Different chemical composition and storage mechanism of soil organic matter between active and permafrost layers on the Qinghai–Tibetan Plateau

Abstract

Although many studies have paid attention to the storage and dynamics of organic carbon (OC) in the Arctic permafrost, there are limited reports for low-latitude alpine permafrost ecosystems like Qinghai–Tibet Plateau (QTP). The aims of this study are to (1) reveal the vertical distribution of OC stocks in permafrost soils; (2) assess the storage and transformation of permafrost OC; and (3) disentangle the effect of mineral protection on OC storage in permafrost soils. A 2-m permafrost profile on the QTP was investigated to understand vertical distribution of organic matter (OM) in different density fractions based on elemental composition, carbon stable isotope (δ13C), mineral grain size, Fe and Al concentrations, and solid-state 13C nuclear magnetic resonance spectroscopy (13C NMR). A positive relationship between light fraction organic carbon (LOC) and root abundance indicates that root is an important contributor for LOC. However, in heavy fractions, the total organic carbon to total nitrogen ratio (TOC/TN) is significantly lower than that in light fractions. This, combined with a negative correlation between TOC/TN and heavy fraction organic carbon (HOC), indicates that microbial input affects the quantity of HOC. In the active layer, the downward decreased δ13C, elevated alkyl/O-alkyl, and decreased ratio of 70–75:52–57 ppm suggest selected decomposition of carbohydrate components. While in the deep permafrost layer, the relatively constant δ13C values and chemical composition of OM suggest a stable environment and minor impact of cryoturbation. The redundancy analysis shows that soil textures and concentration of Fe and Al have weak correlations with OC content, but for deep permafrost soils only, fine soil fraction is associated with aromatic carbon, and Al has strong influence on alkyl carbon, which could be attributed to OM-mineral stabilization. Our results suggest that soil textures and Fe and Al concentration affect SOM preservation in the permafrost soils on the QTP; however, they largely control the quality rather than quantity of OM.