NITROGEN UPTAKE AND CYCLING BY CANOLA, PEA, AND WHEAT: IMPLICATIONS FOR ROTATIONAL NITROGEN USE EFFICIENCY
Maaz, Tai McClellan
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The total soil N supply is often not factored into many N use efficiency (NUE) equations despite its high contribution to plant N nutrition and degree of internal N cycling in soil. Greenhouse, laboratory, and field experiments were conducted to determine differences in soil N uptake and partitioning in wheat (Triticum aestivum L.), field pea (Pisum sativum L.), and canola (Brassica napus L.), and their effects on net N mineralization and N carryover in soil. Multiple nitrogen recovery indices were utilized to track N in the various systems, including the recovery of 15N fertilizer by crops, N uptake efficiency, available N uptake efficiency, apparent fertilizer N recovery, net N mineralization, apparent N mineralization, rotational NUE, rotational N uptake efficiency, and rotational N utilization efficiency. Results from the greenhouse study highlighted the importance of soil derived N supply, which was taken up proportionately more than fertilizer N. The laboratory experiment linked the partitioning of C and N into structural and soluble cell components to the net N immobilization potential of soil. Findings from the field study related the contribution of N carryover and residue N to the N availability of subsequent crops and to enhanced N uptake and recovery efficiencies. Multi-year N balances adequately captured effects of fertilization and inclusion of legumes on increased subsequent N availability and rotational N use efficiencies. Finally, a case study is presented, which examines the driving forces for canola adoption, key factors leading to adoption, and institutionalized programs and organizations that sustain the canola industry in Canada and Australia. Initial public investments in research and market development, competitive prices, and the release of high yielding adapted varieties with advanced agronomics were instrumental for the development of a viable canola industry. The results from these studies will inform Washington growers of agronomic, ecological, social, economic, and political considerations when introducing canola production into wheat-based cropping systems.