Articles | Volume 13, issue 22
Research article
28 Nov 2016
Research article |  | 28 Nov 2016

Soil carbon dioxide emissions controlled by an extracellular oxidative metabolism identifiable by its isotope signature

Benoit Kéraval, Anne Catherine Lehours, Jonathan Colombet, Christian Amblard, Gaël Alvarez, and Sébastien Fontaine

Abstract. Soil heterotrophic respiration is a major determinant of the carbon (C) cycle and its interactions with climate. Given the complexity of the respiratory machinery, it is traditionally considered that oxidation of organic C into carbon dioxide (CO2) strictly results from intracellular metabolic processes. Here we show that C mineralization can operate in soils deprived of all observable cellular forms. Moreover, the process responsible for CO2 emissions in sterilized soils induced a strong C isotope fractionation (up to 50 ‰) incompatible with respiration of cellular origin. The supply of 13C glucose in sterilized soil led to the release of 13CO2 suggesting the presence of respiratory-like metabolism (glycolysis, decarboxylation reaction, chain of electron transfer) carried out by soil-stabilized enzymes, and by soil mineral and metal catalysts. These findings indicate that CO2 emissions from soils can have two origins: (1) from the well-known respiration of soil heterotrophic microorganisms and (2) from an extracellular oxidative metabolism (EXOMET) or, at least, catabolism. These two metabolisms should be considered separately when studying effects of environmental factors on the C cycle because the likelihood is that they do not obey the same laws and they respond differently to abiotic factors.

Short summary
Soil CO2 emissions are a major determinant of the carbon (C) cycle and its interactions with climate. Here, we show that soil CO2 emissions have two origins: (1) the well-known microbial cell respiration and (2) an extracellular oxidative metabolism (EXOMET) carried out by soil-stabilized enzymes and mineral catalysts. These two metabolisms have distinct C isotope signatures, allowing their detection in soil CO2 emissions.
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