In 2021, we published a Perspective paper in Nature Reviews Earth & Environment (1) arguing that, despite mechanistic uncertainty, there is confidence that a strong, positive land carbon–climate feedback can be expected. Several lines of evidence support such a claim, and one of them relates with the accumulation of soil organic carbon in the cold regions of the planet, that is arctic, subarctic and alpine ecosystems. Briefly, the balance between C gains from photosynthesis and C losses from microbial respiration regulates soil organic carbon storage (2). If temperature strongly regulates this balance, C inputs may exceed outputs, leading to a massive accumulation of soil organic carbon in cold regions over millennia. A remaining gap in such an analysis was whether much of the soil organic carbon in these cold regions was in the particulate fraction compared with the mineral-associated fraction.

The dominance of the particulate vs. the mineral-associated fraction has major implications. For instance, the particulate fraction is expected to be less persistent over time as it is not protected from microbial decomposition by interactions with the soil matrix or occlusion within soil aggregates (3). However, despite the distinction of organic carbon fractions has already fostered a more mechanistic interpretation of soil C vulnerability to climate and land use changes in other biomes (4-5), a comprehensive study across the Earth cold regions was lacking. Importantly, cold regions store a third of the global soil organic carbon stocks (6), and are warming 2-4 times faster than the global average (7-8). If soil organic carbon in cold regions is predominantly stored in the particular fraction, likely triggering higher relative soil carbon losses with warming compared to milder biomes, then such a result would build evidence for a potentially dramatic land-C climate feedback involving Earth's cold region soils.

For this endeavor, I joint forces with C. Plaza and M. Bradford to design a literature synthesis gathering a robust dataset. By that time, I was just starting a new position at ICA-CSIC (Spain), and we designed this study as a team collaborative effort where each of us (I. Benavente, JC García-Gil, A. Goñi-Urtiaga, M. Panettieri, A. Rey) reviewed a number of studies. Then, we looked for inputs from reputed soil scientists working in cold regions (T. Schuur and C. Müeller), and experts on remote sensing (J. Gaitán), data analyses (M. de Celis), carbon fractions (N. Sokol), and global databases (M. Delgado-Baquerizo, T. Sáez-Sandino and L. Tedersoo).

We found that soil organic carbon in the first 30 cm of mineral soil is dominated or co-dominated by particulate carbon in both permafrost and non-permafrost soils, and in arctic and alpine ecosystems but not in subarctic environments. Our findings also indicate that the dominance of the particulate carbon increased the vulnerability of soil carbon to climate warming compared with other milder ecosystems. The massive soil carbon accumulation in cold regions appears predominantly distributed in the more vulnerable particulate fraction rather than in the more persistent mineral-associated fraction. Our findings support the likelihood of a strong, positive land-carbon climate feedback driven by Earth’s cold biomes.

 References:

¹García-Palacios, P. et al. Evidence for large microbial-mediated losses of soil carbon under anthropogenic warming. Nature Rev. Earth Environ. 2, 507–517 (2021).

²Davidson, E. A. & Janssens, I. A. Temperature sensitivity of soil carbon decomposition and feedbacks to climate change. Nature 440, 165–173 (2006).

³Lavallee, J. M. et al. Conceptualizing soil organic matter into particulate and mineral-associated forms to address global change in the 21st century. Glob. Chang. Biol. 26, 261–273 (2020).

⁴Lugato, E., Lavallee, J. M., Haddix, M. L., Panagos, P. & Cotrufo, M. F. Different climate sensitivity of particulate and mineral-associated soil organic matter. Nat. Geosci. 14, 295–300 (2021).

⁵Georgiou, K. et al. Global stocks and capacity of mineral-associated soil organic carbon. Nat. Commun. 13, 1–12 (2022).

⁶Schuur, E. A. G. et al. Permafrost and climate change: carbon cycle feedbacks from the warming arctic. Annu. Rev. Environ. Resour. 47, 343–371 (2022).

⁷Rantanen, M. et al. The Arctic has warmed nearly four times faster than the globe since 1979. Commun. Earth Environ. 3, 1–10 (2022).

⁸Jansen, E. et al. Past perspectives on the present era of abrupt Arctic climate change. Nat. Clim. Chang. 10, 714–721 (2020).