LEACHING OF NITROGEN AND PHOSPHORUS FROM THE BIOMASS OF 3 COVER CROP SPECIES

Cover crops, although they may reduce the leaching of NO3-N to groundw ater, may increase nutrient concentration in runoff because of biomass leaching during rainfall events. A simulated rainfall study was condu cted in the laboratory to determine the effects of freezing alone and freezing plus drying treatments, loading rate, and rainfall intensity on the leaching of NH4-N, NO3-N, and inorganic P from the biomass of r ed clover (Trifolium pratense L.), annual ryegrass (Lolium multiflorum L.), and oilseed radish [Raphanus sativus (L.) var. oleifera Dc Metzg ]. Concentration of P increased during early stages of leaching, then decreased slowly but remained greater than 50% of the maximum, even af ter the equivalent of 6.0 cm of simulated rainfall. Concentrations of NO3-N were similar to P but changed less during leaching. Weighted mea n concentrations of P ranged from less than 2.0 to more than 15 mg L(- 1). Concentrations were greatest with oilseed radish and least with re d clover. Drying the sample in addition to freezing increased the P co ncentration, particularly in the initial leaching. About 30% of the bi omass P was leached from oilseed radish and annual ryegrass, but only about 20% from red clover. Because oilseed radish had a greater biomas s P concentration, the total P leached was much greater than for the o ther two species. Between 5 and 9% of the biomass N was leached as eit her NO3-N or NH4-N from ryegrass and red clover while over 10% was lea ched from oilseed radish. The results indicate that cover crop species differed markedly in their potential impact on nutrient concentration , particularly P, in runoff. Of the three cover crops tested, oilseed radish has the greatest potential impact and red clover the least.