Sources, degradation, and thermal stability of organic matter in riparian soils along two major rivers of China

Abstract

Riparian zones function as critical interfaces for the cycling of organic matter (OM) among terrestrial, aquatic, and oceanic environments. However, the characteristics of OM in riparian soils are not yet well understood. This study examined total organic carbon (TOC), stable carbon isotopic composition (δ¹³C), lignin phenols, and thermal gravimetric properties of OM in riparian soils along the Changjiang and Huanghe. The primary objective of this study was to enhance our understanding of the sources, distribution, degradation, and thermal stability of soil OM in these two major river systems in China. The TOC and lignin contents (Λ₈) in riparian soils along the Changjiang were found to be relatively higher than those in the Huanghe soils. Both river systems exhibited considerable spatial heterogeneity and seasonal variability. Analyses of δ¹³C, carbon-to-nitrogen (C/N) ratios, and Λ₈ indicate that the OM in the soils of both river systems primarily originates from terrestrial higher plants and aquatic vegetation, but modified by microbial processes. However, the lignin decay parameters and the ratios of TOC and Λ₈ to specific surface area (SSA) suggest that the degradation of OM in these soils is less extensive compared to that in riverine particulate matters and coastal sediments. This indicates a substantial loss of terrestrial OM and an addition of marine OM during the transport from soils to rivers and estuaries. The carbon reactivity index (CRI) of the soils, as determined by thermogravimetric analysis, ranged from 50% to 100%, which is generally lower than that observed in coastal sediments, suggesting that the OM in these soils exhibits greater thermal reactivity. This study addresses a critical knowledge gap regarding the lignin and thermal stability of OM in China’s terrestrial ecosystems, providing a foundation for a better understanding of the fate of terrestrial OM as it transitions from land to ocean.