A Novel Approach to Estimate the Karst Carbon Sink Captured by Submerged Plants

Abstract

Over the past decades, the stability of the karst carbon sink (KCS) has been controversial due to the reversible chemical reaction. Photosynthesis by submerged plants plays a key role in stabilizing the KCS by directly converting the dissolved inorganic carbon (DIC) into organic matters. However, the calculation of KCS captured by submerged plants is inaccurate, hindering a comprehensive understanding of the karst carbon cycle. To address this issue, a field diel monitoring and a hydrogeochemical measurement was conducted in a typical karst spring-fed river in southwest China. Using 2515 sets of hydrochemical parameters, including water temperature (T), electrical conductivity (Ec), and dissolved oxygen (DO), we developed a novel DO method to calculate the KCS captured by submerged plants and compared it with the traditional calculation method of DIC. Results showed that the photosynthesis of submerged plants is the dominant factor controlling the diel variation of hydrogeochemistry, which imposed negative feedback on the outgassing effect and positive feedback on the precipitation effect. The DO method revealed that the river was highly autotrophic, with a ratio of photosynthesis to respiration (P/R) as 1.53. Submerged plants contributed a mean net ecosystem productivity (NEP) of 0.35 mmol L-1 day-1 equivalent CO2, and the contribution of submerged plants in Zhaidi River to utilized and sequestered DIC in this reach is around 0.62 mg, with an average biomass of 74.18 kg/m2. The results suggested that the mass balance method accounted for only 34.29% compared to the innovative DO method in calculating the KCS sequestered by submerged plants, implying that the traditional method significantly underestimates KCS by submerged plants. It was found that 36.32% of the KCS was captured by submerged plants during the lateral transportation, suggesting a new method for more precisely calculating KCS flux and the global carbon cycle.

Keywords: Karst carbon sink, Submerged plants, Photosynthesis, Hydrochemistry, DO method