Gaseous and fluvial carbon export from an Amazon forest watershed

Abstract

The transfer of carbon (C) from Amazon forests to aquatic ecosystems as CO2 supersaturated in groundwater that outgases to the atmosphere after it reaches small streams has been postulated to be an important component of terrestrial ecosystem C budgets. We measured C losses as soil respiration and methane (CH4) flux, direct CO2 and CH4 fluxes from the stream surface and fluvial export of dissolved inorganic C (DIC), dissolved organic C (DOC), and particulate C over an annual hydrologic cycle from a 1,319-ha forested Amazon perennial first-order headwater watershed at Tanguro Ranch in the southern Amazon state of Mato Grosso. Stream pCO2 concentrations ranged from 6,491 to 14,976 μatm and directly-measured stream CO2 outgassing flux was 5,994 ± 677 g C m−2 y−1 of stream surface. Stream pCH4 concentrations ranged from 291 to 438 μatm and measured stream CH4 outgassing flux was 987 ± 221 g C m−2 y−1. Despite high flux rates from the stream surface, the small area of stream itself (970 m2, or 0.007% of watershed area) led to small directly-measured annual fluxes of CO2 (0.44 ± 0.05 g C m2 y−1) and CH4 (0.07 ± 0.02 g C m2 y−1) per unit watershed land area. Measured fluvial export of DIC (0.78 ± 0.04 g C m−2 y−1), DOC (0.16 ± 0.03 g C m−2 y−1) and coarse plus fine particulate C (0.001 ± 0.001 g C m−2 y−1) per unit watershed land area were also small. However, stream discharge accounted for only 12% of the modeled annual watershed water output because deep groundwater flows dominated total runoff from the watershed. When C in this bypassing groundwater was included, total watershed export was 10.83 g C m−2 y−1 as CO2 outgassing, 11.29 g C m−2 y−1 as fluvial DIC and 0.64 g C m−2 y−1 as fluvial DOC. Outgassing fluxes were somewhat lower than the 40–50 g C m−2 y−1 reported from other Amazon watersheds and may result in part from lower annual rainfall at Tanguro. Total stream-associated gaseous C losses were two orders of magnitude less than soil respiration (696 ± 147 g C m−2 y−1), but total losses of C transported by water comprised up to about 20% of the ± 150 g C m−2 (±1.5 Mg C ha−1) that is exchanged annually across Amazon tropical forest canopies.