Leaching of Nitrogen and Phosphorus from the Biomass of Three Cover Crop Species

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Abstract
Cover crops, although they may reduce the leaching of NO3‐N to groundwater, may increase nutrient concentration in runoff because of biomass leaching during rainfall events. A simulated rainfall study was conducted 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 red 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 after the equivalent of 6.0 cm of simulated rainfall. Concentrations of NO3‐N were similar to P but changed less during leaching. Weighted mean 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 red clover. Drying the sample in addition to freezing increased the P concentration, particularly in the initial leaching. About 30% of the biomass P was leached from oilseed radish and annual ryegrass, but only about 20% from red clover. Because oilseed radish had a greater biomass P concentration, the total P leached was much greater than for the other two species. Between 5 and 9% of the biomass N was leached as either NO3‐N or NH4‐N from ryegrass and red clover while over 10% was leached 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.

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