Dissolved Inorganic Carbon Evolution of Sediment Porewater in the Huixian Wetland, Southwest China.

Abstract

Wetlands, as crucial terrestrial carbon reservoirs, have recently suffered severe degradation due to intense human activities. Lacustrine sediments serve as vital indicators for understanding wetland environmental changes. In the current paper, porewater samples were extracted from lacustrine sediment in three boreholes with a depth of ~75 cm in the Huixian karst wetland, southwest China, to study the chemical and dissolved inorganic carbon (DIC) evolution under anthropogenic influence. Two boreholes are situated beneath the Mudong Lake, while the other one is in the degraded wetland area. The results show that porewater in the central region of Mudong Lake is natural HCO₃-Ca type water and recharged by karst groundwater as evidenced by depleted ²H -¹⁸O isotopes. Methanogenesis prevails in this area, suggested by positive δ¹³C values ranging from 4.29‰ to 7.05‰. However, shallow porewater at the western edge of Mudong Lake and porewater in the degraded wetland exhibit significantly higher concentrations of NO₃ ^- and SO₄ ^2-, resulting from the agricultural input and recharged groundwater influenced by oxidation of pyrite. These processes lead to a decrease in methane production and generate DIC through degradation of organic fertilizer and carbonate weathering by sulfuric acid, thereby significantly altering porewater δ¹³C values. Two types of DIC mixing processes were observed based on the increasing δ¹³C values with depth, which can be attributed to the unique karst groundwater subsystems. This work highlights the potential impact of human-induced porewater chemical variations on the fate of DIC, particularly in karst wetland environments.