Corn Residual Nitrate and Its Implications for Fall Nitrogen Management In Winter Wheat.

Corn (Zea mays, L.) production typically requires supplemental nitrogen (N) to optimize yields.  In dryland corn production systems, where N is applied during the early to mid vegetative growth stages, inappropriate N applications or limited growing season moisture can result in disparities between optimum and applied N rates.  This leads to variable post-harvest residual nitrate (NO₃-N) accumulation, which is susceptible to loss.  However, this NO₃-N could provide the starter N requirement of the subsequent winter wheat (Triticum aestivum, L.) crop.  Accounting for this residual NO₃-N at wheat planting is important to avoid compounding the N loss potential.  The objectives of this study were to 1) examine plant based tools for assessing soil NO₃-N; 2) examine post-harvest residual NO₃-N accumulation patterns following corn; 3)  determine optimum fall starter N rates for winter wheat; and 4) identify a sufficiency soil NO₃-N level above which no starter N would be needed.  We found that plant canopy measurements are useful tools for assessing corn N management and for identifying drought sites which had the greatest NO₃-N accumulations.  The corn stalk nitrate test was significantly (p<0.001) and positively correlated with soil residual NO₃-N (r²=0.41).  Greatest soil residual NO₃-N accumulation occurred where drought conditions reduced production.  The optimum fall starter N rate for winter wheat is 17 to 34 kg N ha^-1 where soil NO₃-N concentration is less than 15 mg kg^-1.  However, the fall starter N response was highly variable and declined significantly (p=0.0017) as fall precipitation after planting increased.  Wheat yield response to fall starter N declined significantly (p=0.0056) as residual soil NO₃-N increased.  Our results determined that residual NO₃-N levels at planting should be considered before applying fall starter N.