Abstract
Global climate change and the spread of COVID-19 have caused widespread concerns about food security. The development of smart agriculture could contribute to food security; moreover, the targeted and accurate management of crop nitrogen is a topic of concern in the field of smart agriculture. Unmanned aerial vehicle (UAV) spectroscopy has demonstrated versatility in the rapid and non-destructive estimation of nitrogen in summer maize. Previous studies focused on the entire growth season or early stages of summer maize; however, systematic studies on the diagnosis of nitrogen that consider the entire life cycle are few. This study aimed to: (1) construct a practical diagnostic model of the nitrogen life cycle of summer maize based on ground hyperspectral data and UAV multispectral sensor data and (2) evaluate this model and express a change in the trend of nitrogen nutrient status at a spatiotemporal scale. Here, a comprehensive data set consisting of a time series of crop biomass, nitrogen concentration, hyperspectral reflectance, and UAV multispectral reflectance from field experiments conducted during the growing seasons of 2017–2019 with summer maize cultivars grown under five different nitrogen fertilization levels in Beijing, China, were considered. The results demonstrated that the entire life cycle of summer maize was divided into four stages, viz., V6 (mean leaf area index (LAI) = 0.67), V10 (mean LAI = 1.94), V12 (mean LAI = 3.61), and VT-R6 (mean LAI = 3.94), respectively; moreover, the multi-index synergy model demonstrated high accuracy and good stability. The best spectral indexes of these four stages were GBNDVI, TCARI, NRI, and MSAVI2, respectively. The thresholds of the spectral index of nitrogen sufficiency in the V6, V10, V12, VT, R1, R2, and R3–R6 stages were 0.83–0.44, −0.22 to −5.23, 0.42–0.35, 0.69–0.87, 0.60–0.75, 0.49–0.61, and 0.42–0.53, respectively. The simulated nitrogen concentration at the various growth stages of summer maize was consistent with the actual spatial distribution.
Highlights
Combined with the Unmanned aerial vehicle (UAV) images, the monitoring and diagnosis of plant nitrogen concentration were undertaken based on the model established by the optimal multispectral index
Ground hyperspectral technology was combined with UAV multispectral technology; thereafter, a multi-index synergy model of nitrogen concentration during the life cycle of summer maize was proposed based on the data of a 3-year systematic field experiment
Based on the critical nitrogen concentration model, the spectral index thresholds of sufficient nitrogen levels were determined at different stages
Summary
Considering global climate change and the COVID-19 pandemic, crops affected by droughts, floods, pests, diseases, and lack of nutrients such as Nitrogen (N) have garnered more attention [1,2,3,4]. Nitrogen is essential for crop growth and development [5]. The fixed-point production and management of nitrogen that considers the entire life cycle of summer maize is of great significance. Nitrogen is a vital nutrient for summer maize production in the North China plain; excessive fertilization has adverse effects on the environment [7,8,9]. The nitrogen demand of summer maize varies spatially across fields, which could lead to localized differences in plant growth [10,11]
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