Microbial communities play multiple weed control roles in green manured agroecosystems
Biological control of weeds may help to reduce the dependence on herbicides and may contribute an integrated, sustainable approach to weed control. The vast diversity of soil microorganisms represents an unexploited resource of potential biocontrol agents for weeds. In order to take advantage of these microbial agents, we need to know more about their identities and activities; about how they can be stimulated to provide weed control services; and about their potential for synergistic interactions with other weed control techniques like green manures. We propose that useful weed control microorganisms will possess one or more of the following desirable traits: 1) they inhibit weed growth or act as specific weed pathogens; 2) they can be enriched by the addition of green manures; and 3) they are keystone players in the soil microbial community. We conducted a weed germination experiment to test how weeds and microbes respond to the addition of whole green manure residues and water-soluble extracts of residue. We employed DNA sequencing of soil microbial communities and multivariate analysis to identify microorganisms associated with stunted and diseased weed seedlings. We identified microorganisms that played the following roles: inhibition of weed growth, promotion of weed disease, and degradation of green manure-derived allelochemicals. We also used network analysis to describe the interactions within the microbial community.
In general, different microorganisms were responsible for weed growth inhibition, disease promotion, and allelochemical degradation, but some microbes played multiple roles. We found that different microorganisms were stimulated to activity by different portions of green manure residues, and the addition of green manures stimulated microbial attack of germinating seedlings. In particular, green manure addition stimulated seedling attack by fungi in the classes Agaricomycetes and Sordariomycetes, and they stimulated bacteria in the orders Sphingobacteriales, Rhizobiales and Pseudomonas. Disease-causing and growth-suppressing organisms appeared to interact in species-poor but clearly defined networks. In contrast, networks of allelochemical degrading microorganisms were much more species rich, with many interconnections. This suggests that allelochemical degradation, which will diminish the weed control effectiveness of green manures, results from a highly connected set of interactions among diverse microorganisms, while microbial attack and growth suppression of weed seedlings results from a small set of key microbes that can be simulated by green manure addition. Maximizing microbial biocontrol of weeds in green manured agroecosystems will require management techniques that stimulate these key microbial antagonists while inhibiting the microbes that degrade green manure-derived allelochemicals.