Leguminous Cover Crops As an Attempt to Combine Catch Crops and Green Manure.

Legumes used as nitrate catch crops and green manures have to fulfil two contradictory simultaneous goals, namely to decrease nitrate leaching risk and to enhance the N supply to the succeeding crop. Mixtures of legumes and non-legumes have been promoted as a strategy to achieve these goals. During 2013-15, two-component mixtures and their pure stands were tested in four Danish field experiments. Plots with bare soil (herbicide-treated) were included as control. Nitrate leaching was measured by ceramic suction cups and the results of species competition in the two mixtures were measured by aboveground biomass sampling. When grown in pure stands, white clover (Trifolium repens) on coarse sand and common vetch (Vicia sativa) on sandy loam were less effective at reducing nitrate leaching than the non-legumes perennial ryegrass (Lolium perenne) or fodder radish (Raphanus sativus). When the competitiveness of the species in mixtures was in balance or favored the non-legume, leaching was not significantly different from the non-legume in the pure stand. In one experiment the non-legume in the mixture was almost outperformed by the legume with uptake of 100 kg N/ha in vetch and 8 kg N/ha in fodder radish, while fodder radish in the pure stand contained 50 kg N/ha. Leaching from the mixture was not significantly different from vetch in pure stand and significantly higher than from fodder radish in pure stand. Yield in the following spring barley was not significantly different from the yield in plots with previously bare soil. It is concluded that the use of mixtures of legumes and non-legumes as nitrate catch crops may not be in competitive balance and they may increase the risk of nitrate leaching without enhancing the N supply for the succeeding crop.