Integrating terrestrial and aquatic ecosystems to constrain estimates of land-atmosphere carbon exchange

Abstract

In this Perspective, we put forward an integrative framework to improve estimates of land-atmosphere carbon exchange based on the accumulation of carbon in the landscape as constrained by its lateral export through rivers. The framework uses the watershed as the fundamental spatial unit and integrates all terrestrial and aquatic ecosystems as well as their hydrologic carbon exchanges. Application of the framework should help bridge the existing gap between land and atmosphere-based approaches and offers a platform to increase communication and synergy among the terrestrial, aquatic, and atmospheric research communities that is paramount to advance landscape carbon budget assessments.

Fig. 1. Carbon fluxes across the atmosphere-land-water continuum of a watershed.

Conceptual diagram of the vertical and lateral flows of biospheric carbon across a watershed, depicting the most common source/sink role of different terrestrial and aquatic ecosystems. Highlighted in color are the elements involved in the calculation of land-atmosphere carbon exchange either as Watershed NEE (∑NEE; in purple) or NWE (Eq. 3; in dark turquoise). Carbon is withdrawn from the atmosphere through the photosynthesis of forests, wetlands, agricultural lands, and other terrestrial ecosystems. Yet not all the carbon accumulates or returns to the atmosphere in situ. A significant fraction of terrestrial productivity and respiratory CO₂ is hydrologically transferred to inland waters, from where it can be emitted back to the atmosphere, stored in aquatic sediments, or routed downstream towards the watershed outlet (inset). Since inland waters receive external carbon inputs, they are generally net sources of carbon to the atmosphere while at the same time they act as long-term sinks of terrestrially derived carbon. The riverine carbon export at the watershed outlet (R_Export) integrates the transfer, routing, and cumulative processing of terrestrial carbon through the entire aquatic network, and it can be seen as the imbalance between the net watershed carbon exchange with the atmosphere and the total watershed carbon accumulation. In managed landscapes, the harvesting and trading (T) of wood and crops involve additional lateral carbon fluxes that need to be accounted for in watershed carbon budget assessments. Non-biospheric carbon fluxes derived from volcanism, chemical weathering, and petrogenic organic carbon oxidation may also significantly contribute to contemporary land-atmosphere carbon exchange. NEE Net Ecosystem Exchange, NWE Net Watershed Exchange, ∆C ecosystem carbon accumulation (or loss), W wetlands, IW inland waters, A agroecosystems, F forests. Figure by Visualizing Science.

Fig. 2. Examples of integrative carbon budgets across different landscape types.

The arrows’ width shows the relative magnitude of carbon flux and is relative within, but not among each case study. a North Temperate Lakes LTER site; b Ombrotrophic peatland catchment in southern Scotland; c Sugarcane plantation in Australia. ∆C_IW in the peatland example (panel b) was not measured and assumed negligible. Units are Gg C yr⁻¹ in panel a, and g C m⁻² yr⁻¹ in panels b and c. NEE: Net Ecosystem Exchange, ∆C: ecosystem carbon accumulation (or loss).