Effects of red clover decomposition on phytotoxicity to wild mustard seedling growth

Previous studies have suggested that phenolics from legume green manures may contribute to weed control through allelopathy. The objective of this study was to determine how decomposition of red clover (Trifolium pratense L.) residue affects phytotoxicity to a weed species, wild mustard (Sinapis arvensis L.). Red clover (1.25 g), 30 g acid washed sand, and 5 ml of microbial inoculant were incubated in the dark at 14◦C. The incubations were conducted in sand to allow the determination of decomposition phytotoxicity effects without being confounded by soil sorption effects. Aqueous extracts were collected at 0, 1, 2 and 5 weeks. The extracts were analyzed for dissolved organic carbon (DOC) and phenolic carbon (PhC). Phytotoxicity of the extracts was estimated by comparing root elongation under sterile laboratory conditions. The red clover extract significantly reduced radicle growth in the bioassay, but there was no significant change in the degree of phytotoxicity over the 5-week decomposition period. Although the concentration of DOC decreased with increasing decomposition, the PhC concentration was not significantly affected. This suggests that in the absence of soil sorbants, the phytotoxic compounds are stable throughout the 5-week incubation period and are phenolic in nature. This study demonstrates that the potential exists for using legume green manures to be a component of a sustainable weed management system, but indicates that soil characteristics need to be factored in evaluating the potential for this weed control tool. © 2001 Elsevier Science B.V. All rights reserved.