Net landscape carbon balance of a tropical savanna: Relative importance of fire and aquatic export in offsetting terrestrial production

Abstract

The magnitude of the terrestrial carbon (C) sink may be overestimated globally due to the difficulty of accounting for all C losses across heterogeneous landscapes. More complete assessments of net landscape C balances (NLCB) are needed that integrate both emissions by fire and transfer to aquatic systems, two key loss pathways of terrestrial C. These pathways can be particularly significant in the wet–dry tropics, where fire plays a fundamental part in ecosystems and where intense rainfall and seasonal flooding can result in considerable aquatic C export (ΣFaq). Here, we determined the NLCB of a lowland catchment (~140 km2) in tropical Australia over 2 years by evaluating net terrestrial productivity (NEP), fire‐related C emissions and ΣFaq (comprising both downstream transport and gaseous evasion) for the two main landscape components, that is, savanna woodland and seasonal wetlands. We found that the catchment was a large C sink (NLCB 334 Mg C km−2 year−1), and that savanna and wetland areas contributed 84% and 16% to this sink, respectively. Annually, fire emissions (−56 Mg C km−2 year−1) and ΣFaq (−28 Mg C km−2 year−1) reduced NEP by 13% and 7%, respectively. Savanna burning shifted the catchment to a net C source for several months during the dry season, while ΣFaq significantly offset NEP during the wet season, with a disproportionate contribution by single major monsoonal events—up to 39% of annual ΣFaq was exported in one event. We hypothesize that wetter and hotter conditions in the wet–dry tropics in the future will increase ΣFaq and fire emissions, potentially further reducing the current C sink in the region. More long‐term studies are needed to upscale this first NLCB estimate to less productive, yet hydrologically dynamic regions of the wet–dry tropics where our result indicating a significant C sink may not hold.

Partitioning carbon sources between wetland and well-drained ecosystems to a tropical first-order stream – implications for carbon cycling at the watershed scale (Nyong, Cameroon)

“…Johnson et al, 2008). Therefore, F D (MgC-CO 2 yr −1 ) was estimated from a mass balance that calculates the loss of the dissolved CO 2 between non-flooded forest groundwater (F D-GW ) (or wetland, F D-WL ) and stream water, using CO 2 concentrations and drainage data, a method similar to that of and Duvert et al (2020a), as the following:…”

“…In a small tropical catchment (140 km 2 ) in Australia, in which the land use was shared between dry savanna and wetland, the contribution of the total hydrological export of C to the stream relative to the local net terrestrial C sink was 7 % (Duvert et al, 2020a). However, Duvert et al (2020a) did not partition the hydrological export of C to the river between dry savanna and wetland. Furthermore, to the best of our knowledge, partitioning the hydrological export of C to rivers between well-drained ecosystems and wetland has never been done in tropical Africa.…”

“…At the plot scale and in temperate climate, the very few studies that compare the local net terrestrial C sink with direct measurements of the hydrological export of C from welldrained ecosystems showed less than 3 % of the local net terrestrial C sink is actually exported to the aquatic environment Kindler et al, 2011). In a small tropical catchment (140 km 2 ) in Australia, in which the land use was shared between dry savanna and wetland, the contribution of the total hydrological export of C to the stream relative to the local net terrestrial C sink was 7 % (Duvert et al, 2020a). However, Duvert et al (2020a) did not partition the hydrological export of C to the river between dry savanna and wetland.…”

“…In tropical watersheds, considering the importance of lateral inputs in sustaining riverine C fluxes, quantifying hydrological C fluxes resulting from the drainage of well-drained ecosystems and wetlands is thus fundamental to close the riverine C budget. Still, in tropical watersheds, questions remain about the quantification and partitioning of hydrological C fluxes resulting from the drainage of well-drained ecosystems and wetland and their significance in comparison to the local net terrestrial C sink (Duvert et al, 2020a). At the plot scale and in temperate climate, the very few studies that compare the local net terrestrial C sink with direct measurements of the hydrological export of C from welldrained ecosystems showed less than 3 % of the local net terrestrial C sink is actually exported to the aquatic environment Kindler et al, 2011).…”

“…Furthermore, to the best of our knowledge, partitioning the hydrological export of C to rivers between well-drained ecosystems and wetland has never been done in tropical Africa. As the warmer and wetter conditions expected in tropical Africa in a near future will likely modify C fluxes at the watershed scale, integrative studies on C cycling in tropical watersheds are required to get a better grasp of the present drivers of riverine CO 2 emissions and thus to better predict future changes (Duvert et al, 2020a).…”