Stable hydrogen isotope ratios in soil constituents: New insights into the formation of organic matter and clay minerals
Final Report Abstract
The nonexchangeable H pool of soil organic matter and soil clay fractions bears a stable H isotope signal that can be used for environmental forensics to assign an unknown sample to a defined source region. However, the stable signal of soil organic matter could be destroyed by the microbial replacement of the nonexchangeable C-bonded H via microbial glycolysis during decomposition. The current methods to determine δ2H values of nonexchangeable H to soil clay fractions are limited by a varying removal of nonstructural, exchangeable water-H from the hygroscopic minerals. It is not clear, whether there is a relationship of the δ2H value of precipitation with the nonexchangeable δ2H values of soil clay fractions. We determined to which degree ambient water-H is incorporated into C-bonded H during assimilation of favorable (glucose) and unfavorable (lysine) substrate by the two heterotrophic bacteria Bacillus atrophaeus and Escherichia coli, during litter decomposition (Fagus sylvatica L., Acer pseudoplatanus L.) in laboratory incubations with the litter-inherent microbial community, and during litter decomposition (Fagus sylvatica, Tilia platyphyllos Scop.) in the field. Moreover, we tested steam equilibration as a new method to determine the nonexchangeable δ2H values of soil clay fractions and applied it to determine whether there is unique relationship with the δ2H values of precipitation. Because it turned out that for the correct determination of nonexchangeable δ2H values of soil clay fractions by steam equilibration, the fractionation factor between steam-H and exchangeable H in the soil clay fractions (αex-w) needs to be known, we determined sample-specific αex-w values. Our incubations of two microorganism species on favorable and unfavorable substrate and of leaf litter in the laboratory and in the field revealed that ambient water-H can indeed be incorporated into the C-bonded H by microbial transformation of the organic matter. The experiment with the favorable and unfavorable substrate demonstrated that this incorporation is only possible under favorable conditions in which the glycolysis pathway was active. However, during our incubations of litter () under optimum conditions only a small percentage of the C-bonded H (<10%) experienced incorporation, with a higher degree of incorporation in the dissolved organic matter than in the solid phase. The degree of H incorporation under field conditions was larger (15% to 48%) and seemed to depend on the ratio between organic matter transformation reactions and mineralization. We successfully adapted the steam equilibration method to determine nonexchangeable δ2H values of soil clay fractions and demonstrated with the help of clay mineral standards that the classical particle-size fractionation method did not change the nonexchangeable δ2H value. Applying the steam equilibration method to a global data set of soil clay fractions showed that there was a similar correlation of the δ2H values of precipitation with the nonexchangeable δ2H values of the clay fractions as for soil organic matter. However, this relationship showed a large scatter which was attributable to the fact that the nonexchangeable δ2H values of different clay minerals, particularly 1:1 and 2:1 clay minerals showed different relationships with the δ2H values of precipitation. Moreover, we found that the latter two mineral groups also differed with respect to the αex-w values so that part of the relationship between the δ2H values of precipitation and the nonexchangeable δ2H values of the soil clay fractions was attributable to the with latitude systematically varying clay mineralogy.
Publications
- (2017): Stable hydrogen isotope ratios in soil organic matter. Congress of the German Soil Science Society (DBG), 02- 07 Sep. 2017, Göttingen, Germany
Kessler, A., S. Merseburger, W. Wilcke & Y. Oelmann
- (2017): Stable isotope ratios in crystal water of clay minerals. Congress of the German Soil Science Society (DBG), 02- 07 Sep. 2017, Göttingen, Germany
Merseburger, S., A. Kessler, Y. Oelmann & W. Wilcke
- (2019): A new method to determine nonexchangeable stable hydrogen isotope ratios in crystal water of clay minerals. Congress of the German Soil Science Society (DBG), 24-29 Aug. 2019, Berne, Switzerland
Merseburger, S., A. Kessler, Y. Oelmann & W. Wilcke
- (2019): Incorporation of hydrogen from ambient water into the nonexchangeable H fraction during litter decomposition. Congress of the German Soil Science Society (DBG), 24-29 Aug. 2019, Berne, Switzerland
Kessler, A., K. Kreis, S. Merseburger, W. Wilcke & Y. Oelmann
- (2021): Incorporation of hydrogen from ambient water into the C-bonded H pool during litter decomposition. Soil Biol. Biochem. 162, 108407
Kessler, A., K. Kreis, S. Merseburger, W. Wilcke & Y. Oelmann
(See online at https://doi.org/10.1016/j.soilbio.2021.108407)
