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Acta Agronomica Sinica ›› 2026, Vol. 52 ›› Issue (6): 1788-1801.doi: 10.3724/SP.J.1006.2026.53085

• TILLAGE & CULTIVATION · PHYSIOLOGY & BIOCHEMISTRY • Previous Articles     Next Articles

Estimation of leaf nitrogen content in dryland forage maize using UAV-based hyperspectral imaging and machine learning

Liang Jin-Yu**(), Yin Jia-De**(), Wang Hong-Li, Zhang Guo-Ping, Hou Hui-Zhi, Dong Bo, Ma Ming-Sheng*()   

  1. Institute of Dry Land Farming, Gansu Academy of Agricultural Sciences / Key Laboratory of High Water Utilization on Dryland of Gansu Province / Northwest Dryland Agriculture Green Low Carbon Key Laboratory, Ministry of Agriculture and Rural Affairs, Lanzhou 730070, Gansu, China
  • Received:2025-10-29 Accepted:2026-02-27 Online:2026-06-12 Published:2026-03-10
  • Contact: * Ma Ming-Sheng, E-mail: mamingsh@163.com
  • About author:

    **Contributed equally to this work

  • Supported by:
    Gansu Provincial Science and Technology Program Projects(23JRRA1340);Gansu Provincial Science and Technology Program Projects(23JRRA1335);Gansu Provincial Science and Technology Program Projects(24JRRA731);Basic Research Program Project of Gansu Academy of Agricultural Sciences(2026GAAS22-7);National Natural Science Foundation of China(32360532);Longyuan Young Talents Program

Abstract:

This study integrates UAV-based hyperspectral imaging with ensemble learning to identify an optimal spectral estimation model for predicting leaf nitrogen content (LNC) in dryland forage maize, providing a methodological reference for improving production efficiency and quality. The study was conducted on the Loess Plateau in central Gansu province, China, with forage maize as the target crop. Hyperspectral data were acquired using a V185G integrated gimbal hyperspectral imaging system mounted on a UAV. Spectral indices were generated from all possible two-band combinations using original reflectance spectra, first-derivative spectra, and continuum-removed spectra. Six machine-learning algorithms were evaluated, and Voting and Stacking ensemble models were further developed to select the best-performing approach. Transformed spectra substantially strengthened the relationships between spectral indices and LNC compared with the original bands. Among the six individual models, random forest regression (RFR), K-nearest neighbors (KNN), XGBoost, and gradient boosting decision tree (GBDT) achieved relatively high accuracy across maize growth stages, with test-set R2 values of 0.7165-0.7713 and RMSE values of 2.4265-2.8296. These four models were then combined to build the ensemble models, both of which achieved test-set R2 > 0.7459 and RMSE < 2.6358. The Voting ensemble based on first-derivative spectra delivered the best performance (R2 = 0.8152, RMSE = 2.1253), indicating improved predictive accuracy and robustness through model integration. Overall, the Voting-first-derivative spectra (Voting-FDS) model enables rapid estimation of LNC at key growth stages, supporting in-season nutrient management and high-quality production in dryland forage maize.

Key words: dryland forage maize, leaf nitrogen content, hyperspectral, band selection, machine learning

Table 1

Description of the experimental treatments"

处理
Treatment
种植密度
Planting density (×104 plant hm-2)
施肥方式
Fertilization method
CK1 6.0 全部基施 All applied as basal fertilizer
T1 6.0 40%追施 40% applied as topdressing
T2 6.0 60%追施 60% applied as topdressing
CK2 7.5 全部基施 All applied as basal fertilizer
T3 7.5 40%追施 40% applied as topdressing
T4 7.5 60%追施 60% applied as topdressing

Table 2

Camera parameters for flight route planning"

指标
Specification
参数
Parameter
分辨率Resolution 1000×1000 pix
探测器尺寸Detector size 6.45×6.45 mm
焦距Focal length 16 mm
最小拍照间隔Minimum capture interval 1 s

Fig. 1

Location of the study area and sampling sites This map is based on the standard map downloaded from the Standard Map Service website of the Ministry of Natural Resources, with the review approval number GS (2019) 3333. The boundaries of the base map remain unaltered. Treatments are the same as those given in Table 1."

Fig. 2

Workflow for hyperspectral image mosaicking Cuvis Export 3.2 was used to export hyperspectral images; Agisoft Metashape was used for image mosaicking; ENVI 5.6 was used to convert TIFF files to BIL format. TIFF, tagged image file format; BIL, band interleaved by line."

Fig. 3

Workflow of the ensemble machine-learning approach RFR, random forest regression; KNN, K-nearest neighbors regression; XGBoost, extreme gradient boosting; GBDT, gradient boosting decision tree; RR, ridge regression; PLSR, partial least squares regression; Voting, voting ensemble; Stacking, stacking ensemble."

