Elaboration of a nitrogen nutrition indicator for winter wheat based on leaf area index and chlorophyll content for making nitrogen recommendations

Remote sensing in the solar domain provides indirect measurements of green leaf area index and chlorophyll content on a within-field scale. This information can be used to build an indicator related to the nitrogen status of the crop itself used to make nitrogen recommendations, varying on the within-field scale. We propose as an indicator for winter wheat crops the nitrogen absorption deficit determined by reference to optimal nitrogen status. Three methods for calculating the nitrogen absorption deficit of the crop from the green leaf area index (GLAI) and the chlorophyll content (Cab) are proposed and compared. The first method consists in evaluating the nitrogen nutrition index from the chlorophyll content. The second evaluates the nitrogen content of the canopy and the third directly estimates the quantity of nitrogen absorbed by the canopy from the quantity of chlorophyll that the canopy contains. The methods were calibrated and validated on three datasets obtained on 2 years and two sites, on experiments were nitrogen dressing varied from 0 to 300 kg ha−1. Measurements consisted in time evolution from end of winter to earing stage of dry matter (range 5.9–17.5 t ha−1), GLAI (range 0.1–6), chlorophyll content (range 0.3–0.9 g m−2 leaf or 0.1–4.5 g m−2 soil) and nitrogen absorption (range 38–234 kg ha−1). We found that the relationship between absorbed nitrogen and chlorophyll was more stable when considering integrated quantities of chlorophyll in the canopy (g m−2 soil) than when considering chlorophyll content (g cm−2 leaf). As a consequence, the third method gave the best results, both for description and prediction, with an error of about 20 kg N ha−1.