Mycorrhizal Fungi Colonization and Community Dynamics in a Long-Term Rotation As Influenced By Previous Crop and Weed Management Intensity.

Arbuscular mycorrhizal (AM) fungi are an important component of the soil-plant continuum and are influenced by many factors.  The effects of previous crop (flax or canola) and in-crop weed management intensity (low, medium, or high) on mycorrhizal colonization and community structure were investigated in a durum trap crop that was grown in soil from a 10 year-old rotation study.  The rotation study consists of a four year annual rotation (canola-wheat-flax-oats) with three different in-crop herbicide use intensities (2 of 4 –low, 3 of 4 – medium, or 4 of 4 years - high) that have established three different weed population densities over time.  In addition to mycorrhizal colonization and community structure using T-RFLP analysis, above and below-ground durum biomass and root length were determined at four developmental stages of the durum trap crop (3-leaf, 5-leaf, flowering and physiological maturity).  Few differences were found in the proportion of root colonization by AM fungi, although at the reproductive developmental stages of durum, roots were longer when grown in soil with reduced in-crop weed management and this influenced total colonized root length.  The structure of the community of AM fungi was affected by weed management intensity and previous crop; the latter changed over time.  During the early vegetative stage, the structure of the AM fungal community was similar after both flax or canola and then diverged during the later vegetative and early reproductive stages before converging again at physiological maturity. A number of the observed differences in AM fungal community structure were related to differences in community richness.  This study reiterated that AM fungal communities are complex and indicated that subtle changes in weed management contribute significantly to root growth and the dynamics of AM fungi.