Interactions between allelochemicals and the microbial community affect weed suppression following cover crop residue incorporation into soil

Authors: LOU, Y - University Of Illinois; Davis, Adam; YANNARELL, A - University Of Illinois

Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/25/2015
Publication Date: 2/2/2016
Citation: Lou, Y., Davis, A.S., Yannarell, A.C. 2016. Interactions between allelochemicals and the microbial community affect weed suppression following cover crop residue incorporation into soil. Plant and Soil. 399:357-371.

Interpretive Summary: Diversification of weed management practices within an integrated weed management approach can benefit both organic production systems, where lack of effective tools makes weed management a serious challenge, and conventional systems for which herbicide resistant weeds are becoming problematic. Use of green manures to suppress weeds is potentially useful weed management tactic with highly variable results. To better understand the factors underlying the variation in weed suppression by green manures, we set out to study how soil microorganisms interact with cover-crop derived phytotoxic compounds after green manure incorporation into the soil. Cover crop-derived allelochemicals strongly suppressed weed germination and growth during the early portions of the experiment, but this effect was short-lived. In contrast, soil microbial communities consistently suppressed weed germination. There was a negative interaction between microbes and allelochemicals, such that the combination was less suppressive than their separate contributions. Microbial activity can directly suppress weed germination and growth, but microorganisms can also indirectly help weeds by rapidly degrading cover crop-derived allelochemicals. As a result of these different roles, cover crop-derived weed suppression in agricultural soils shifts from an early allelochemical-dominated phase to a later phase where microbial suppression is more important. Future work that examines how methods of green manure incorporation into the soil affect the accessibility of allelochemicals to microbial modification may help maximize and prolong cover crop-derived weed suppression.

Technical Abstract: The objective of this study is to understand how soil microorganisms interact with cover crop-derived allelochemicals to suppress weed germination and growth following cover crop residue incorporation. We conducted a time series experiment by crossing sterilized and non-sterilized soil with four different residue treatments (no residues, water-soluble chemicals only, leached straw residues, and whole red clover residues). We measured weed seed germination rates, radicle elongation and disease incidence in these treatments using 7-day seed germination bioassays. We extracted and measured total phenolic compounds and nine cover crop-derived isoflavones in the soil. We partitioned the total weed suppression into three sources: microbial only inhibition, residue only inhibition, and the microbe-residue interaction. Cover crop-derived allelochemicals strongly suppressed weed germination and growth during the early portions of the experiment, but this effect was short-lived. In contrast, soil microbial communities consistently suppressed weed germination. There was a negative interaction between microbes and allelochemicals, such that the combination was less suppressive than their separate contributions. The negative interaction was strongest for water-soluble components of cover crop residues, while residue fractions containing intact plant tissues retained their suppressiveness over 30 days even in the presence of a live microbial community. Microbial activity can directly suppress weed germination and growth, but microorganisms can also indirectly help weeds by rapidly degrading cover crop-derived allelochemicals. As a result of these different roles, cover crop-derived weed suppression in agricultural soils shifts from an early allelochemical-dominated phase to a later phase where microbial suppression is more important. The timing of this phase transition depends in part on the accessibility of allelochemicals to microbial modification, and we propose that understanding microbe-chemical interactions can help maximize and prolong cover crop-derived weed suppression.