Belowground Contributions of Field Pea and Canola to Soil Nitrogen Pools and Processes.

Crop production in the Canadian Prairies has become increasingly diversified with the inclusion of pulse and oilseed crops into traditional cereal-based crop rotations. Differences in quality and quantity of the belowground residues that remain in the soil following harvest of these dissimilar crop species may have implications for subsequent nutrient cycling. The purpose of this study was to quantify root and root-derived N in soil and examine its relationship to the abundance of N cycling microorganisms in the rhizospheres of field pea and canola. Field pea and canola were grown in soil collected from an agricultural field under controlled conditions in a greenhouse.  Plants were labeled with ¹⁵N-enriched urea using the cotton-wick method over a 5-week period to quantify root-derived N. Following harvest at physiological maturity, grain, pods, straw, roots and rhizosphere and bulk soils were analyzed for atom % ¹⁵N excess. DNA from rhizosphere soils has been extracted and the abundance of nitrifying and denitrifying genes is being determined using quantitative real-time PCR (qPCR). A higher proportion of the recovered ¹⁵N was found in belowground components (roots, in particular) of canola compared to pea.  However, total N was greater for pea than for canola on a per plant basis and a higher proportion of N was allocated to grain.  As a result, of the total residue-N remaining in the soil following harvest, belowground N comprised 35% and 65% for canola and pea, respectively.  Preliminary results from the qPCR of canola and pea rhizosphere soils indicate greatest abundance of nirS and nosZ genes in pea, while potential denitrification activity was similar between crops.  Full results will be presented and discussed. There are clear distinctions in plant partitioning of N belowground between pea and canola, with implications for N cycling in soils growing pulse and oilseed crops.