4 July 2023

Subsoil carbon is particularly vulnerable to climate change


International study shows how, at one meter depth, soil gives up more than one-third of its pyrogenic carbon after experiencing a 4°C temperature rise in less than five years

by Matteo Cavallito


Soil is an invaluable resource for storing carbon. But the stress generated by climate change makes the soil itself particularly vulnerable by reducing its ability to store the element. This is highlighted in a recent study involving researchers from Lawrence Berkeley National Laboratory, California, and the University of Zurich.

“Our study shows that climate change will affect all aspects of soil carbon and nutrient cycling. It also shows that in terms of carbon sequestration, there’s no silver bullet,” explained Margaret Torn, Berkeley Lab researcher and lead author of the investigation in a note released by the U.S. university. “IIf we want soil to sustain carbon sequestration in a warming world,” she added, “we will need better soil management practices, which can mean minimal disturbance of soils during forest management and agriculture.”

Grasslands, pastures and forests capture a quarter of global CO2

The carbon captured by forests, grasslands and pastures, researchers recall, amounts to about 25 percent of global emissions. The role of plants, which store the element in cell walls and soil during photosynthesis, is crucial. Carbon stored by soil is about twice the total in the atmosphere. Half of that amount lies below the top 20 centimeters.

The problem is that a vicious cycle is created between soil and climate change. Deforestation and agriculture, the United Nations Intergovernmental Panel on Climate Change recalls, are responsible for about one-fifth of global greenhouse gases.

These emissions promote rising temperatures, which, in turn, lead to a significant decline in carbon stocks stored in soils. By 2021, the same group of researchers had estimated a 33 percent loss over five years in forest soils. The new study highlights in more detail the dynamics that lead to the loss of organic carbon created by plants during photosynthesis.

The study

In the experiment, conducted at the University of California’s Blodgett Forest Research Station in the Sierra Nevada, researchers warmed soil layers up to one meter deep by 4 degrees Celsius, simulating the conditions predicted for the end of the 21st century (+4°C precisely) in the case of no significant reduction in greenhouse gas emissions.

“After 4.5 years of warming, the absolute concentration of pyrogenic carbon (per gram soil) was 37 ± 8% lower in warmed subsoils,” the research states.

In addition, “the wood-derived compound lignin was 17% lower. Finally, hydrolysable lipids were 28 ± 3% lower in warmed compared with control subsoils.” These lipids, the researchers point out, are typically composed of cutin and suberin, the compounds found in leaves, stems and roots that protect plants from pathogens.

Variazioni della presenza di carbonio pirogenico e di polimeri vegetali a diverse profondità del suolo in risposta al riscaldamento. Le concentrazioni di carbonio pirogenico (a), lignina (b) e lipidi idrolizzabili (c) per grammo di suolo non erano significativamente diverse negli strati superficiali (0-20 cm), ma erano decisamente più basse nei sottosuoli riscaldati rispetto a quelli di controllo (20-90 cm). I valori indicati sono le differenze tra le parcelle riscaldate e quelle di controllo espresse in percentuale (valori negativi in rosso, valori positivi in blu). I quadrati neri mostrano la media (n = 3), mentre i cerchi grigi indicano i singoli punti dati. Le barre rappresentano l'errore standard della media. Fonte: Zosso, C.U., Ofiti, N.O.E., Torn, M.S. et al. Rapid loss of complex polymers and pyrogenic carbon in subsoils under whole-soil warming. Nat. Geosci. 16, 344–348 (2023). https://doi.org/10.1038/s41561-023-01142-1 Attribution 4.0 International (CC BY 4.0)

Changes in abundance of PyC and plant polymers at different depths in response to warming. ac, Concentrations of PyC (a), lignin (b) and hydrolysable lipids (c) per gram soil were not significantly different in the topsoil (0–20 cm) but were significantly lower in warmed compared with control subsoils (20–90 cm). Values shown are the differences between the warmed and control plots expressed as a percentage ((warmed – control)/control × 100%), with negative values in red and positive values in blue. Black squares show the mean (n = 3), and grey circles indicate the single data points. Error bars represent the standard error of the mean (s.e.m.). Source: Zosso, C.U., Ofiti, N.O.E., Torn, M.S. et al. Rapid loss of complex polymers and pyrogenic carbon in subsoils under whole-soil warming. Nat. Geosci. 16, 344–348 (2023) Attribution 4.0 International (CC BY 4.0)

New studies on soil carbon planned

The study looked at pyrogenic carbon, which is that soil organic carbon that originates from burned vegetation and other remnants of organic matter that have undergone fire. This subsurface element is considered to be very stable, that is, not very prone to leakage that would lead to additional CO2 emissions.

However, the study concludes, pyrogenic carbon is “vulnerable to decomposition and propose that molecular structure alone may not protect compounds from degradation under future warming.”

The researchers let it be known that they plan to sample the soil under study again in another four and a half years to determine the impact of nine years of warming on soil composition and health. A similar new experiment is also planned in the grasslands of the Point Reyes National Seashore in northern California.