RESUMO
Nitrogen is one of the essential nutrients for the production of agricultural crops, participating in a complex interaction among soil, plant and the atmosphere. Therefore, its monitoring is important both economically and environmentally. The aim of this work was to estimate the leaf nitrogen contents in sugarcane from hyperspectral reflectance data during different vegetative stages of the plant. The assessments were performed from an experiment designed in completely randomized blocks, with increasing nitrogen doses (0, 60, 120 and 180 kg ha-1). The acquisition of the spectral data occurred at different stages of crop development (67, 99, 144, 164, 200, 228, 255 and 313 days after cutting; DAC). In the laboratory, the hyperspectral responses of the leaves and the Leaf Nitrogen Contents (LNC) were obtained. The hyperspectral data and the LNC values were used to generate spectral models employing the technique of Partial Least Squares Regression (PLSR) Analysis, also with the calculation of the spectral bands of greatest relevance, by the Variable Importance in Projection (VIP). In general, the increase in LNC promoted a smaller reflectance in all wavelengths in the visible (400-680 nm). Acceptable models were obtained (R2 > 0.70 and RMSE <1.41 g kg-1), the most robust of which were those generated from spectra in the visible (400-680 nm) and red-edge (680-750 nm), with values of R2 > 0.81 and RMSE <1.24 g kg-1. An independent validation, leave-one-date-out cross validation (LOOCV), was performed using data from other collections, which confirmed the robustness and the possibility of LNC prediction in new data sets, derived, for instance, from samplings subsequent to the period of study.
RESUMO
This study applied spectroradiometry techniques with hyperspectral data to identify the correlations between sugarcane leaf reflectance and the contents of Nitrogen (N), phosphorus (P), Potassium (K), Sulfur (S), Calcium (Ca) and Magnesium (Mg). During the harvests 2019/20 and 2020/21, sugarcane was introduced to nutritional stress by the application of limestone doses. Liming was applied in a fractional way and, at the end of five years, the amounts corresponded to 0, 9, 15 and 21 t ha-1 of dolomitic limestone. The leaf hyperspectral reflectance data and the state of nutrients in the exponential growth phase of the culture were registered. The wavelengths correlated with N, P, K, S, Ca and Mg were identified using the Spearman's correlation analysis. The test of similarity (ANOSIM) and the Principal Component Analysis (PCA) were applied to evaluate data variability, as well as the Partial Least Squares Regression (PLSR) for the prediction of the nutritional contents. The order of the degree of correlation in the region of visible was: P > K > N > Ca > S > Mg and for the region of the near infrared: P > K > Ca > N > S > Mg. P presented peaks with high correlations in the wavelengths 706-717 nm (-0.78) and 522-543 nm (-0.76). The values of the PLSR registered the best spectral responses in the region of VIS and red-edge, regions that are more sensitive to the deficiency of sulfur, potassium and phosphorus.
Este estudo aplicou técnicas de espectrorradiometria com dados hiperespectrais para identificar as relações da reflectância foliar da cana-de-açúcar com os teores de Nitrogênio (N), Fósforo (P), Potássio (K), Enxofre (S), Cálcio (Ca) e Magnésio (Mg). Durante as safras 2019/20 e 2020/21 a cana foi induzida ao estresse nutricional a partir da aplicação de doses de calcário. A calagem foi aplicada de forma fracionada e ao final de cinco anos as quantidades corresponderam a 0, 9, 15 e 21 t ha-1 de calcário do tipo dolomítico. Foram registrados os dados de reflectância hiperespectral da folha e o estado de nutrientes na fase de exponencial crescimento da cultura. Os comprimentos de onda correlacionados ao N, P, K, S, Ca e Mg foram identificados usando análise de correlação de Spearman. Aplicou-se o teste de similaridade (ANOSIM) e Análise de Componentes Principais (ACP) para avaliar a variabilidade dos dados, assim como, a Regressão por Mínimos Quadrados Parciais (PLRS) para a predição dos teores nutricionais. A ordem do grau de correlação na região do visível foi: P > K > N > Ca > S > Mg e para região do infravermelho próximo: P > K > Ca > N > S > Mg. O P teve picos com alta correlação nos comprimentos de onda 706-717 nm (-0,78) e 522-543 nm (-0,76). Os valores do PLRS registraram melhores respostas espectrais na região do VIS e red-edge, regiões mais sensíveis a deficiência do enxofre, potássio e fósforo.