Global changes alter the amount and composition of land carbon deliveries to European rivers and seas

ARTICLE https://doi.org/10.1038/s43247-022-00575-7 OPEN Global changes alter the amount and composition of land carbon deliveries to European rivers and seas 1234567890():,; Haicheng Zhang1,2 ✉, Ronny Lauerwald Pierre Regnier1 ✉ 3, Philippe Ciais 4, Kristof Van Oost5, Bertrand Guenet6 & Lateral carbon transfer along the land-ocean continuum is a key component of global carbon cycle, yet its response to global change is poorly quantified. Here, we use a land-surface model to simulate vertical (soil-plant-atmosphere) and lateral (land-river-ocean) carbon exchanges in Europe between 1901–2014 and investigate the effect of atmospheric carbon dioxide, climate and land use changes on lateral carbon transfer. We find that global change during 1901–2014 led to a significant increase in the total terrestrial carbon delivery to European rivers (33% increase) and to the sea (20% increase). Carbon delivery increased in the dissolved phase and decreased in the particulate phase. Climate change, increased atmospheric carbon dioxide, and land-use change explain 62%, 36% and 2% of the temporal change in European lateral carbon transfer during the study period, respectively. Our findings suggest that redistribution of soil carbon due to lateral carbon transfer induced a 5% reduction in the net land carbon sink in Europe. 1 Department Geoscience, Environment & Society-BGEOSYS, Université libre de Bruxelles, 1050 Bruxelles, Belgium. 2 School of Geography and Planning, Sun Yat-Sen University, Guangzhou, China. 3 Université Paris-Saclay, INRAE, AgroParisTech, UMR ECOSYS, 78850 Thiverval-Grignon, France. 4 Laboratoire des Sciences du Climat et de l’Environnement, IPSL-LSCE CEA/CNRS/UVSQ, Orme des Merisiers, 91191 Gif sur Yvette, France. 5 UCLouvain, TECLIM - Georges Lemaître Centre for Earth and Climate Research, Louvain-la-Neuve, Belgium. 6 LG-ENS (Laboratoire de géologie) – CNRS UMR 8538 – École normale supérieure, PSL University – IPSL, 75005 Paris, France. ✉email: ; COMMUNICATIONS EARTH & ENVIRONMENT | (2022)3:245 | https://doi.org/10.1038/s43247-022-00575-7 | www.nature.com/commsenv 1 ARTICLE L COMMUNICATIONS EARTH & ENVIRONMENT | https://doi.org/10.1038/s43247-022-00575-7 ateral carbon (C) transfer (LCT) along the land-to-ocean continuum has been a topic of broad interest as it is an important component of the global C cycle1–6 and can strongly affect the function of aquatic ecosystems including the coastal zone (e.g., oxygen concentrations, nutrients availability, and turbidity)7,8. Global soil erosion and leaching release large amounts of C from land to river and the sea every year5,6. The subsequent deposition and burial of particulate C in river channels and floodplains contribute to the global C sequestration9–11. In contrast, the re-emission of carbon dioxide (CO2), which is leached from upland soils to rivers or produced in-transit by the mineralization of riverine organic C usually constitutes a closed C loop with the atmosphere6,12–14. The fraction that is not buried or recycled back to the atmosphere is ultimately exported to the ocean, partly contributing to the recalcitrant organic C pool, or to trophic status of marine waters and the air-sea CO2 exchange8,15–20. Moreover, previous studies have revealed that the magnitude of global lateral C flux (2.35 ± 0.70 PgC yr−1) is comparable to the net C sink of global terrestrial ecosystems (2.30 ± 1.5 PgC yr−1)6, and ignoring LCT in land surface models results in notable biases (~10%) in the simulated terrestrial C budget21–23. Accurately estimating LCTs along the land-toocean continuum thus is vital for better constraining the C budget of terrestrial and aquatic ecosystems, as well as their response to fossil fuel emissions, land use and climate changes. Nonetheless, there are still notable uncertainties in existing estimates of large-scale LCTs, in particular regarding their temporal trends under global change4–6. Previous estimates of global C loss from soil to rivers vary from 1.6 to 4.1 Pg (=1015 g) C per year (yr−1)5,6 while estimates of global riverine C export to the sea are better constrained and mostly fall in the range 0.7–1.2 Pg C yr−15,6,24 (Supplementary Table S1). Estimates of CO2 emission from global inland waters also vary widely from 0.75 to 2.5 Pg C yr−15,6,25. These assessments are mostly based on inventory and extrapolation of observed riverine organic C and CO2 concentrations, river discharges, and surface area of inland waters1,3,8,26. However, existing observational data are still too sparse to provide a global and unbiased spatial coverage while long-term trends (e.g., before 1980s) are essentially unknown, especially for the particulate organic C (POC)4. Due to the scarcity of observational data, only a few studies have investigated the temporal evolution of LCTs over the past decades at large spatial scales1,6,27. Process-based land-surface models (LSMs) with explicit representation of LCTs, in conjunction with sparse observational data for model calibration and evaluation, have proven to be a suitable approach to estimate the long-term land-to-ocean C fluxes at large spatial scales22,28–30. As LSMs simulate both vertical (atmosphere-plant-soil) and lateral (land-river-ocean) carbon cycles, they can also be used to explore the complex interactions between vertical and lateral C fluxes, from vegetation C uptake to C exports to the ocean. In addition, global changes, most importantly atmospheric CO2 increase, climate change and land use change, have together resulted in notable alterations of LCTs by impacting not only surface runoff and belowground drainage31, but also vegetation growth, soil C stocks and the decomposition rate of riverine organic C4,32–34. As the few observed changes in LCTs result from the combined effects of different global/regional change factors, it is difficult to attribute the overall temporal evolution to each of these factors. By being able to conduct factorial simulations, LSMs are ideal tools for attribution analyses, allowing to distinguish the effect of each global change factor on past LCTs and predict their future evolution under different climate change or land use change scenarios. 2 At the European scale, the magnitude of the lateral C flux from land to river (113 Tg C yr−1) has been estimated to be comparable to the carbon accumulation in European forests1, and the C concentrations in the waters of many European rivers have been observed to have changed drastically over the past decades33,35–37. Even so, an integrated view of the magnitude and composition of LCTs through the European river network and how they evolved over the past century as a result of changes in climate, land-use, and atmospheric CO2 concentration, is still lacking. In the absence of such temporal trends, the river C fluxes cannot be decomposed into natural and anthropogenic perturbation terms, precluding their inclusion in anthropogenic CO2 budget analyses, as performed by the Global Carbon (...truncated)