1. Citrus Research Institute, Southwest University, Chongqing 400712，China 2. Citrus Research Institute, Chinese Academy of Agricultural Sciences, Chongqing 400712, China 3. National Engineering Research Center for Citrus Technology, Chongqing 400712, China
Abstract：The relationship between the spectrum characteristics and nitrogen content of soils in citrus orchard of the Three Gorges Reservoir Area was studied by analyzing the visible-near infrared spectrum. The results showed that the soil reflectivity increased lineally as the wavelength increases across the visible spectrum and reached a stable plateau in the short wavelength near-infrared region(780-1 750 nm)without much fluctuation. In the long wavelength near-infrared region (1 750-2 400 nm)the reflectivity of the soils was higher with higher fluctuation. There were three strong absorbance peaks around 1 416, 1 913 and 2 209 nm, respectively, in the long wavelength infrared region. Soil available nitrogen content and total nitrogen content were positively correlated with soil light reflectivity but negatively correlated with catoptric-spectrum values reciprocal logarithm. At 541 nm of visible light region, a high positive correlation was found between the available nitrogen content and the first derivative of the soil reflective spectrum with a correlation coefficient of +0.605** and the best fitting equation was y=2E+09x2-3E+06x+890.49, where R2=0.5，and x is the first derivative of the soil reflective spectrum. At 1 909 nm of the near-infrared long wavelength region, the correlation between the total nitrogen content and the reciprocal-log values of the reflective spectrum of the soils was the best with a correlation coefficient of -0.612**, and the best fitting equation was y=1.372 1x2-2.107 5x+0.859 2, where R2=0.4, and x is the reciprocal values of the log reflective spectrum of the soils.
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