Fig. 4

Original spectral features"

Fig. 5

Contour maps of correlations between narrowband spectral indices and leaf nitrogen content in forage maize OS, original spectra; FDS, first-derivative spectra; CRS, continuum-removed spectra; NDSI, normalized difference spectral index; RSI, ratio spectral index; DSI, difference spectral index."

Table 3

Optimal band combinations for narrowband spectral indices"

变换光谱
Transform spectrum
光谱指数
Spectral indices
相关系数
Correlation coefficient
原始光谱
Original spectrum (OS)
NDSI (R630, R506) 0.7796**
RSI (R746, R750) 0.7137**
DSI (R630, R506) 0.7806**
一阶导数光谱
First derivative spectrum (FDS)
NDSI (R710, R742) 0.7904**
RSI (R706, R750) 0.7851**
DSI (R710, R738) 0.7995**
连续统去除光谱
Continuum removal spectrum (CRS)
NDSI (R570, R498) 0.8074**
RSI (R738, R454) 0.7879**
DSI (R566, R490) 0.8094**

Table 4

Accuracy comparison of six machine learning models"

模型
Model
变换光谱
Transform spectrum
训练集R2
Train R2
测试集R2
Test R2
训练集RMSE
Train RMSE
测试集RMSE
Test RMSE
RFR 原始光谱Original spectrum (OS) 0.7370 0.7432 2.4977 2.5975
一阶导数光谱First derivative spectrum (FDS) 0.7503 0.7640 2.4336 2.4901
连续统去除光谱Continuum removal spectrum (CRS) 0.7352 0.7576 2.4980 2.5404
KNN 原始光谱Original spectrum (OS) 0.7201 0.7165 2.5300 2.8296
一阶导数光谱First derivative spectrum (FDS) 0.7500 0.7501 2.4350 2.5623
连续统去除光谱Continuum removal spectrum (CRS) 0.7198 0.7346 2.5695 2.6582
XGBoost 原始光谱Original spectrum (OS) 0.7892 0.7454 2.2361 2.5866
一阶导数光谱First derivative spectrum (FDS) 0.8156 0.7671 2.0991 2.4490
连续统去除光谱Continuum removal spectrum (CRS) 0.7886 0.7461 2.1655 2.7398
GBDT 原始光谱Original spectrum (OS) 0.7455 0.7432 2.4571 2.5976
一阶导数光谱First derivative spectrum (FDS) 0.7521 0.7713 2.4336 2.4265
连续统去除光谱Continuum removal spectrum (CRS) 0.7583 0.7373 2.4469 2.4995
RR 原始光谱Original spectrum (OS) 0.5577 0.6642 3.3320 2.7835
一阶导数光谱First derivative spectrum (FDS) 0.6025 0.7375 3.1384 2.4982
连续统去除光谱Continuum removal spectrum (CRS) 0.5627 0.6729 3.3303 2.7129
PLSR 原始光谱Original spectrum (OS) 0.6007 0.6984 3.1416 2.6837
一阶导数光谱First derivative spectrum (FDS) 0.6082 0.7333 3.1041 2.5445
连续统去除光谱Continuum removal spectrum (CRS) 0.6232 0.7391 3.0714 2.4592

Fig. 6

Measured vs predicted values for the six machine-learning models Abbreviations are the same as those given in Fig. 3 and Fig. 5. R2: coefficient of determination; RMSE: root mean squared error."

Table 5

Accuracy comparison of two ensemble machine learning models"

模型
Model
变换光谱
Transform spectrum
训练集R2
Train R2
测试集R2
Test R2
训练集RMSE
Train RMSE
测试集RMSE
Test RMSE
Voting 原始光谱Original spectrum (OS) 0.7899 0.7608 2.1835 2.5920
一阶导数光谱First derivative spectrum (FDS) 0.8251 0.8152 2.0115 2.1253
连续统去除光谱Continuum removal spectrum (CRS) 0.8236 0.7717 2.0906 2.3304
Stacking 原始光谱Original spectrum (OS) 0.6958 0.7530 2.6865 2.5476
一阶导数光谱First derivative spectrum (FDS) 0.7589 0.7529 2.3907 2.4502
连续统去除光谱Continuum removal spectrum (CRS) 0.7377 0.7459 2.4712 2.6358

Fig. 7

Scatter plots of measured vs predicted values for Voting and Stacking ensemble machine learning models Abbreviations are the same as those given in Fig. 3 and Fig. 5. R2: coefficient of determination; RMSE: root mean squared error."

Fig. 8

Spatial inversion of leaf nitrogen content in forage maize in 2024 using the Voting-FDS model"

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