Flux and source of dissolved inorganic carbon in a headwater stream in a subtropical plantation catchment

Abstract

Dissolved inorganic carbon (DIC) in headwater streams significantly contributes to the riverine CO2 emission to the atmosphere and oceans. However, understanding of the sources and mechanisms of DIC generation and mass balances of DIC flux in headwater streams at the scale of catchments remain poor and add to uncertainty in regional carbon estimates. In this study, the input and output of water fluxes and their chemical and isotopic compositions in a headwater stream were investigated from 2016 to 2019 in a subtropical plantation catchment. Our results showed that the headwater stream catchment experienced a net carbon loss of 262–4352 kg C km−2 yr−1 based on the water balance of annual precipitation, evapotranspiration, headwater stream and groundwater discharge, and change in soil water storage. The headwater stream exhibited comparable DIC flux (3561 ± 933 kg C km−2 yr−1) and higher DIC concentration (7.91 ± 3.13 mg L−1) than precipitation (3662 ± 682 kg C km−2 yr−1 and 2.63 ± 1.22 mg L−1, respectively). Differences in DIC concentration between precipitation and headwater stream were mainly controlled by soil CO2 dissolution, carbonate weathering, and their synergistic effects. Relative contributions to DIC fluxes in this headwater stream were 25.3 ± 5.2% by precipitation, 47.3 ± 6.2% by soil biological CO2, and 27.4 ± 7.3% by carbonate weathering. Our results indicated that the DIC-generating processes may result in an underestimation of soil biological CO2 efflux to the atmosphere. Further, soil carbonate weathering, and a net carbon loss at the scale of catchments may be overlooked in acid soils.