Abstract
Abstract. Soil organic matter (OM) represents a key C pool for climate regulation but also an essential component for soil functions and services. Scientific research in the 21st century has considerably improved our knowledge of soil organic matter and its dynamics, particularly under the pressure of the global disruption of the carbon cycle. This paper reviews the processes that control C dynamics in soil, the representation of these processes over time, and their dependence on variations in major biotic and abiotic factors. The most recent advanced knowledge gained on soil organic matter includes the following. (1) Most organic matter is composed of small molecules, derived from living organisms, without transformation via additional abiotic organic polymerization; (2) microbial compounds are predominant in the long term; (3) primary belowground production contributes more to organic matter than aboveground inputs; (4) the contribution of less biodegradable compounds to soil organic matter is low in the long term; (5) two major factors determine the soil organic carbon production “yield” from the initial substrates: the yield of carbon used by microorganisms and the association with minerals, particularly poorly crystalline minerals, which stabilize microbial compounds; (6) interactions between plants and microorganisms also regulate the carbon turnover time and therefore carbon stocks; (7) among abiotic and biotic factors that regulate the carbon turnover time, only a few are considered in current modeling approaches (i.e., temperature, soil water content, pH, particle size, and sometimes C and N interactions); and (8) although most models of soil C dynamics assume that the processes involved are linear, there are now many indications of nonlinear soil C dynamics processes linked to soil OM dynamics (e.g., priming). Farming practices, therefore, affect soil C stocks not only through carbon inputs but also via their effect on microbial and organomineral interactions, yet it has still not been possible to properly identify the main mechanisms involved in C loss (or gain). Greater insight into these mechanisms and their interdependencies, hierarchy and sensitivity to agricultural practices could provide future levers of action for C sequestration in soil.
Highlights
Increasing organic carbon stocks in agricultural soils has emerged as an effective means to improve soils, increase plant productivity, and delay the rise of atmospheric carbon dioxide and the rate of climate change while maintaining the quality of downstream ecosystems
Part of the criticism is focused on the potential for a political delay in the transition to renewable energies (Baveye et al, 2018b; Baveye and White, 2020), as well as on the calculation itself, which is based on several assumptions, some of which are highly debatable
In order to foster progress in this area and facilitate proper interpretation of experimental results, this paper aims to provide a comprehensive and upto-date review of processes that control soil C dynamics, a time-course representation of these processes, and their response to variations in major biotic and abiotic factors
Summary
Increasing organic carbon stocks in agricultural soils has emerged as an effective means to improve soils, increase plant productivity, and delay the rise of atmospheric carbon dioxide and the rate of climate change while maintaining the quality of downstream ecosystems. Considering that the total amount of organic carbon in soils at the global scale is about 2400 Gt of C and that anthropogenic CO2 emissions are about 9.4 Gt of C per year, a simple calculation suggests that an annual 4 per 1000 increase in the soil C stock could theoretically offset annual emissions (2400 × 0.04 = 9.6) (Minasny et al, 2017) This initiative has given rise to a scientific controversy
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