Nitrate Leaching under a Cereal Rye Cover Crop

Abstract Winter cover crops hold potential to capture excess NO 3 − and reduce leaching by recycling nutrients. The objective of this study was to compare winter NO 3 ‐N leaching losses under winter‐fallow and a winter cereal rye ( Secale cereale L.) cover crop following the harvest of sweet corn ( Zea mays L.) or broccoli ( Brassica oleracea var. italica Plenck). Leachate was sampled with passive capillary wick samplers that apply a suction of 0 to 5 kPa to the soil‐pore water and intercept leachate in a pan of known area. Without disturbing the over‐laying soil profile, 32 samplers (0.26 m 2 ) were installed at a depth of 1.2 m in a Willamette loam (fine‐silty mixed mesic Pachic Ultic Argixeroll). The randomized complete‐block split plot design of this cover crop‐crop rotation study (initiated in 1989) has cropping system (winter fallow vs. winter cereal rye) as main plots and three N application rates, ranging from 0 to 280 kg N ha −1 yr −1 , as subplots. At the recommended N rate for the summer crops, NO 3 leaching losses were 48 kg N ha −1 under sweet corn‐winter‐fallow for winter 1992–1993, 55 kg N ha −1 under broccoli‐winter‐fallow for winter 1993–1994, and 103 kg N ha −1 under sweet corn‐winter‐fallow for winter 1994–1995, which were reduced to 32, 21, and 69 kg N ha −1 , respectively, under winter cereal rye. For the first two winters, most of the variation (61%) in NO 3 − leaching was explained by N rate (29%), cereal rye N uptake (17%), and volume of leachate (15%). Seasonal, flow‐weighted concentrations at the recommended N rate were 13.4 mg N L −1 under sweet corn‐winter‐fallow (1992–1993), 21.9 mg N L −1 under broccoli‐winter‐fallow, and 17.8 mg N L −1 under sweet corn‐winter‐fallow (1994–1995), which were reduced by 39, 58, and 22%, respectively, under winter cereal rye.