Optimising non-destructive sampling methods to study nitrogen use efficiency throughout the growth-cycle of giant C4 crops

AIMS: The improvement of nitrogen use efficiency (NUE) of crops allows crop nitrogen (N) demands to be met while reducing N supply, and so reducing excess N which has potential negative environmental implications. NUE is often determined destructively at the end of crop growth-cycles without considering temporal variability. Here we present a methodological study which optimises the determination of NUE throughout the sugarcane growth-cycle using minimally destructive methods, and suggest the use of these methods for non-destructive NUE determination over the growth-cycle of other giant C4 crops. METHODS AND RESULTS: The determination of the NUE relied on the optimisation of three methods: the estimation of aboveground biomass, N content and N derived from fertiliser (NdfF). First, the ability of different allometric relationships to estimate sugarcane biomass was investigated by selecting a relationship based on height and diameter to estimate aboveground biomass along the crop growth-cycle. Secondly, we assessed the minimum number of harvested sugarcane required to construct a dilution curve to predict N content from biomass and found that a sampling of 5 sugarcane at 3 dates was sufficient to represent aboveground N content over the growth-cycle. Finally, the ability of ¹⁵N content of individual leaves to represent the NdfF in ¹⁵N-fertilised cane was tested. The first and second leaf below the top visible dewlap were the most representative. Based on a variance analysis, we assessed the level of influence of each method on the NUE calculation. Crop age accounted for 54% of the variance of NUE, the choice of ¹⁵N leaf 13%, with the choice of model to estimate biomass and the number of plants harvested for the N dilution curve, each accounting for less than 2% over the four sampling dates. CONCLUSIONS: This study highlighted the importance of evaluating NUE not only at the point of harvest. We propose, therefore, a set of methods to study NUE throughout the sugarcane growth-cycle by using minimally destructive sampling.