Evaluating the critical nitrogen dilution curve for storage root crops

Criteria for N diagnostics have been developed for a range of agricultural crops. The Nitrogen Nutrition Index (NNI) is a widely used indicator of N status. The NNI represents the relationship between above-ground N concentration and an estimated critical N concentration (N-c), which is the minimum N concentration allowing maximum crop growth rate at a given above-ground biomass (W). Although the NNI has been determined for a wide range of agricultural crops, the conceptual basis that explains N dilution patterns have not yet been validated in crops with large storage organs such as fodder beet (Beta vulgaris L. ssp. vutgaris). In this study, we assessed the validity of the Nc and the NNI for fodder beet based on two field datasets with a wide range of N treatments (0-200 kg ha(-1)) and water supply (rain fed and irrigated) conditions in New Zealand. The N, and NNI concepts were shown to be valid for crops with large storage organs. A pattern of N dilution, similar to other agricultural crops, was represented by N-c = 4.9 W-0.52 (R-2 = 0.97) for biomass (W) ranging from 1.1 to 28 tW ha(-1). The dynamics of N dilution was explained by a simultaneous relative increase in the biomass of storage roots, which have the lowest N concentrations, and a decline in N concentration of all organs (leaves, petioles and storage roots) as W increased. The NNI values calculated from the Nc curve ranged from 0.6 to 1.74 with relative biomass declining at NNI < 1. Water stressed crops showed luxury N uptake (NNI up to 1.74) highlighting need for careful interpretation of NNI values depending on growth conditions. These results can be used to manage N supply in fodder beet, and to support analysis and modelling of physiological responses to N supply in crops with large storage organs. (C) 2016 Elsevier B.V. All rights reserved.