Uptake and Release—What Is Driving Change in the Net Carbon Budget in Forest Soils?

It is well established that soils are a significant store of carbon in terrestrial ecosystems—very few papers on soil carbon dynamics fail to mention the fact that there is more carbon stored in global soils than in vegetation and the atmosphere combined. This is generally correctly cited to justify research into the processes driving changes in soil carbon storage, and how management of land use can help protect this store. Meanwhile, mitigation of climate change by enhanced carbon sequestration through increasing tree cover across Earth's biomes is regularly discussed in scientific literature and policy. There has been a vigorous debate over the validity of tree establishment in specific contexts and the usefulness of tree planting targets for carbon off-sets in net-zero policies. These results highlight the importance of including deep- soil C in forest ecosystem assessments, as it plays a key role in the overall C balance. (Mayer et al. 2025, Global Change Biology) This paper adds to a growing number of recent publications that challenge the assumption that plant growth and CO2 drawdown in productive forests results in parallel increases in organic matter stored in soils (e.g., Joly et al. 2025; Lutter et al. (2023); Mayer et al. (2024); Quartucci et al. (2023)). Plant–soil interactions that link the carbon sequestered from the atmosphere to different pools of carbon in vegetation and soil are intricate. This means that otherwise meaningful carbon management through vegetation change can result in less obvious longer-term consequences for whole-ecosystem carbon sequestration. For the most part, increases in carbon uptake by vegetation result in higher respiration fluxes from soil and vegetation back to the atmosphere (Jiang et al. 2020). Net carbon uptake in ecosystems is generally a small imbalance between photosynthesis and respiration, tipped towards the former; actual sequestration results from carbon being fixed in pools with long turnover times. Importantly, these offsetting carbon losses are not yet considered when planning management options such as tree planting, and when valorising land-use management through rewards such as carbon credits. Soil carbon is stored in a range of forms, from particulate—fragments of dead plant matter—to mineral associated. The latter are stabilised forms of organic matter that have resulted from microbial processing and leaching through the soil profile and subsequent precipitation onto surfaces of mineral particles. These forms of carbon pools represent a spectrum of biochemical stability, associated with a wide range in turnover rates for each of them. Carbon stores in deep soils are generally dominated by more stable forms of carbon, with slow-cycling, mineral associated forms dominating over the more biochemically labile, particulate forms of organic matter (Lavallee et al. 2020). Mayer et al. (2025) observed an accumulation of topsoil carbon which is typically dominated by particulate carbon. It is a less reliable carbon stock for climate change mitigation, due to its accessibility to microbes and vulnerability to perturbations such as fire, forestry operations and pest outbreaks (Mayer et al. 2024). So, whilst vegetation management can provide new inputs of organic matter to soil, it is important to look beyond just the total change in organic matter to take account of where this is found within the profile, and how changes in specific soil organic matter fractions affect overall turnover. Changes in forest productivity have a direct impact on belowground biota and the way in which carbon is added, processed and lost from soils. Changes in the quantity and quality of litter cause shifts in the type and abundance of soil fauna, fungi and bacteria, impacting carbon cycling. Rhizosphere priming, where relatively labile compounds from root exudations lead to enhanced decomposition of organic matter to meet the nitrogen demand of plants (often via mycorrhizal fungi), has been described in numerous studies. Ectomycorrhizal fungi associated with most coniferous and deciduous European tree species have particular potential to enhance decomposition because of nutrient extraction from organic matter (Choreño-Parra and Treseder 2024). Nutrient extraction and associated decomposition to support tree growth could have contributed to the deep soil carbon losses observed. The European beech forests investigated by Meyer et al. are located within the lowest rainfall zones of Northern and Central Europe (Pisut 2021). The region has undergone significant climate change in recent decades, with increased annual average temperatures and annual precipitation coinciding with deep soil carbon loss (Mayer et al. 2025). Soil decomposer activity and communities are closely linked to their climate. Heterotrophic respiration peaks at 50%–60% soil water saturation, allowing sufficient water for metabolic and dissolved reactions but not so much that aerobic processes shut down. Additionally, the increase in respiration following a moisture increase is most pronounced in the driest soils (Moyano et al. 2012). Priming may be very moisture sensitive, as enzymes and exudates need to be in dissolved form to stimulate microbial activity in soil organic matter. Therefore, as soil moisture (or the frequency at which soil moisture thresholds are reached) increases, priming-related losses of carbon could also increase. Higher average temperatures would further add to any moisture-driven enhancement of organic matter turnover. An observed increase of around 2°C over the study period could explain an increase in microbial decomposition by 15%, assuming a simplistic doubling of decomposition rates for a 10°C increase in temperature (Q10 = 2). Known relationships of moisture and temperature dependence would thus lend support to Mayer et al.'s interpretation of a climate-driven change in deep soil carbon. Forests and the deep soils that they occur on are often considered our ‘safest’ option to deposit and store long term carbon. However, Mayer et al. show that this carbon sink is losing its strength and should not be relied on. The mechanisms of carbon loss at depth are unclear and must be resolved to predict the future trajectory of carbon storage in forests and the wider terrestrial carbon sink. This paper emphasises that we cannot over-rely on forests to mitigate climate change because there is still so much that we do not understand; despite accumulating carbon in tree biomass, they may be losing carbon ‘capital’ to the atmosphere. Jens-Arne Subke: conceptualization, writing – original draft, writing – review and editing. Jens-Arne Subke: conceptualization, writing – original draft, writing – review and editin. The authors declare no conflicts of interest. This article is a Invited Commentary on Mayer et al., https://doi.org/10.1111/gcb.70446. The authors have nothing to report.