Legume Cover Crop Residues for Weed Management through Chemical Suppression of Weed Seedling Germination and Root Growth

Legume cover crops and residues perform useful agroecological functions including improvement of soil texture, enhancement of soil N and water availability, and reduction of erosion. Covers are also beneficial for the suppression of weeds either through competition for resources or release of phytotoxic metabolites from crop residues. Allelochemicals associated with cover crop residues generally suppress weed seed germination and seedling establishment by impacting root growth and their activity may be modulated by soil microbiota. A unique in vitro soil bioassay was utilised to evaluate the suppression provided by Australian legume residues with or without soil microbiota. Dried field-grown cover crop residues (including clover spp., biserrula, French and yellow serradella, vetch, field and chick pea) were evaluated for weed suppression at various residue rates using a comparative control treatment of inert cellulose. Soil type and presence of soil microbiota on residue activity was evaluated with selected indicator species [barnyard grass (Echinochloa crus-galli), annual ryegrass (Lolium rigidum), cress (Lepidium sativum), and radish (Raphanus raphanistrum)]. Methanolic extracts of cover crop residues were further subjected to untargeted metabolic profiling by UPLC- ESI MS QToF to identify phytotoxic metabolites using bioassay-guided stepwise linear regression. Reduction in seed germination and radicle length was dependent on residue rate and crop, with biserrula, yellow serradella, subterranean clover and vetch most suppressive of seedling root growth. Residues selectively impacted growth of seedling indicators, i.e. chickpea and field pea suppressed dicot weeds more than monocots. Soil type impacted suppression, with greatest growth reduction observed in red sodosol soils vs. those with higher OM content. Removal of microbiota by soil pasteurization resulted in decreased phytotoxicity of residues, indicating that microbial activity was instrumental in suppression of weed seedling germination and root growth. Metabolic profiling revealed that phytotoxicity was strongly correlated with the presence of certain flavonoids in extracts of legume residues.