Stable Hydrogen Isotope Ratios: New Insights into the Formation of Soil Organic Matter

Abstract

Stable hydrogen (H) isotope ratios can be used to elucidate biogeochemical processes, serve as a paleo climate proxy, or identify soil organic matter (SOM) dynamics. Previous research has shown that stable H isotope ratios of nonexchangeable H (δ2Hn) of SOM are closely related to δ2H values of precipitation. These findings can be explained by a large contribution of root biomass carrying an isotope signal close to ambient water or by the incorporation of ambient water-H into C-bonded H during SOM decomposition. We assessed the plausibility of the latter process and quantified the incorporation of ambient water-H into C-bonded H of organic matter (OM). In more detail, we quantified (i) the incorporation of ambient water-H into C-bonded H in the biomass of two bacteria species during substrate-specific metabolism and the incorporation of ambient water-H into C-bonded H of leaf litter and leached total OM (TOM) during decomposition under (ii) laboratory conditions and (iii) under field conditions. For the quantification of the incorporation of ambient water-H into C-bonded H of OM we used 2H-enriched ambient water (i, ii) and 2H-enriched leaf litter exposed to precipitation at natural abundance levels (iii). Our results show that the incorporation of ambient-water H into C-bonded H of OM decreased in the order bacteria under optimum conditions (up to 80% of C-bonded H) > leaf litter decomposition under field conditions (18 to 48%) > leaf litter decomposition under laboratory conditions (3 to 8%). The incorporation of ambient-water H into C-bonded H of OM was larger for TOM than for the remaining solid leaf litter. The bacteria species as well as the OM type (glucose/lysine; leaf litter of Fagus sylvatica versus Acer pseudoplatanus or Tilia platyphyllos) influenced the extent of incorporation. Therefore, non-optimum conditions in terms of OM type and temperature/moisture can reduce the extent of incorporation of ambient-water H into C-bonded H of OM. Nevertheless, high incorporation rates into bulk bacterial biomass during anabolic metabolism together with an nearly overprinting of δ2Hn values of leached TOM under natural conditions point towards the importance of microbial decomposition products as a source of SOM and might contribute to explain the close correlation between δ2Hn values of SOM and δ2H values of precipitation.