Impact of arbuscular mycorrhizal fungi on crop and weed growth: potential implications for integrated weed managment

With the growing problem of herbicide-resistant weeds, efficient and environmentally friendly non-chemical weed control approaches are needed to develop multi-tactic integrated weed management (IWM) programs. Using soil microbes for weed suppression has been considered as one of the non-chemical approaches, owing to the microbial potential for reducing weed growth and influencing weed community dynamics. However, successful use of soil microbes as classical biocontrol agents, which rely on importing control agents in the introduced environment, has been rare in agricultural fields. This is because agricultural practices can strongly change the soil and environmental properties in a way that reduce the population density of an introduced agent and result in an insufficient level of weed control. Therefore, conservation biocontrol approaches based on manipulating the population of residential enemies of target species that are well adapted to local environments, may hold more promise. In this study, I investigated whether arbuscular mycorrhizal fungi (AMF), a group of widely distributed soil fungi, can have suppressive effects on weeds under field conditions varying in soil management practices, and thereby contribute to IWM in agricultural systems. AMF form symbiotic associations with more than 80% of land plants, and can strongly influence plant growth through impacts on nutrient and water uptake, stress tolerance, and pathogen defense. AMF can negatively influence the growth of some weeds, including many non-host weeds and some host weeds. Non-host weeds, which are resistant to the colonization of AMF, tend to have negative responses when interacting with AMF, based on several mechanisms. AMF can also inflict costs on host weeds when external nutrients, particular phosphorus (P), are abundant, or when the host weed is physically near a host crop species. Based on these facts, I hypothesized that AMF have the potential to contribute to weed control through direct and indirect pathways: directly suppress non-host weeds by influencing their root development, and indirectly suppress some host weeds mediated by competitive effects exerted by strong neighboring host crops. In this study, I used three quantitative and experimental approaches to test these hypotheses. First, a systematic meta-analysis was conducted to identify patterns in crop and weed responses to AMF from an intensive survey of published data. The effects of crop and weed host status (strong and weak AMF hosts are divided in this study by a 10% root length colonization threshold), AMF diversity (single vs. mixed), and soil nitrogen (N) and P fertility management on plant mycorrhizal growth responses (MGR) was quantified. Results from meta-analysis supported the first hypothesis demonstrating that AMF had a significantly negative impact on the growth of weak host weeds. In addition, in the presence of N and P fertilizers, strong host crops had a stronger positive response to AMF than strong host weeds, which supported the second hypothesis. Second, a field experiment was carried out to test whether the suppressive effects of AMF on weed growth were consistent under field conditions when soil properties were manipulated by tillage and cover crop treatments. Five weed species were planted within combinations of two tillage and cover crop treatments at two locations: Illinois (IL) and Minnesota (MN). Structural equation modeling approach was used to quantify the direct and indirect impact of AMF, soil inorganic N, and soil physical properties on weed growth. Results showed that although AMF colonization rate showed slightly negative effects on the growth of some weed species, the relationships were not significant under field conditions. In addition, the relative importance of AMF on weed growth was weaker than that of soil properties. Third, I investigated whether managing the community structure of AMF can enhance their potential for weed control. A two-year field experiment was conducted in IL, and five weed species were planted in a maize field. The experimental design was the same as the second study. AMF community DNA in weed and maize roots was extracted and sequenced. Multivariate analyses were carried out to identify AMF species that have selective effects on the growth of maize and weed species as well as quantify the relative importance of plant identity, tillage, and cover cropping in influencing AMF community structure. The results demonstrated that several AMF taxa could suppress the growth of specific weeds without influencing the growth of maize. Tillage and cover cropping made significant contributions to AMF community structure, but their effect sizes were small, indicating a limited possibility of using tillage and cover cropping to manage AMF communities. Approaches to weed management based on direct augmentation (e.g., inoculation with particular AMF strains) of the weed-suppressing AMF taxa may hold more promise. Overall, the results from three studies demonstrated that AMF have direct or indirect impacts on weed growth based on weed identity, host status, and environmental context. Although the effect of the AMF community as a whole on weed growth may be not significant under field conditions, increasing the abundance of potential weed-suppressive AMF species in the AMF community through direct augmentation (e.g., inoculation with particular AMF strains) may help to improve the control potential of AMF on specific weed species. Additional research supporting improved weed control by AMF, and the incorporation of other biological, physical, and chemical control methods will be necessary to make AMF a viable part of the IWM toolkit