Soil carbon sequestration for climate change mitigation: Mineralization kinetics of organic inputs as an overlooked limitation

Abstract

Over the last few years, the question of whether soil carbon sequestration could contribute significantly to climate change mitigation has been the object of numerous debates. All of these debates so far appear to have entirely overlooked a crucial aspect of the question. It concerns the short-term mineralization kinetics of fresh organic matter added to soils, which is occasionally alluded to in the literature, but is almost always subsumed in a broader modelling context. In the present article, we first summarise what is currently known about the kinetics of mineralization of plant residues added to soils, and about its modelling in the long run. We then argue that in the short run, this microbially-mediated process has important practical consequences that cannot be ignored. Specifically, since at least 90% of plant residues added to soils to increase their carbon content over the long term are mineralized relatively rapidly and are released as CO 2 to the atmosphere, farmers would have to apply to their fields 10 times more organic carbon annually than what they would eventually expect to sequester. Over time, because of a well-known sink saturation effect, the multiplier may even rise significantly above 10, up to a point when no net carbon sequestration takes place any longer.

A well‐established fact: Rapid mineralization of organic inputs is an important factor for soil carbon sequestration

“…We fully agree with Berthelin et al (2022) when they write: ‘it is not reasonable to ask of soil carbon sequestration to compensate all of the greenhouse gas emissions of other anthropogenic sectors’ and ‘one should also ensure that soils will be sufficiently resilient to adapt to a rapidly changing climate in the near future, and still be able to fulfill their essential functions, on which humanity depends crucially’. Yet, we think it is important to add here that increasing SOC and enhancing resilience to climate change is crucial to minimise the negative feedback effect of climate change on net primary production and on resulting C inputs to soils (IPCC, 2019; Lal, 2016).…”

“…We thank our colleagues Berthelin et al (2022) for fostering this discussion. Dominique Arrouays and Budiman Minasny are members and Manuel P. Martin is the collaborator of the GLADSOILMAP research consortium supported by LE STUDIUM Institute for Advanced Research Studies, France.…”

“…This is now a well-established pathway of SOC formation and stabilisation (e.g., Cotrufo et al, 2013;Kallenbach et al, 2016;Sokol et al, 2022). Berthelin et al (2022) emphasise the role that organic inputs can play in storing additional SOC. The impact of crop residue retention on SOC is generally positive but indeed variable depending on soil, climate and agronomic contexts (e.g., review by Bolinder et al, 2020).…”

“…In brief, after a short review, mainly citing papers that question or criticise the potential of soils to sequester CO 2 to mitigate climate change, Berthelin et al (2022) claim that, for the ‘very first time’, they ‘analyze in detail’ the short‐term mineralization kinetics of fresh organic inputs added to soils, which according to them ‘is occasionally alluded to in the literature, but almost always subsumed in a broader modelling context’. However, the authors simply put on the table a fact that has been known for nearly a century, that is, all organic carbon (C) added to the soil is subject to mineralization and is not entirely stabilised or sequestered into the soil, just a small fraction of it is.…”

“…We have read with interest ‘Soil carbon sequestration for climate change mitigation: Mineralization kinetics of organic inputs as an overlooked limitation’ recently published in the European Journal of Soil Science (Berthelin et al, 2022). This paper presents some interesting considerations, at least one of which is already well known to soil scientists working on soil organic carbon (SOC), that is, a large portion (80%–90%) of fresh carbon inputs to soil is subject to rapid mineralization.…”