A considerable fraction of soil-respired CO2 is not emitted directly to the atmosphere

Enrique P Sánchez-Cañete,Greg A Barron-Gafford,Jon Chorover

doi:10.1038/s41598-018-29803-x

Enrique P Sánchez-Cañete, Greg A Barron-Gafford + Show 1 more

Open Access

https://doi.org/10.1038/s41598-018-29803-x

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Abstract

Soil CO2 efflux (Fsoil) is commonly considered equal to soil CO2 production (Rsoil), and both terms are used interchangeably. However, a non-negligible fraction of Rsoil can be consumed in the subsurface due to a host of disparate, yet simultaneous processes. The ratio between CO2 efflux/O2 influx, known as the apparent respiratory quotient (ARQ), enables new insights into CO2 losses from Rsoil not previously captured by Fsoil. We present the first study using continuous ARQ estimates to evaluate annual CO2 losses of carbon produced from Rsoil. We found that up to 1/3 of Rsoil was emitted directly to the atmosphere, whereas 2/3 of Rsoil was removed by subsurface processes. These subsurface losses are attributable to dissolution in water, biological activities and chemical reactions. Having better estimates of Rsoil is key to understanding the true influence of ecosystem production on Rsoil, as well as the role of soil CO2 production in other connected processes within the critical zone.

Highlights

Soil carbon dioxide (CO2) efflux is the second largest contributor to terrestrial CO2 exchanges, similar in scale to uptake by terrestrial photosynthesis[1,2]
Our goals were (i) to quantify the values, patterns, and seasonality of apparent respiratory quotient (ARQ) at different soil depths within a semi-arid coniferous forest and (ii) to estimate the amount of soil CO2 removed through biological and non-biological processes (Rsoil_ARQ) (iii) in order to illustrate the disparity between from the soil surface (Fsoil) using traditional assumptions that result of soil CO2 production (Rsoil) = Fsoil and an estimate of Fsoil that takes into account CO2 losses (Rloss) and actual rates of Rsoil, as determined using the ARQ
The low ARQ values we found here (ARQ ≈ 0.3, Fig. 2 and Table 1S) in comparison to oxidative ratios expected for natural organic matter, highlight the important role of subsurface biological and non-biological processes in removing CO2 from Rsoil

Summary

Introduction

Soil carbon dioxide (CO2) efflux is the second largest contributor to terrestrial CO2 exchanges, similar in scale to uptake by terrestrial photosynthesis[1,2]. Misrepresenting Fsoil as Rsoil can have significant consequences for interpretation of both biotic and abiotic processes because it underestimates the contributions of aboveground function to belowground processes, but it yields a misguided understanding of the rates and drivers of subsurface biogeochemistry and the potential for carbon exports from the system through hydrological transport. The importance of these alternative CO2 loss pathways is illustrated when considering that soil can store an order of magnitude greater CO2 as dissolved inorganic carbon (DIC, inclusive of dissolved CO2, carbonic acid, bicarbonate, and carbonate) in the aqueous-filled relative to gas-filled pore space[22]. Having better estimates of Rsoil is key to understanding the true influence of aboveground production on Rsoil, CO2-induced mineral weathering, and other biologically-driven processes within the critical zone

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