The ability to predict spatially explicit nitrogen uptake (NUP) in maize (Zea mays L.) during the early development stages provides clear value for making in-season nitrogen fertilizer applications that can improve NUP efficiency and reduce the risk of nitrogen loss to the environment. Aerial hyperspectral imaging is an attractive agronomic research tool for its ability to capture spectral data over relatively large areas, enabling its use for predicting NUP at the field scale. The overarching goal of this work was to use supervised learning regression algorithms-Lasso, support vector regression (SVR), random forest, and partial least squares regression (PLSR)-to predict early season (i.e., V6-V14) maize NUP at three experimental sites in Minnesota using high-resolution hyperspectral imagery. In addition to the spectral features offered by hyperspectral imaging, the 10th percentile Modified Chlorophyll Absorption Ratio Index Improved (MCARI2) was made available to the learning models as an auxiliary feature to assess its ability to improve NUP prediction accuracy. The trained models demonstrated robustness by maintaining satisfactory prediction accuracy across locations, pixel sizes, development stages, and a broad range of NUP values (4.8 to 182 kg ha-1). Using the four most informative spectral features in addition to the auxiliary feature, the mean absolute error (MAE) of Lasso, SVR, and PLSR models (9.4, 9.7, and 9.5 kg ha-1, respectively) was lower than that of random forest (11.2 kg ha-1). The relative MAE for the Lasso, SVR, PLSR, and random forest models was 16.5%, 17.0%, 16.6%, and 19.6%, respectively. The inclusion of the auxiliary feature not only improved overall prediction accuracy by 1.6 kg ha-1 (14%) across all models, but it also reduced the number of input features required to reach optimal performance. The variance of predicted NUP increased as the measured NUP increased (MAE of the Lasso model increased from 4.0 to 12.1 kg ha1 for measured NUP less than 25 kg ha-1 and greater than 100 kg ha-1, respectively). The most influential spectral features were oftentimes adjacent to each other (i.e., within approximately 6 nm), indicating the importance of both spectral precision and derivative spectra around key wavelengths for explaining NUP. Finally, several challenges and opportunities are discussed regarding the use of these results in the context of improving nitrogen fertilizer management.
Bibliographical noteFunding Information:
This research was possible because of funding provided by the Minnesota Department of Agriculture through the Minnesota Clean Water Land and Legacy Act, grant number 153761 PO 3000031069 (Waseca experiments), and the Minnesota Soybean Research and Promotion Council, grant numbers 00079668 and 00071830 (Wells experiment). Minnesota's Discovery, Research, and InnoVation Economy (MnDRIVE) and the University of Minnesota also provided financial support in the form of student fellowships and/or research assistantships. We thank Ali Moghimi for his contributions toward helping to integrate the hyperspectral imager with the UAV, as well as his valuable perspectives regarding image processing and model training. We thank the staffat the University of Minnesota Southern Research and Outreach Center, especially JeffVetsch, for help with field activities for the Waseca experiments. We also thank the Soil, Water, & Climate field crew, J?nos Forg?cs, Xiaolei Deng, Abdullahi Abdullahi, and Leanna Leverich for assistance with sample collection, and Wenhao Su for assistance with image pre-processing.
Funding: This research was possible because of funding provided by the Minnesota Department of Agriculture through the Minnesota Clean Water Land and Legacy Act, grant number 153761 PO 3000031069 (Waseca experiments), and the Minnesota Soybean Research and Promotion Council, grant numbers 00079668 and 00071830 (Wells experiment). Minnesota’s Discovery, Research, and InnoVation Economy (MnDRIVE) and the University of Minnesota also provided financial support in the form of student fellowships and/or research assistantships.
© 2020 by the authors.
- Feature selection
- Hyperparameter tuning
- Image processing
- Image segmentation
- Nitrogen fertilizer recommendation
- Supervised regression