Legacy Effects of Long-Term Nitrogen Fertilizer Rate on Nitrogen Use Efficiency in Continuous Corn.

By increasing crop residue inputs to the soil, N fertilization helps to build soil organic C (SOC) in cereal-based cropping systems. However, the impact of long-term N management on crop N use efficiency is not well-understood. We evaluated the legacy effect of N fertilizer rate on N use efficiency of corn (Zea mays L.) at two long-term N fertilization experiments in Iowa where the ranges in N fertilizer rate have generated gradients of SOC concentration after 15 years of continuous corn production. We applied isotopically-labeled N fertilizer at a site-specific optimal rate uniformly across N rate treatments and measured fertilizer N recovery in the crop and soil organic matter pools. Aboveground crop recovery of ¹⁵N at physiological maturity differed drastically between the two locations, averaging 88 kg N ha^-1 in central Iowa and 37 kg ha^-1 in southern Iowa (44% and 14% of fertilizer applied at the two sites, respectively). At both locations, fertilizer N recovery in the mature crop exhibited a curvilinear response to historical N rate, with ~10% or 30% greater recovery at the lowest and highest historical N rates than intermediate rates for the central and southern Iowa sites, respectively. At the intermediate historical N rate, which had an intermediate amount of SOC, recovery of fertilizer N in the crop was lowest. However, historical N rate had no effect on fertilizer N recovery in the crop plus soil (0-120 cm) system. At the five-leaf stage, we observed decreasing recovery of fertilizer N in mineral-associated organic matter and increasing recovery of fertilizer N in particulate organic matter with increasing historical N rate. Our results suggest that historical N management affects crop fertilizer N use efficiency, possibly by controlling topsoil retention of fertilizer N in different organic matter pools.