C/N content ratio of rice leaf monitoring based on canopy hyperspectral parameters

Carbon (C) content and nitrogen (N) content ratio of leaf is a key index of carbon and nitrogen metabolic status, thus it is important for precision diagnosis and management of plant growth and yield formation. In order to study the feasibility of estimating the ratio of C and N contents of rice leaves with properties of canopy reflectance spectra, two field experiments were conducted with different nitrogen levels and rice cultivars (Oryza sativa L.) in two years. Then the relationships of C/N ratio of leaf to reflectance of single bands, different vegetation indices, derivative indices and parameters normalized by the continuum were analyzed comprehensively. The results showed that there was consistent correlation between C/N ratio and canopy reflectance after rice jointing stage. The C/N ratio of leaf was positively correlated to reflectance at 350～742 nm, and negatively correlated at 750～1143 nm, while highly correlated to eight spectral parameters among all cultivars and growth stages. After comparing the R2 and SE of regression equations, the spectral index of ND672 was found to be the best parameter for predicting C/N ratio of leaf in rice. The derived equation was tested with the observed data of all growth stages in the field experiment. The estimation precision ranged 0.687～0.986, estimation accuracy 0.907～1.126, and RMSE 7.851～18.25, indicating a good fit between the predicted and observed values of C/N ratio of leaf. Tests with other independent dataset showed that the estimation precision was 0.857～0.967, estimation accuracy 0.970～1.049, and RMSE 7.07～16.01. It was concluded that the present hyperspectral model was feasible and reliable for estimating C/N ratio of rice leaf with different cultivars and nitrogen levels.