Overwinter Nitrogen Cycling in Winter Canola.

Winter canola has developed mechanisms to mitigate N losses associated with physiologic leaf senescence, including the mineralization of N from leaves dropped in the winter during spring regrowth and the synthesis and remobilization of vegetative storage proteins in the taproot. However, previous studies examining the N remobilization have typically been conducted after stem elongation, and the role of overwinter N storage in the taproot has largely been ignored. An ¹⁵N tracer study was conducted in Davenport, WA, in which the cotton-wick method was used to introduce ¹⁵N-enriched urea (98 atom%) into canola plants at two to three day intervals beginning with the six leaf stage and prior to the onset of freezing air temperatures. Plants were harvested prior to freezing, after 5 consecutive days of subfreezing temperatures, and at stem elongation during the spring. Results indicate that plant growth was not affected by the cotton wick technique. However, within the freezing period, root weight and width increased while above-ground biomass, crown height, and root length remained the same. Freezing had no significant effect on the quantity of ¹⁵N recovered in the shoots and roots. However, a greater portion of the ¹⁵N was stored in the roots and crown after freezing and early spring, constituting 32% of the assimilated N. Recycling of senesced leaf ¹⁵N was also observed in neighboring plants. These results indicate that overwinter N storage, in addition to the recycling of senesced leaf N, are important mechanisms to mitigate N losses by winter canola.