resulted in significant seawater intrusion, endangering aquifers used for both irrigation and urban needs (Harter, 2015).
Extensive research has been conducted to improve N fertilization and irrigation management of lettuce (Lactuca sativa L.), the predominant crop in this region (Bottoms et al., 2012; Breschini and Hartz, 2002; Hartz et al., 2000; Jackson et al., 1994). Less attention has been given to cole crops (broccoli, cabbage, and cauliflower), which are important rotational crops. Cole crops represent both an opportunity and a challenge. They are relatively deep rooted (Greenwood et al., 1982; Thorup-Kristensen, 2001), have the ability to take up more than 300 kg N/ha (Bakker et al., 2009b; Bowen et al., 1999; Thompson et al., 2002a), and consequently may be able to scavenge residual SMN from a substantial depth. Conversely, a large amount of N is contained in crop residue after harvest, and rapid mineralization from this N-rich residue has been recognized as a nitrate leaching hazard (Bakker et al., 2009b; Congreves and Van Eerd, 2015; Everaarts, 2000).
Yield-maximizing N fertilization rates for cole crops have been reported to range from <150 to >400 kg·ha−1 (Bakker et al., 2009a; Bowen et al., 1999; Cutcliffe and Munro, 1976; Kahn et al. 1991; Sorenson, 1999; Thompson et al., 2000a, 2002a; Zebarth et al., 1991). This wide range undoubtedly reflects differences among studies in field environment, soil fertility, plant population, and yield potential. Residual SMN has been recognized as a major determinant of the N fertilizer requirement of cole crops (Bakker et al., 2009b; Everaarts and van den Berg, 1996; Heckman et al., 2002; Zebarth et al., 1995). In an intensive production system like that of the Salinas Valley, where high levels of residual SMN are common (Bottoms et al., 2012; Breschini and Hartz, 2002), efficient N management requires consideration of field-specific factors rather than prescriptive recommendations.
Irrigation management is closely linked to N efficiency in cole crop production (Thompson et al., 2000b, 2002b). Marketable yield is maximized by maintaining relatively low soil moisture tension (10–25 kPa; Smittle et al., 1994; Thompson et al., 2000a, 2002a), but irrigation in excess of crop evapotranspiration (ETc) can result in significant NO3-N leaching in heavily fertilized fields. Documentation is lacking on cole crop irrigation requirements under coastal California conditions, and on current irrigation practices.
Improved cole crop N management requires a more complete understanding of soil and crop N dynamics of these crops within the intensive crop production scheme of this region. This study was undertaken to document cole crop growth, rooting depth, N uptake and partitioning, and patterns of soil N availability in representative commercial fields of the Salinas Valley.
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