SOIL MICROBIAL COMMUNITY RESPONSE TO OILSEED CROPS INTRODUCED IN LONG-TERM MONOCULTURE WHEAT ROTATIONS
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Introduction of Cruciferous oilseed crops into monoculture wheat (Triticum aestivum L.) cropping systems has increased in the Inland Pacific Northwest. Canola (Brassica napus L.), and camelina (Camelina sativa L. Crantz) are members of the Brassicaceae family. 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 the soil 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 winter canola rotation study conducted near Reardan, WA, yields of spring wheat following winter canola declined compared to yields following winter wheat. Objectives of this research were 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 higher in winter canola compared to the winter wheat. Subsequent spring wheat crops had greater microbial biomass following winter wheat compared to winter canola indicating a residual effect. Similarly, the biomarker groups of 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 phospholipid fatty acid 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. Similar patterns of reduced abundance and shifts in microbial community structure were perceived in a canola/winter wheat rhizosphere study. Camelina, in a wheat-camelina-summer fallow rotation appeared to provide a primer effect on the subsequent fallow microbial community. Microbial abundance decreased from winter wheat, to camelina, to summer fallow. Summer fallow reached significantly lower levels of microbial abundance compared to winter wheat. Given the importance of microbially-mediated soil processes, any decline in members of the community or alteration of the community as a whole could potentially impact the performance of subsequent crops.