A JRC-led Nature Geoscience paper shows that, by 2080, extreme climate change could lead to carbon losses from mineral topsoils in the order of 2.5 ± 1.2 Pg in the EU and UK.
While this estimate is lower than previously reported potential losses, it highlights the importance of adopting tailored practices for each ecosystem due to the different sensitivity of organic carbon fractions to climate change.
Not all topsoils are equal when it comes to carbon
The European Green Deal’s roadmap for carbon neutrality requires a significant contribution of land: forests must continue to act as carbon sinks, while agricultural soils should be managed in such a way as to reverse carbon losses and sequester organic carbon.
However, not all the carbon in soil is made equal.
Soil organic matter can be separated into two types: particulate organic matter (POM) and mineral-associated organic matter (MAOM).
This separation provides a useful framework for scientists to understand the mechanisms that drive carbon transformation in soils.
POM and MAOM
Particulate organic matter (POM) is predominantly made up of plant-based particulates, while mineral-associated organic matter (MAOM) consists of single molecules or microscopic pieces of organic material that have either leached directly from plants or been chemically transformed by the soil biota.
Particulate organic matter is often used as an indicator of soil quality as it is a readily available source of soil nutrients, contributes to soil structure, is highly sensitive to soil management and has a low nitrogen content.
Mineral-associated organic matter is largely made up of microbial products that are richer in nitrogen, and more persistent in time.
Mapping POM and MAOM
The authors assessed the current geographical distribution of mineral topsoil carbon in POM and MAOM by land cover using a machine-learning approach, based on European-wide databases and the physical fractionation of soil organic matter from 352 Land Use and Coverage Area frame Survey (LUCAS) samples.
The derived high-resolution maps allowed for an unprecedented quantification of the nitrogen and organic carbon stock partitioned into POM and MAOM in the mineral topsoils of the EU and UK.
POM and MAOM carbon stock changes under climate change
Previous experimental and modelling studies already suggested likely losses of carbon from soil under warming and climate change.
In this study, the authors applied a space-for-time approach to assess the potential vulnerability of POM and MAOM carbon stocks to climate change.
Using a linear multiregression model between the measured carbon in MAOM and POM against the current mean annual temperature and precipitation in interaction with soil texture and land cover, they estimated the model regression slopes within the broader forest, cropland, grassland and shrubland categories.
The slopes, which represent the change in soil organic carbon per unit change of temperature and precipitation, were upscaled using the projections under three different global climate models (HadGEM2-ES, CNRM-CM5 and IPSL-CM5A-LR) for the RCP8.5 (high-emissions) scenario.
This high-emissions scenario is frequently referred to as “business as usual”, suggesting that is a likely outcome if society does not make concerted efforts to cut greenhouse gas emissions.
Projected drop in soil carbon, particularly from forest- and croplands
In their paper, the scientists observed a general decrease of POM and MAOM across the European Union and United Kingdom by 2080 under the simulated climate changes.
The greatest relative decrease (−20%) in the carbon stocks was observed for the POM fraction in forest, which is the only land cover type to show a significant negative relationship between carbon in POM and temperature.
The next largest soil carbon losses forecasted were from croplands, with important losses of both MAOM (−11%) and POM (−6%).
Policy recommendations: more forests, and better land management
Under the worst-case scenario, these new estimates do not suggest a catastrophic carbon loss, but do suggest a potential long-term carbon dioxide (CO2) debt (9.2 PgCO2e) which would necessitate a greater abatement of the current EU emissions (in the order of 4.5 Pg yr−1 of CO2) to achieve carbon neutrality.
The scientists urge the implementation of coniferous forest management practices that increase plant inputs to soils to offset POM losses, and the adoption of best land management practices to avert the loss of and to build up both POM and MAOM in agricultural soils.
- Different climate sensitivity of particulate and mineral-associated soil organic matter
- Soil Organic Matter (SOM) fractions
- Land Use and Coverage Area frame Survey (LUCAS)
- 19 Maj 2021