Sediment particle size distribution, source of organic matter and environmental implications in the Liao River, northeast China

Abstract

The Liao River Basin, one of China’s most severely affected coastal regions by soil erosion, necessitates a thorough quantitative evaluation of the sources of organic matter (OM) across sediment particle sizes to enhance understanding of carbon biogeochemical cycling. This study examined total organic carbon (TOC), total nitrogen (TN), the TOC/TN ratio (C/N ratio), and δ¹³C and δ¹⁵N isotope signatures in four distinct sediment particle size fractions from Liao River sediments, sampled during the summer and winter of 2023. The results demonstrated that the highest concentrations of TOC, TN, and C/N ratio were found in the <2 µm particle fraction (p <0.05), with summer values of 2.50 ± 0.43 %, 0.19 ± 0.06 %, and 13.89 ± 2.40, respectively, and winter values of 2.71 ± 0.42 %, 0.19 ± 0.05 %, and 14.66 ± 2.55. OM sources were characterized using δ¹³C and C/N ratio analysis, and the MixSIAR Bayesian model was employed to quantify their contributions. Aquatic plants were identified as the dominant OM source in the <2 µm fraction, whereas soil and sewage predominated in the >2 µm fractions. These results indicate that exogenous OM in the Liao River’s mainstem is primarily influenced by sewage discharge and soil erosion. This process facilitates the continuous release of nutrients from sediment particles into the water column, contributing to secondary pollution. Therefore, enhanced management of sewage discharge and erosion control is essential. Carbon and nitrogen isotope fingerprinting within sediment particle size fractions provides novel insights into biogeochemical processes within soil microenvironments.