Rotational Effects of Oilseed Crops on Subsequent Wheat Crops As Related to the Soil Microbial Community.

With increasing acreage of canola (Brassica napus L.) in the Inland Pacific Northwest, it becomes necessary to investigate the effect of this relatively new rotational crop on soil microbial communities and the performance of subsequent crops. Canola plants contain glucosinolates, which upon cell rupture and during the decay of residue hydrolyze to produce isothiocyanates. The production of isothiocyanates is the mechanism responsible for what has been termed the biofumigation effect. This effect is commonly considered positive; however, the non-selectivity of isothiocyanates has potential to impact beneficial soil organisms. In a 7-year on-farm canola-wheat (Triticum aestivum L.) rotation study conducted near Reardan, WA, yields of spring wheat following winter canola were reduced compared to yields following winter wheat. The objective of this research was to determine the differences and similarities in the soil microbial communities associated with winter canola and winter wheat, and if those differences were correlated to spring wheat yield response. Microbial biomass and community composition were determined using phospholipid fatty acid analysis (PLFA). Microbial biomass was greater in the winter canola compared to the winter wheat treatment. In the subsequent spring wheat crop, greater microbial biomass was observed following winter wheat over winter canola indicating a residual effect. Similarly, the biomarker groups for fungi, mycorrhizae, gram negative, and gram positive bacteria were observed to be greater in the winter wheat treatment over winter canola with the same residual effect in the subsequent spring wheat. Discriminant function analysis of the PLFA composition differentiated communities associated with winter wheat from winter canola as well as the communities of spring wheat following winter wheat compared to winter canola. Given the importance of microbially-mediated soil processes, any decline in members of the community or the community as a whole could potentially impact the performance of subsequent crops.