Sediment organic carbon and nitrogen trapping in a small reservoir within a typical agricultural granite catchment

Abstract

Small reservoirs are more widely distributed globally than larger reservoirs, and they have characteristic sediment total organic carbon (TOC) and total nitrogen (TN) trapping patterns that differ from those of larger reservoirs. Few studies have quantified the TOC and TN trapping in a small reservoir within a catchment underlain by a single lithology, particularly in non-karst terrain, from the perspective of environmental change. Based on the chronologies established using 137Cs and 210Pbex, we assessed the temporal patterns of sediment TOC and TN trapping in a small reservoir within a typical agricultural granite catchment (Yimeng Mountains, China) by examining depth trends in TOC and TN concentrations in three sediment cores and temporal trends in TOC and TN deposition rates in the reservoir over the past ∼60 years. The TOC and TN concentrations and C/N ratios show an overall upward increase in the three cores. The TOC and TN deposition rates were relatively high during 1954–1962, decreased to the lowest rates during 1963–1972, and generally increased thereafter. Anthropogenic sources associated with agriculture were the dominant factor in the up-core increases in TOC and TN concentrations, although natural sources were the main contributor to TOC and TN deposition. The temporal changes in TOC and TN deposition rates were mainly due to the sediment yield rather than anthropogenic sources associated with agriculture. The changes in TOC and TN concentrations with depth, and temporal changes in the TOC and TN deposition rates determined from the three cores, reflect the temporal patterns of sediment TOC and TN trapping in the small reservoir over the past ∼60 years. Sediment yield exerted a greater effect on the TOC and TN deposition in the studied reservoir, as compared with a previously reported reservoir within a small agricultural limestone catchment, and the different results of the two studies originated from the contrasting lithologies in the two catchments. Moreover, a comparison with data obtained for a previously studied large reservoir suggests that differences in TOC and TN deposition (including concentrations, specific deposition rates, and temporal trends in deposition rates) are related to catchment size, and the comparison highlights the characteristic patterns of TOC and TN trapping in the small and large reservoirs. The present findings improve our understanding of the temporal patterns of sediment TOC and TN trapping in a small reservoir within a typical agricultural granite catchment in terms of environmental changes influenced by human activity. This study also enhances our knowledge of the barrier effects of a dam on the downstream transport of TOC and TN in a small catchment in a non-karst setting, and the redistribution of sediment TOC and TN in a large river, which can help guide strategies for water resource management on the river basin-scale.