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

Previous studies have suggested that phenolics from legume green manures ma y contribute to weed control through allelopathy. The objective was to dete rmine if red clover (Trifolium pratense L,) residue amended field soils exp ressed 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 l aboratory wild mustard bioassay. There was a 20% reduction of radicle growt h in the green manure treatment in comparison with the wheat stubble treatm ent, but only at the first sample date after residue incorporation (8 DAI). The radicle growth reduction had the highest correlation with the concentr ation of soluble phenolics in the soil, water extracts. Bioassays using aqu eous extracts of the clover shoots and roots alone predicted a radicle grow th reduction of 18% for the quantity of clover amendment rate used in the f ield plots. The close agreement of the predicted and observed root growth r eduction at 8 DAI further supports clover residue as the source of the phyt otoxicity. This study demonstrates that the potential exists for using legu me green manures to reduce the amounts of synthetic herbicides needed for w eed control. (C) 2000 Elsevier Science B.V. All rights reserved.