Concern over nitrate contamination in drinking water has led to increased regulatory scrutiny of N fertilizer use by regional water quality agencies. A recent assessment of nitrate in groundwater for two of the most intensive agricultural production regions in California (the Salinas Valley and Tulare Basin) found that 51% of all N applied to cropland is leached to groundwater (Harter and Lund, 2012). As a result, many wells in these areas exceed the U.S. Environmental Protection Agency (EPA) drinking water standard for nitrate-N of 10 mg·L−1.
The high N requirement of cool-season vegetables produced in the coastal valleys of California results in a high nitrate leaching hazard. The mild year-round climate allows for the cultivation of two to three cool-season vegetable crops per year, with lettuce (Lactuca sativa) being the dominant crop in both value and acreage (Monterey County Agricultural Commission, 2012). Vegetable production fields often receive N fertilizer applications in excess of crop uptake or the amount of N needed for maximum yield and product quality (Bottoms et al., 2012; Breschini and Hartz, 2002; Hartz et al., 2000; Heinrich et al., 2013). This tendency for overfertilization occurs because N fertilizer is a small part of the overall production budget for the high-value vegetable crops grown in the region (e.g., Tourte and Smith, 2010) and growers do not want to risk an economic loss for these crops that have exacting market standards for size, color, and quality. Another factor leading to high postharvest nitrate levels is the fact that less than 50% of the crop biomass may be removed during harvest for some crops (Bottoms et al., 2012; Heinrich et al., 2013). The remaining crop residues typically have a high N content and rapidly breakdown after incorporation into the soil and release nitrate. As a result of these factors, high concentrations of residual nitrate may be present in the soil at the end of the vegetable cropping cycle and before the onset of the winter rainy season. Unless a winter vegetable or cover crop is planted, much of this end of season nitrate can be leached by winter rains (Wyland et al., 1996).
In the vegetable producing regions on the central coast of California, farmers almost exclusively grow cereal rye as a winter cover crop because of the low seed cost and it does not set seed too early in the growth cycle, eliminating the potential for it to become a weed hazard. In this region, a full-season cereal rye cover crop has been shown to effectively reduce nitrate leaching during the winter by 65% to 70% relative to land left bare fallow (Jackson et al., 1993; Wyland et al., 1996). Nitrogen scavenging cover crops reduce nitrate leaching through increased evapotranspiration (i.e., reducing soil moisture thus increasing water storage between rain events) and nitrate scavenging (Brennan et al., 2013; Jackson et al., 1993; Wyland et al., 1996). After incorporation into soil, decomposing cover crop can increase soil N available for subsequent crops (Jackson, 2000; Lundquist et al., 1999; Wyland et al., 1996), and over time may increase soil organic matter (Kong et al., 2005), and improve long-term nutrient cycling as well as many soil chemical and physical properties. An additional benefit is that winter cover crops have been shown to increase infiltration (Joyce et al., 2002) thereby reducing runoff, nutrient, and sediment losses (Smuckler et al., 2012).
Despite the benefits of an N scavenging winter cover crop, the proportion of vegetable acreage in cover crops in the coastal valleys of California is low (we estimate between 5% and 7%). During the winter, most of the vegetable acreage is left in listed (peaked/unshaped), bare-fallow beds. This simplifies soil preparation operations and allows for timely access to fields to meet late winter and early spring planting schedules. A Full-season cover crop requires additional tractor operations (i.e., mowing, multiple disking passes, listing, and bed shaping), and there is the risk that rain will prevent access to fields and delay plantings. Furthermore, by allowing a cover crop to grow to full-season, this may reduce the number of cash crops that can be planted in a year, which is difficult to do in the region because of the high land value resulting in high rental costs. A Full-season cover crop is therefore not compatible with much of the vegetable producing acreage in this region.
One possible strategy to derive some of the benefits of a winter cover crop yet have minimal residue to impede ground preparation operations for subsequent vegetable crops is to kill the cover crop with a herbicide when it is young [i.e., low carbon (C) to N ratio and low lignin content] so that it rapidly decomposes. For this strategy to most effectively reduce nitrate leaching, the cover crop should grow long enough to reduce soil nitrate concentrations through crop uptake and also deplete soil moisture (i.e., reduce soil moisture between rain events) before it is killed. But, it should not be allowed to grow too long and produce too much residue that could impede early spring land preparation operations. By killing when young, the cover crop will have a low C:N and low lignin content, and break down rapidly after killing. The objective of this study was to evaluate if an early killed cover crop management strategy could reduce nitrate leaching relative to vegetable production fields left bare fallow during the winter.
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