Defining upper limits of nitrogen uptake and nitrogen use efficiency of potato in response to crop N supply

Sound nitrogen management seeks to ensure crop yield, quality and profit, while avoiding over-fertilization lead to excess reactive nitrogen entering the agro-ecosystem or under-fertilization leading to mining of soil organic matter. Here we advance a framework to compare crop nitrogen use efficiency (NUE) across genotypes, environments and management practices that accounts for the non-linearity between N supply and N uptake, and between N uptake and crop dry matter or yield. We used published data of potato dry matter production, yield, N uptake and N inputs to demonstrate the efficacy of simple frontier analysis to describe the upper limits of the first order components of nitrogen use efficiency. Using quantile regression, frontier curves were derived for the response of apparent nitrogen recovery to the rate of applied N and for the responses of crop dry matter and tuber yield (dry matter) to crop N uptake. The analysis captured the known higher NUE for tuber production of late relative to early maturing potato varieties. The maximum achievable apparent nitrogen recovery did not differ between N fertilization methods. Simple frontier analysis provides a practical approach to benchmark the nitrogen use efficiency of crops. This then allows valid comparison of treatments or commercial crops when there are differences in the amount of N applied or differences in crop dry matter accumulation.