Articles | Volume 13, issue 24
Research article
15 Dec 2016
Research article |  | 15 Dec 2016

Hydrogen dynamics in soil organic matter as determined by 13C and 2H labeling experiments

Alexia Paul, Christine Hatté, Lucie Pastor, Yves Thiry, Françoise Siclet, and Jérôme Balesdent

Abstract. Understanding hydrogen dynamics in soil organic matter is important to predict the fate of 3H in terrestrial environments. One way to determine hydrogen fate and to point out processes is to examine the isotopic signature of the element in soil. However, the non-exchangeable hydrogen isotopic signal in soil is complex and depends on the fate of organic compounds and microbial biosyntheses that incorporate water-derived hydrogen. To decipher this complex system and to understand the close link between hydrogen and carbon cycles, we followed labeled hydrogen and labeled carbon throughout near-natural soil incubations. We performed incubation experiments with three labeling conditions: 1 – 13C2H double-labeled molecules in the presence of 1H2O; 2 – 13C-labeled molecules in the presence of 2H2O; 3 – no molecule addition in the presence of 2H2O. The preservation of substrate-derived hydrogen after 1 year of incubation (ca. 5 % in most cases) was lower than the preservation of substrate-derived carbon (30 % in average). We highlighted that 70 % of the C–H bonds are broken during the degradation of the molecule, which permits the exchange with water hydrogen. Added molecules are used more for trophic resources. The isotopic composition of the non-exchangeable hydrogen was mainly driven by the incorporation of water hydrogen during microbial biosynthesis. It is linearly correlated with the amount of carbon that is degraded in the soil. The quantitative incorporation of water hydrogen in bulk material and lipids demonstrates that non-exchangeable hydrogen exists in both organic and mineral-bound forms. The proportion of the latter depends on soil type and minerals. This experiment quantified the processes affecting the isotopic composition of non-exchangeable hydrogen, and the results can be used to predict the fate of tritium in the ecosystem or the water deuterium signature in organic matter.

Short summary
The terrestrial environment has been affected by tritium contamination. There is a need to assess the dynamics of organic hydrogen in soils in order to predict the fate of tritium. In the present study we traced carbon and hydrogen from plant-derived molecules and hydrogen from water in different soil types. The main findings of the work are that water is the main donor of organic hydrogen and the long-term fate of hydrogen (and tritium) will depend on the status of soil carbon dynamics.
Final-revised paper