FUNGAL DIVERSITY IN WHEAT-BASED ROTATIONS

Crop rotation is a key strategy of sustainable agriculture in the Canadian Prairie. Improving crop productivity and yield stability in pulses-based cropping systems with better soil biology is the ultimate goal of this research. Firstly, my studies provide information on the effect of pulses on the biodiversity of soil fungi: arbuscular mycorrhizal fungi (AM) and non-AM fungi, associated with the main pulse species grown in the Canadian Prairie (field pea, lentil, and chickpea), and their influence on wheat-based cropping systems. Secondly, the optimum 4-yr crop rotation for wheat production was determined, based on the relationship among fungal communities associated to the different crops and the yield and quality of these crops. My research included two experiments. First, in a field experiment replicated in time and site, the effect of previous pulse crops on wheat root-associated microbial communities and crop performance was assessed in four 2-yr rotation systems. Second, a 4-yr field experiment evaluated the relative influence of eight different crop rotations on root-associated microbial communities and on wheat productivity in the last year of the rotations. A greenhouse assay was conducted to evaluate, under controlled conditions, the influence of the microbial communities selected by these previous field crop rotations on wheat performance, using soil from the field as inoculant. The response of root-associated microbial communities was characterized using next generation sequencing technologies, phospholipid fatty acid markers, microscopic observation of roots and soil dehydrogenase assay. Plant response was evaluated based on crop density, biomass, yield and tissue nutrient content. My studies showed that community composition of AM and non-AM fungal communities in the roots of wheat were largely influenced by host plant identity and environmental conditions. The structure of the overall fungal community in wheat roots was not affected by the previous crops. The soil microbial legacies of previous crops were different from the fungal communities found in the roots of the following wheat, suggesting that wheat, as a host plant, selects and associates with a specific fungal community. Seasonal variations in soil moisture, temperature, pH, and nutrient cycling between sampling times have a great influence on soil microbes and could also be influencing these effects. The 2-yr crop rotation experiment revealed that wheat after a pulse crop had higher plant density and produced more seed biomass and total yield. The 4-yr crop rotation studies revealed that, in the field, diversified rotations including pea or lentil in alternate years, largely contributed to wheat performance. However, rotations including chickpea contributed little to the rotation benefits, suggesting that a careful selection of plant species is essential to improve the performance of the agroecosystem. Contrary to the field results’ findings, under greenhouse conditions, rotations that included chickpea before wheat contributed the best to wheat productivity, suggesting that in the field, factors other than the microbial community selected by chickpea were responsible for the poor performance of chickpea-wheat rotations in the field. Soil bacterial and fungal biomasses were positively correlated with wheat yield in the field experiments, which suggests that an abundant and diversified microbial community positively influences wheat productivity. Also, possible antagonistic and synergistic interactions between different AM species and root pathogens could be inferred. These results suggest that many AM fungi can potentially contribute to combat pathogens and enhance plant performance, whereas other might produce detrimental effects on the plants. Overall my studies revealed that host plant identity and environmental conditions influence the fungal community structure and dynamics. The frequency and sequence of crops in the rotations strongly influences productivity in wheat based agroecosystems. Lentil and pea alternating with wheat largely contribute to wheat performance. Thus, the productivity of wheat can be improved by selecting and including the plant species most beneficial to the rotation in order to increase soil available water and N, while promoting beneficial microbial associations and reducing disease incidence