Abstract
Vegetation indices (VIs) can be useful tools to evaluate crop nitrogen (N) status. To be effective, VIs measurements must be related to crop N status. The nitrogen nutrition index (NNI) is a widely accepted parameter of crop N status. The present work evaluates the performance of several VIs to estimate NNI in sweet pepper (Capsicum annuum). The performance of VIs to estimate NNI was evaluated using parameters of linear regression analysis conducted for calibration and validation. Three different sweet pepper crops were grown with combined irrigation and fertigation, in Almería, Spain. In each crop, five different N concentrations in the nutrient solution were frequently applied by drip irrigation. Proximal crop reflectance was measured with Crop Circle ACS470 and GreenSeeker handheld sensors, approximately every ten days, throughout the crops. The relative performance of VIs differed between phenological stages. Relationships of VIs with NNI were strongest in the early fruit growth and flowering stages, and less strong in the vegetative and harvest stages. The green band-based VIs, GNDVI, and GVI, provided the best results for estimating crop NNI in sweet pepper, for individual phenological stages. GNDVI had the best performance in the vegetative, flowering, and harvest stages, and GVI had the best performance in the early fruit growth stage. Some of the VIs evaluated are promising tools to estimate crop N status in sweet pepper and have the potential to contribute to improving crop N management of sweet pepper crops.
Highlights
Vegetable crops production is characterized by nitrogen (N) losses and the associated environmental problems [1,2,3]
The highest R2 values were obtained in phenological stage corresponding to early fruit growth, where the average R2 value across all vegetation indices was 0.71 ± 0.04, with a range from 0.52 to 0.84 (Table 3)
The R2 and root mean square error (RMSE) of linear regressions between vegetation indices and crop nutrition index (NNI) were variable between phenological stages throughout the crop and between the eight vegetation indices evaluated
Summary
Vegetable crops production is characterized by nitrogen (N) losses and the associated environmental problems [1,2,3]. The most common environmental problems include ground and surface water contamination, eutrophication of surface water, and nitrous oxide (N2O) emission [4,5] These problems are often a consequence of the high use of N fertilizer as a way to ensure optimal growth and production [6], which generally exceeds the demand of the crops [3,7,8]. A traditional tool is leaf nutrient analysis, which requires laborious and time-consuming laboratory work, and which generally cannot characterize the temporal and spatial variability of N status [12,13] These are major drawbacks, because knowledge of temporal and spatial variability of crop N status appreciably assists the matching of N supply to crop N requirements [14]. They enable frequent assessment throughout a crop, and assessment of spatial variability
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