Increasing soil carbon storage: mechanisms, effects of agricultural practices and proxies. A review

Abstract

The international 4 per 1000 initiative aims at supporting states and non-governmental stakeholders in their efforts towards a better management of soil carbon (C) stocks. These stocks depend on soil C inputs and outputs. They are the result of fine spatial scale interconnected mechanisms, which stabilise/destabilise organic matter-borne C. Since 2016, the CarboSMS consortium federates French researchers working on these mechanisms and their effects on C stocks in a local and global change setting (land use, agricultural practices, climatic and soil conditions, etc.). This article is a synthesis of this consortium's first seminar. In the first part, we present recent advances in the understanding of soil C stabilisation mechanisms comprising biotic and abiotic processes, which occur concomitantly and interact. Soil organic C stocks are altered by biotic activities of plants (the main source of C through litter and root systems), microorganisms (fungi and bacteria) and 'ecosystem engineers' (earthworms, termites, ants). In the meantime, abiotic processes related to the soil-physical structure, porosity and mineral fraction also modify these stocks. In the second part, we show how agricultural practices affect soil C stocks. By acting on both biotic and abiotic mechanisms, land use and management practices This synthesis of the CarboSMS French consortium's first seminar was already published in French: Derrien D, Dignac M-F, Basile-Doelsch I, Barot S, Cécillon L, Chenu C, Chevallier T, Freschet GT, Garnier P, Guenet B, Hedde M, Klumpp K, Lashermes G, Maron P-A, Nunan N, Roumet C, Barré P (2016) (choice of plant species and density, plant residue exports, amendments, fertilisation, tillage, etc.) drive soil spatiotemporal organic inputs and organic matter sensitivity to mineralisation. Interaction between the different mechanisms and their effects on C stocks are revealed by meta-analyses and long-term field studies. The third part addresses upscaling issues. This is a cause for major concern since soil organic C stabilisation mechanisms are most often studied at fine spatial scales (mm-μm) under controlled conditions, while agricultural practices are implemented at the plot scale. We discuss some proxies and models describing specific mechanisms and their action in different soil and climatic contexts and show how they should be taken into account in large scale models, to improve change predictions in soil C stocks. Finally, this literature review highlights some future research prospects geared towards preserving or even increasing C stocks, our focus being put on the mechanisms, the effects of agricultural practices on them and C stock prediction models.

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?

“…It will need a sustained effort to maintain and develop soil monitoring programs worldwide and research efforts for an adequate use of such a large amount of data. Moreover, this approach would allow little progress in understanding the mechanisms explaining soil C storage potential (Dignac et al 2017). Research on these aspects should 30 therefore be carried out in parallel.…”

A model based on Rock-Eval thermal analysis to quantify the size of the centennially persistent organic carbon pool in temperate soils

“…Mapping persistent SOC at large scales may allow for the identification of regional hotspots of centennially persistent SOC that may contribute little to climate change by 2100. It may also provide information on the sustainability of additional SOC storage from soil carbon sequestration strategies such as those promoted by the international 4 per 1000 initiative in agriculture and forestry (https://www.4p1000.org/, last access: 6 May 2018; Dignac et al, 2017;Minasny et al, 2017;Soussana et al, 2018). A global map of centennially persistent SOC based on this empirical RE6 thermal analysis model could also be useful for improving the parameterization of SOC dynamics in Earth system models (Falloon and Smith, 2000;Luo et al, 2014;He et al, 2016).…”

Accounting for Carbon Stocks in Soils and Measuring GHGs Emission Fluxes from Soils: Do We Have the Necessary Standards?

“…Proxies for the capacity of soils to store/lose C and to enhance/reduce GHGs emissions can help to make choices on land use and soil management Soil C saturation capacity (Angers et al, 2011;Wiesmeier et al, 2014) has been proposed to estimate the potential gain of soil organic C. Its relevance for predicting soil C storage remains to be evaluated (O'Rourke et al, 2015;Dignac et al, 2017) Physical fractionation of soil organic C (particulate organic matter fraction) and thermal analysis have been proposed to estimate the potential for soil C loss (Dignac et al, 2017 and references therein) Soil capacity to reduce N 2 O (Hénault et al, 2001;Jones et al, 2014) Guidelines to estimate GHGs emissions from soil using models…”

“…Standardized proxies to estimate the capacity of soils to store/lose C and to enhance/reduce GHGs emissions can also help land managers and policy makers to make choices on land use and soil management (Dignac et al, 2017). For example, some authors have proposed that we could calculate a soil C deficit from soil properties including fine particles content (Hassink, 1997;Six et al, 2002;Feng et al, 2013;Beare et al, 2014), and thus estimate the ability of a soil to store additional C. Specific organic matter fractions such as particulate organic matter have been proposed as proxy or early indicators for soil C changes and thermal analysis can indicate the potential for SOC loss on large soil sample sets (Saenger et al, 2015;da Silva Oliveira et al, 2017;Dignac et al, 2017).…”