Expression of allelopathy in the soil environment: soil concentration and activity of benzoxazinoid compounds released by rye cover crop residue

Rye (Secale cereale L.) residue is known to suppress weeds in agronomic environments and to produce benzoxazinoid (BX) compounds which are phytotoxic. Experiments were conducted to determine the duration of indicator plant inhibition and BX soil concentrations in response to field incorporated or surface rye residue. Surface rye residue was highly inhibitory to lettuce (Lactuca sativa L.) and smooth pigweed (Amaranthus hybridus L.) throughout an experimental period of 4 weeks, whereas soil removed from beneath residue and assayed in pots had little phytotoxicity, suggesting that physical rather than chemical mechanisms were involved. Incorporated residue inhibited lettuce and pigweed for approximately 2 weeks after incorporation, which corresponded to the period when elevated BX levels were detected in soil, suggesting potential allelopathy. The most toxic BX compounds, APO, DIBOA, and DIMBOA, were present at relatively low levels, whereas the less toxic compounds, BOA and MBOA, and the non-toxic compounds, HBOA and HMBOA, were the predominant BX species in amended soils. When the benzoxazolinones BOA and MBOA were exogenously added to soils to maintain extractable levels of up to 10 mu g g(-1) soil (100-500 times higher than measured BX in field soils), no significant inhibition of pigweed plants was observed. This result indicated that the observed association between the duration of plant inhibition and BX from incorporated rye was not causal, and that other compounds released with similar dynamics were likely responsible. This approach provides a sound basis for demonstrating the presence of allelopathy in natural or managed ecosystems.