Abstract
AbstractLegume cover crops can supply a significant amount of nitrogen (N) for cash crops, which is particularly important for organic farmers. Because N mineralization from cover crop residue depends on the amount of biomass, cover crop quality, as well as environmental conditions such as soil moisture and temperature, predicting the amount of N mineralized and the timing of release has been difficult. We have developed a Cover Crop Nitrogen Calculator based on the N subroutine of the CERES crop model and evaluated the use of the predicted N credits on yields of fall broccoli [Brassica oleracea L. (Italica group)] at a research farm and two working farms. Research farm trials consisted of a cowpea (Vigna unguiculata L. Walp.) cover crop and no cover crop treatments, each at four N rates (0N, 0.5N, 1N and 1.5N, with 1N the target N rate of 112 kg N ha−1 in 2013 and 168 kg N ha−1 in 2014 and 2015) in a randomized complete block design. On-farm trials consisted of a cowpea or sunn hemp (Crotolaria juncea L.) cover crop at four N rates (0N, 0.5N, 1N and 1.5N) and no cover crop treatment at the 1N rate in a completely randomized design. Cover crop biomass and quality (N%, carbohydrates%, cellulose% and lignin%) were measured and used with a 5-yr average soil moisture and soil temperature from a local weather station to predict an N credit. In the cover crop treatments, the N rate was modified by the predicted credit, while the no cover crop treatment received the full N fertilizer rate either as feathermeal (certified organic fields) or as urea (conventional field). At the research farm, broccoli yield increased up to the 0.5N rate, and there was no difference in yield between the no cover 0.5N rate and the cover crop 0.5N rate in 2013, 2014 and 2105. On-farm, we saw an N response in two site-years. In these site-years, there was no difference between the no cover 1N rate and the cover crop 1N rate. At the third site-year, no N response was seen. Overall, our results showed using the cover crop credit predicted by the Calculator did not reduce yields. The use of a decision support tool such as the Calculator may help farmers better manage N fertilizer when cover crops are used, and increase cover crop adoption.
Highlights
Cover crops provide many benefits to producers (Abdul-Baki et al, 1997; Burket et al, 1997; Wang et al, 2008; Coombs et al, 2017)
There were two on-farm experiments: Crystal Organics Farm (CFarm)—a certified organic farm located in Newton County, Georgia (Piedmont physiographic region) and Wide Bottom Farm (WBFarm)—a small-scale, conventional truck farm located in Habersham County, Georgia in the foothills of the Appalachian Mountains
Environmental conditions In 2013, volumetric soil water content (SWC) and soil temperature (ST) immediately after cover crop termination were similar to the 5-yr average (Figs. 2a and 3a)
Summary
Cover crops provide many benefits to producers (Abdul-Baki et al, 1997; Burket et al, 1997; Wang et al, 2008; Coombs et al, 2017). Yields may be increased or decreased depending on the cover crop species and quality (Wyland et al, 1996; Burket et al, 1997; Muramoto et al, 2011; Brennan and Boyd, 2012; Brennan et al, 2012; Luna et al, 2020), soil fertility (Muramoto et al, 2011), cover crop management such as surface residue vs incorporation (Wang et al, 2008; Finney et al, 2009; Jahanzad et al, 2016) as well as synchronization of N release with the N demand of the crop (Griffin and Hesterman, 1991; Schellenberg et al, 2009). It is difficult to meet the entire N requirement for high N demand crops with N derived from cover crops (Schellenberg et al, 2009); in certified organic production systems, cover crops are an economically important source of N (Muramoto et al, 2011)
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