Nitrogen efficiency indices for sustainable sweet potato production in tropical environments

Improving nitrogen (N) use efficiency (NUE) is a key goal for develop sustainable production of sweet potatoes. Therefore, understanding the variability of crop traits and their impact on the main component of NUE are crucial to improve the agronomic management of this crop. Here we aimed to determine the associations among crop traits related N utilization efficiency for storage root production (NUTEroot), N recovery efficiency (NRE), and N nutrition index (NNI), as well as to assess the upper limit of the main components of NUE through frontier analysis. Twelve field experiments were carried out between 2015 and 2023 in sandy-textured Oxisols from different locations in São Paulo State, southeastern Brazil. In all experiments, the sweet potato cv. Canadense was assessed under four to five N fertilization rates (0–200 kg N ha−1). Treatments were arranged in a randomized complete block design with four replicates. High variation was observed across and within the experiments in total dry matter (DM) biomass (3.9–14.4 Mg DM ha−1), storage root biomass (0.34–10.7 Mg DM ha−1), N concentration (0.35–1.79 %), N uptake (19–235 kg ha−1), NUTEroot (2.9–172 kg DM kg−1 N), and NRE (-114–208 %). The NRE decrease from around 175 % at 17 kg N ha−1 to around 60 % and 36 % for N rates of 100 and 200 kg N ha−1, respectively. The principal component analysis reveals that total N concentration, NNI, and total N uptake were highly correlated, while NNI was strongly negatively correlated with NUTEroot. The NNI was positively correlated with NRE. However, different ranges of NNI were observed depending on the critical N dilution curve used for its calculation. A reference NUTEroot of 51–71 kg DM kg−1 is proposed for sweet potatoes grown at an optimal N status (NNI = 1). This study highlights the utilization of the NNI as a useful tool to interpret the NUTE in response to environments × N availability interaction, and to improve the N fertilization management in sweet potato production systems.