Ideas and perspectives: Can we use the soil carbon saturation deficit to quantitatively assess the soil carbon storage potential, or should we explore other strategies?

Abstract

Abstract. An increase in soil organic carbon stock can contribute to mitigate climate change. International negotiation mechanisms and initiatives call for countries to consider land use change and soil management to achieve atmospheric CO2 removal through storage in terrestrial systems (http://4p1000.org/). As a result, policy makers raised a specific operational question to the soil science community: how much and at which annual rate additional carbon can be stored in soils in different 20 locations? It has been suggested that the ability of a soil to store additional organic carbon can be estimated from its carbon saturation deficit (Csat-def), which is defined as the difference between the maximum amount of carbon that can be associated to its fine (<20 µm) fraction and the current amount of carbon associated to its fine fraction. In this opinion paper, we explain why, for conceptual reasons, the soil Csat-def is not appropriate, at least in its present form, for assessing quantitatively the whole-soil (total) organic carbon storage potential for operational purposes. We then propose alternative approaches based on 25 new opportunities offered by the development of national and international soil monitoring programs (possibly coupled with modelling) that can provide quantitatively relevant estimates of soil total carbon storage potential. This pragmatic approach will require a sustained effort to maintain and develop soil monitoring programs worldwide and research allowing proper use of such a large amount of data.

Topsoil organic matter build‐up in glacier forelands around the world

“…The increase in CH‐POC 2 with soil age and SOC concentration also highlights the accumulation of labile organic matter enriched in hydrogen moieties over time (Barré et al, 2016; Gregorich et al, 2015; Poeplau et al, 2019; Saenger et al, 2015; Soucémarianadin et al, 2018). Finally, the consistent decrease in SOC stability with SOC accumulation in glacier forelands supports the increasing body of evidence that SOC storage in most terrestrial ecosystem is driven by the accumulation of labile SOC (see e.g., Barré et al, 2017, and references therein). This labile SOC fraction is considered potentially more vulnerable to microbial decomposition and thus to loss (Cotrufo et al, 2019; Viscarra Rossel et al, 2019).…”

Intensification enhances litter carbon and nitrogen decomposition dynamics in subtropical grazinglands

“…The greater proportion of C-Min fraction with the addition of N suggests a possible C transfer from light to mineral fractions and possible C accumulation in the C-Min fraction as decomposition progressed. Carbon associated with mineral fraction in most soils consists of recalcitrant, chemically and physically protected C pools (Barrè et al, 2017;Christensen, 1992;Six et al, 2002). However, in coarse, sandy-textured soils this fraction can often be dynamic and susceptible to short-term changes in soil management (Xu et al, 2017).…”

Response to the comments of Reviewer 2

“…We disagree with the definitions of storage and sequestration proposed by Reviewer 2. We consider following Olson et al (2014) that Carbon sequestration is "the process of transferring CO2 from the atmosphere into the soil of a land unit, through plants, plant residues and other organic solids which are stored or retained in the unit as part of the soil organic matter with a long residence time", i.e. several decades.…”