Phytotoxic effects of red clover amended soils on wild mustard seedling growth

Previous studies have suggested that phenolics from legume green manures may contribute to weed control through allelopathy. The objective was to determine if red clover (Trifolium pratense L,) residue amended field soils expressed phytotoxicity to a weed species, wild mustard (Sinapis arvensis L.). Field plots involving incorporation treatments of wheat (Triticum aestivum L,) stubble or wheat stubble plus 2530 kg ha(-1) red clover residue, were sampled at -12, 8, 21, 30, 41, 63, and 100 days after residue incorporation (DAI). Soil-water extracts (1 : 1, m: v) were analyzed for plant nutrients and phenolic content. Phytotoxicity of the extracts was measured using a laboratory wild mustard bioassay. There was a 20% reduction of radicle growth in the green manure treatment in comparison with the wheat stubble treatment, but only at the first sample date after residue incorporation (8 DAI). The radicle growth reduction had the highest correlation with the concentration of soluble phenolics in the soil, water extracts. Bioassays using aqueous extracts of the clover shoots and roots alone predicted a radicle growth reduction of 18% for the quantity of clover amendment rate used in the field plots. The close agreement of the predicted and observed root growth reduction at 8 DAI further supports clover residue as the source of the phytotoxicity. This study demonstrates that the potential exists for using legume green manures to reduce the amounts of synthetic herbicides needed for weed control. (C) 2000 Elsevier Science B.V. All rights reserved.