A Mechanism Study of Reflectance Spectroscopy for Predicting Soil Total Nitrogen
Reflectance spectroscopy has been widely used for predicting soil properties due to its rapidity and convenience.In past decades,the application of soil spectroscopy on soil science studies has increased exponentially.The total nitrogen (TN)content in soil is an important index for soil fertility and the rapid prediction of TN content with spectroscopy serves an important func-tion in precision agriculture.However,whether the TN content in soil is predicted through its relationship with soil organic car-bon (SOC)or on its specific absorption is still debatable.The obj ective of this study was to explore the mechanism of reflectance spectroscopy for predicting TN in soils.Soils used for calibration were sampled from coastal soil in the north of Jiangsu prov-ince.Partial least squares regression (PLSR)analysis was used for the calibration datasets with different TN content when the sample number is the same in every dataset.In order to explore the mechanism of reflectance spectroscopy for predicting total ni-trogen in soil,the changes of model accuracies and the correlation of TN and SOC were analyzed.The results indicated that the contents of TN and SOC in soil were relatively lower because the soil was derived from coastal sediments in the past 1 000 years and formed during cultivation.There was strong correlation between TN and SOC (R=0.98).The prediction accuracy of TN increased at first and then decreased slightly with the increase of mean,standard deviation of TN content.Meanwhile,the chan-ges of prediction accuracy comply well with coefficients of variation.In conclusion,when the TN content is relatively low (mean TN<0.27 g·kg-1 ),the correlation coefficient between TN and SOC was moderately-high and TN was predicted on the basis of N absorbers.When the TN content is relatively high (mean TN>0.29 g·kg-1 ),strong correlation coefficients were obtained for TN and SOC and the model accuracy of SOC were better than TN.The effect of SOC to spectroscopy enhanced with the in-crease of SOC content,which masked the spectral features of N.Therefore,TN was predicted through the correlation with SOC when the TN content is high.This study revealed the mechanism of reflectance spectroscopy for predicting TN in soil and it could provide a theoretical basis for predicting soil TN content rapidly using reflectance spectroscopy.
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