Submitted to: Plant Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/28/2012
Publication Date: 5/26/2012
Citation: Teasdale, J.R., Rice, C., Cai, G., Mangum, R.A. 2012. Expression of allelopathy in the soil environment: Soil concentration and activity of benzoxazinoid compounds released by rye cover crop residue. Plant Ecology. DOI: 10.1007/s11258-012-0057-x. Interpretive Summary: Rye cover crops are known to provide many ecological services to agroecosystems including weed suppression in subsequent cash crops caused by decomposing rye residue. Rye produces phytotoxic benzoxazinoid compounds which are thought to explain observed weed suppression by this species. ARS researchers at Beltsville, Maryland, showed that the most abundant benzoxazinoids found in soils were not present in sufficiently high concentrations to account for the observed levels of weed suppression. Results suggest that, within the soil environment, the associations between weed suppression and rye benzoxazinoid concentrations were probably not causal, and that other compounds released with a similar dynamics were likely responsible. These results will require plant scientists to reevaluate the phytotoxins responsible for weed suppression by rye and give closer attention to the behavior and availability of these compounds in soil.
Technical Abstract: The activity of allelopathic compounds is often reduced in the soil environment where processes involving release from donor plant material, soil adsorption and degradation, and uptake by receptor plants naturally result in complex interactions. Rye (Secale cereale L.) cover crops are known to suppress weeds in agronomic environments and to produce benzoxazinoid (BX) compounds which are phytotoxic to indicator plants in laboratory assays. Experiments were conducted to determine the duration of indicator plant suppression and BX soil concentrations in response to field incorporated or surface applied rye residues. Surface rye residue was highly suppressive to lettuce (Lactuca sativa L.) and smooth pigweed (Amaranthus hybridus L.) indicator plants throughout the four weeks of this assay whereas soil removed from under residue had little phytotoxicity, suggesting that physical rather than chemical mechanisms were involved. However, suppression of lettuce and smooth pigweed persisted for approximately two weeks after rye incorporation, which corresponded to the period when elevated levels of BX compounds were detected in soil, suggesting the potential for allelopathic involvement. Total maximum soil concentrations of all BX compounds ranged from 27 to 136 ng g-1 across sites and years during the first two weeks after rye termination. The benzoxazolinones, BOA and MBOA, two of the most abundant BX species found in these soils, were exogenously added to soils to maintain continuous extracted levels up to 10 µg g-1 soil (70 to 370 times higher than total extracted BX compounds measured in field soils), but no significant inhibition was observed on smooth pigweed assay plants. This result indicates that the association between the duration of indicator plant suppression and BX concentration for field incorporated rye was not causal, and other compounds released with a similar dynamics as BX were likely responsible.