Excess N Supply Impacts on Single Season and Multiple Year N Use Efficiencies in a Canola- Pea- Wheat Cropping Sequence.

In annual cropping systems, nitrogen (N) use efficiency is typically estimated on a single crop basis. However, this approach ignores the dynamic nature of N cycling within multi-year crop rotations featuring a diverse set of crops. The objective of our research was to develop a multiple-year N balance approach to track N dynamics over a three-year spring canola-spring pea-winter wheat cropping sequence in the annual cropping wet-cool zone near Pullman, WA, and fallow-transition zone near Davenport, WA. Rotational N uptake efficiency (rNUpE = sum of crop uptake/sum of N supply) and rotational apparent N recovery (ANR = incremental increases in crop N uptake from fertilization/N fertilizer rate) indices were calculated to determine the effect of canola fertilization on the N uptake of two subsequent crops. The N supply included changes in residual N, apparent N mineralization, apparent N fixation, and fertilizer N additions. Apparent N mineralization was estimated using unfertilized wheat as a bioassay, while apparent biological N fixation was assessed by the N difference method. Over-fertilization resulted in the accumulation of 20 to 40 kg N ha^-1 in canola residue and inorganic N in soil at harvest.  In the first and second year following high fertilization, 31 to 41 kg N ha^-1 more N was recycled, while N uptake by the two subsequent crops increased by 44 to 67 kg N ha^-1.  In the second year, residual N was enhanced by 12 to 33 more kg N ha^-1 following spring peas, which directly contributed to the N supply and uptake by the subsequent winter wheat crop. Pea had no effect on apparent N mineralization in the following year. Optimal fertilization of canola led to the highest rNUpE for the three year cropping sequence.  At the highest rate of fertilization, the single-season canola N uptake efficiencies ranged from 23 to 37%; and ANR, 9 to 13%. After considering effects on subsequent crops, rotational NUpE increased by 8 to 13%; and rotational ANR, by 22 to 48%.  Our findings suggest that the residual N may be effectively recovered by subsequent crops in the dryland region of the inland Pacific Northwest.