Land carbon sink of the Tibetan Plateau may be overestimated without accounting for the aquatic carbon export

November 8, 2021
118 (46) e2114694118
Wei et al. (1) report a net carbon dioxide (CO2) sink of the Tibetan Plateau (TP) by using a regional-scale eddy covariance observation network. They also project a substantial enhancement of the CO2 sink under a future warming and increasingly wet climate across the TP with modeling efforts. The CO2 sink is attributed to the exceedance of plant carbon assimilation to permafrost carbon loss and plant respiration (1). Although the broad points are robust, we believe the carbon balance of the TP can be a different picture with the inclusion of aquatic carbon export, since not all permafrost carbon losses are captured by the eddy covariance towers.
Terrigenous carbon, including organic carbon from net primary production and inorganic carbon from chemical weathering or soil respiration, actively enters aquatic ecosystems with water flow. The river networks constantly export dissolved and particulate carbon from terrestrial ecosystem to the downstream area. Both lotic and lentic water bodies outgas a large amount of gaseous carbon, including CO2 and methane, to the atmosphere (2). When considering an ecosystem or a region as a volume, the aquatic export of carbon represents a carbon loss. As such, the terrestrial carbon sink can be significantly offset by aquatic carbon removal (3, 4). Our earlier study found that the aquatic carbon export can offset 30% of annual net ecosystem production in a TP watershed (5). Unfortunately, most eddy covariance systems fail to measure the aquatic carbon losses. The gaseous carbon flux by aquatic systems can barely be captured by eddy covariance flux towers installed on land, which are most cases for the Wei et al. (1) study sites. The lateral carbon export in river water must also be supplemented to get the full picture of ecosystem carbon balance.
The TP is also called the “water tower of Asia” and is the source region of many large Asian rivers including Yangtze, Yellow, Lancang, Nu, Yarlung Tsangpo, and Indus. The river networks can export a significant amount of terrestrial carbon out of the TP via waterways (6, 7). The abundant lakes, ponds, and wetlands across the TP may also emit a large amount of carbon. In a warming and wetter climate, the intensified hydrological cycles may increase the aquatic carbon export. For instance, warming-induced permafrost thaw causes substantial lateral soil carbon export (8). More precipitation and increased streamflow may enhance the erosion and transport of soil carbon. Hence, the strength of the projected land CO2 sink (1) is uncertain without considering the magnitude and change of aquatic carbon export.
The idea of a net ecosystem carbon budget is the best concept to consider all carbon gains and losses in an ecosystem or region (9, 10). The complete inventory accounting for carbon flows from both terrestrial and aquatic ecosystems is pivotal to evaluating the ecosystem or regional carbon balance. Although the importance of aquatic carbon fluxes is well-established, land-based carbon flux measurements are more prevalent across the TP. If the permafrost carbon loss by aquatic pathways is poorly quantified or overlooked, the carbon sink may be overestimated.

Acknowledgments

We acknowledge financial support from the National Natural Science Foundation of China (grant 42107062), the Sanjiangyuan National Park Joint Research Program of the Chinese Academy of Sciences and The People’s Government of Qinghai Province (grant LHZX-2020-11), and Fundamental Research Funds for the Central Universities.

References

1
D. Wei et al., Plant uptake of CO2 outpaces losses from permafrost and plant respiration on the Tibetan Plateau. Proc. Natl. Acad. Sci. U.S.A. 118, e2015283118 (2021).
2
T. W. Drake, P. A. Raymond, R. G. M. Spencer, Terrestrial carbon inputs to inland waters: A current synthesis of estimates and uncertainty. Limnol. Oceanogr. Lett. 3, 132–142 (2018).
3
D. Butman et al., Aquatic carbon cycling in the conterminous United States and implications for terrestrial carbon accounting. Proc. Natl. Acad. Sci. U.S.A. 113, 58–63 (2016).
4
J. R. Webb, I. R. Santos, D. T. Maher, K. Finlay, The importance of aquatic carbon fluxes in net ecosystem carbon budgets: A catchment-scale review. Ecosystems 22, 508–527 (2019).
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C. Song et al., Net ecosystem carbon budget of a grassland ecosystem in central Qinghai-Tibet Plateau: Integrating terrestrial and aquatic carbon fluxes at catchment scale. Agric. For. Meteorol. 290, 108021 (2020).
6
C. Song, G. Wang, N. Haghipour, P. A. Raymond, Warming and monsoonal climate lead to large export of millennial-aged carbon from permafrost catchments of the Qinghai-Tibet Plateau. Environ. Res. Lett. 15, 074012 (2020).
7
L. Zhang et al., Significant methane ebullition from alpine permafrost rivers on the East Qinghai-Tibet Plateau. Nat. Geosci. 331, 349–354 (2020).
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C. Plaza et al., Direct observation of permafrost degradation and rapid soil carbon loss in tundra. Nat. Geosci. 12, 627–631 (2019).
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F. S. Chapin et al., Reconciling carbon-cycle concepts, terminology, and methods. Ecosystems 9, 1041–1050 (2006).
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T. F. Keenan, C. A. Williams, The terrestrial carbon sink. Annu. Rev. Environ. Resour. 43, 219–243 (2018).

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Proceedings of the National Academy of Sciences
Vol. 118 | No. 46
November 16, 2021
PubMed: 34750270

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Submission history

Accepted: September 16, 2021
Published online: November 8, 2021
Published in issue: November 16, 2021

Acknowledgments

We acknowledge financial support from the National Natural Science Foundation of China (grant 42107062), the Sanjiangyuan National Park Joint Research Program of the Chinese Academy of Sciences and The People’s Government of Qinghai Province (grant LHZX-2020-11), and Fundamental Research Funds for the Central Universities.

Authors

Affiliations

State Key Laboratory of Hydraulics and Mountain River Engineering, College of Water Resource and Hydropower, Sichuan University, Chengdu 610065, China
State Key Laboratory of Hydraulics and Mountain River Engineering, College of Water Resource and Hydropower, Sichuan University, Chengdu 610065, China

Notes

1
To whom correspondence may be addressed. Email: [email protected].
Author contributions: C.S. and G.W. designed research; C.S. performed research; C.S. analyzed data; and C.S. and G.W. wrote the paper.

Competing Interests

The authors declare no competing interest.

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    Land carbon sink of the Tibetan Plateau may be overestimated without accounting for the aquatic carbon export
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