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Sensors 2017, 17(3), 538; doi:10.3390/s17030538

Non-Destructive Evaluation of the Leaf Nitrogen Concentration by In-Field Visible/Near-Infrared Spectroscopy in Pear Orchards

Jiangsu Provincial Key Lab for Organic Solid Waste Utilization; Ministry of Agriculture, Key Laboratory of Plant Nutrition and Fertilization in Low-Middle Reaches of the Yangtze River; College of Resources and Environmental Sciences, Nanjing Agricultural University, Nanjing 210095, China
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Academic Editor: Lammert Kooistra
Received: 12 January 2017 / Revised: 2 March 2017 / Accepted: 3 March 2017 / Published: 8 March 2017
(This article belongs to the Special Issue Precision Agriculture and Remote Sensing Data Fusion)
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Abstract

Non-destructive and timely determination of leaf nitrogen (N) concentration is urgently needed for N management in pear orchards. A two-year field experiment was conducted in a commercial pear orchard with five N application rates: 0 (N0), 165 (N1), 330 (N2), 660 (N3), and 990 (N4) kg·N·ha−1. The mid-portion leaves on the year’s shoot were selected for the spectral measurement first and then N concentration determination in the laboratory at 50 and 80 days after full bloom (DAB). Three methods of in-field spectral measurement (25° bare fibre under solar conditions, black background attached to plant probe, and white background attached to plant probe) were compared. We also investigated the modelling performances of four chemometric techniques (principal components regression, PCR; partial least squares regression, PLSR; stepwise multiple linear regression, SMLR; and back propagation neural network, BPNN) and three vegetation indices (difference spectral index, normalized difference spectral index, and ratio spectral index). Due to the low correlation of reflectance obtained by the 25° field of view method, all of the modelling was performed on two spectral datasets—both acquired by a plant probe. Results showed that the best modelling and prediction accuracy were found in the model established by PLSR and spectra measured with a black background. The randomly-separated subsets of calibration (n = 1000) and validation (n = 420) of this model resulted in high R2 values of 0.86 and 0.85, respectively, as well as a low mean relative error (<6%). Furthermore, a higher coefficient of determination between the leaf N concentration and fruit yield was found at 50 DAB samplings in both 2015 (R2 = 0.77) and 2014 (R2 = 0.59). Thus, the leaf N concentration was suggested to be determined at 50 DAB by visible/near-infrared spectroscopy and the threshold should be 24–27 g/kg. View Full-Text
Keywords: non-destructive; visible/near-infrared spectroscopy; leaf nitrogen; PLSR; pear non-destructive; visible/near-infrared spectroscopy; leaf nitrogen; PLSR; pear
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Wang, J.; Shen, C.; Liu, N.; Jin, X.; Fan, X.; Dong, C.; Xu, Y. Non-Destructive Evaluation of the Leaf Nitrogen Concentration by In-Field Visible/Near-Infrared Spectroscopy in Pear Orchards. Sensors 2017, 17, 538.

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