Factors affecting spectral characteristics of typical takyr solonetzs

Abstract: Spectral reflectance of soil can be used to accurately predict the degree of soil salinization, however, it has not been widely used in the Typical Takyr Solonetzs of northern Yinchuan city, Ningxia Province. Although it has been reported that soil spectral reflectance can be affected by multiple factors, it is still lack of enough evidences to test which factor plays main role in soil spectral reflectance. Therefore, we conducted an experiment using several treatments in Typical Takyr Solonetzs of northern Yinchuan city, including alkalinity degree (extremely alkaline soil, strongly alkaline soil, moderately alkaline soil, slightly alkaline soil, and non-alkaline soil), water content (3.93%, 11.71%, 19.24%, 22.33%, 25.39% and 26.45%), particle size (3 mm, 2 mm, 1 mm, 0.25 mm and 0.1 mm) and roughness (coarse and smooth). We measured soil spectral reflectance both in field and in laboratory under different treatments. By analyzing these variables, we wanted to systematically determine how these factors affect spectral reflectance of Takyr Solonetzs. The results showed that reflectance in the top soil (0-20 cm) observed in the field had a positive relationship with the degree of soil alkalinity, indicating that the soil with the most serious alkalinity had the highest reflectance. In the wavelength range of 450-925 nm, the reflectance of extremely alkaline soil was 7.36%、23.18%、32.10% and 39.97% respectively higher than those of strongly alkaline soil, moderately alkaline soil, slightly soil and non-alkaline soil. Reflectance at 765 nm, 945 nm and 974 nm was sensitive to diagnose the alkalinity of Takyr Solonetzs. Soil reflectance can be significantly decreased by pretreatment (including air dried, ground and sifted before spectral measurement) in laboratory. The reflectance of extremely alkaline soil, strongly alkaline soil, moderately alkaline soil and slightly alkaline soil measured in the field were 0.1645, 0.1312, 0.0575 and 0.0246 higher than that with pretreatment. Spectral reflectance showed a decreasing trend with the increase of soil water content. However, it increased gradually when soil water content was over 22.85% (field capacity). Spectral reflectance in the soil with 26.45% of water content increased by 39.68% and 19.79% than those with 22.33% and 25.39% of water content, respectively. We also observed that particle size of soil sample had obvious impact on soil reflectance. When the particle size was smaller than 0.25 mm, the difference of reflectance between 0.25mm and 0.1mm were smaller than the particle size with 3 mm, 2 mm and 1 mm. The reflectance of soil particle size below 0.10 mm were 30.39%, 28.62%, 23.87% and 8.10% higher than 3 mm, 2 mm, 1 mm and 0.25 mm of particle size, and the bigger particle size had a special twin - peak phenomenon. Surface roughness had a negative effect on soil reflectance. We concluded that salinity degree can significantly influenced the field spectral reflectance of Takyr Solonetzs; the soil water content was the major factor that influenced the spectral reflectance of strongly alkaline soil by pretreatment, and there were little influences of the salinity degree and surface roughness on the soil spectral reflectance. Our results will be helpful for in situ prediction of the degree of soil alkalinity of Takyr Solonetzs.