NITRATE LEACHING UNDER A CEREAL RYE COVER CROP

Winter cover crops hold potential to capture excess NO3- and reduce le aching by recycling nutrients. The objective of this study was to comp are winter NO3-N leaching losses under winter-fallow and a winter cere al rye (Secale cereale L.) cover crop following the harvest of sweet c orn (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 profil e, 32 samplers (0.26 m(2)) were installed at a depth of 1.2 m in a Wil lamette loam (fine-silty mixed mesic Pachic Ultic Argixeroll). The ran domized complete-block split plot design of this cover crop-crop rotat ion study (initiated in 1989) has cropping system (winter fallow vs. w inter 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 r ate for the summer crops, NO3 leaching losses were 48 kg N ha(-1) unde r 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 NO3- leaching w as explained by N rate (29%), cereal rye N uptake (17%), and volume of leachate (15%). Seasonal, Bow-weighted concentrations at the recommen ded N rate were 13.4 mg N L(-1) under sweet corn-winter-fallow (1992-1 993), 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.