1. Fujian Provincial Key Laboratory Eco-physiology, Fujian Normal University, Fuzhou 350007, China
2. College of Geographical Sciences, Fujian Normal University, Fuzhou 350007, China
3. Beijing Research Institute of Uranium Geology, National Key Laboratory of Science and Technology on Remote Sensing Information and Image Analysis, Beijing 100029, China
4. State Key Laboratory for Subtropical Mountain Ecology of the Ministry of Science and Technology and Fujian Province, Fujian Normal University, Fuzhou 350007, China
Abstract：Nature and Science have revealed the interaction between surface rocks and the global nitrogen cycle, considering that rock is an important reservoir of nitrogen. Nitrogen excretion during rock weathering is one of the main sources of nitrogen in soil ecosystems, and has a major impact on the local and even global nitrogen cycle. Sedimentary rock, one of the important rock types, occupies 75% of the land surface. Danxia landform is one of the important geomorphologic types formed by the weathering and erosion of sedimentary rocks. Studies on Danxia landform sedimentary rock weathering and nitrogen excretion contribute to better understanding of the global nitrogen cycle. In this paper, we systematically collected red sandstone samples from Danxia landform in Longhushan Mountain of Jiangxi Province, and studied the mineral composition, spectral characteristics and nitrogen content of the rock samples using polarizing microscope, ASD spectrometer and carbon-nitrogen analyzer. The experimental results showed that the sandstones in the Longhushan Mountain area are dominated by feldspar sandstones. The granular minerals include feldspar and quartz, and the cement is mainly composed of iron and calcium with minor muddy. The chemical weathering of sandstone in Longhushan Mountain area includes cement dissolution and hydration. The dissolution of cement is mainly caused by iron dissolution, and the hydration of granular mineral is mainly represented by feldspar altering to clay mineral. The absorption index at 902 nm (the characteristic absorption band of Fe3+)decreased when iron dissolved, while that at 2 210 nm (the characteristic absorption band of clay Al—OH) increased after feldspar altered to clay minerals. For the same sample, the nitrogen content in the chemically weathered part is lower than that in the unweathered part, and the nitrogen content has a negative relationship with the absorption index at 2 210 nm (R2=0.802 6), indicating that rock weathering contributes to nitrogen excretion. For different samples, the correlation between nitrogen content and the absorption index at 2 210 nm is very poor (R2=0.025 6), indicating that the different mineral compositions and structures of rocks will decrease the correlation between absorption index and nitrogen content. The above studies showed that the reflectance spectroscopy provides a potential approach to study sandstone weathering and nitrogen excretion, but samples used in the study should have same lithology property.
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