Abiotic and Biotic Changes of Sulfur and Iron Speciation After Aeration of Intertidal Soils in Germany and Thailand as Assessed by Synchrotron-Based K-Edge X-Ray Absorption near-Edge Spectroscopy (XANES).

For four intertidal wetland soils from Germany (sandy, loamy, and clayey intertidal marsh soils) and Thailand (sandy intertidal soil), the kinetics of speciation changes of sulfur and iron after aeration was studied.  The speciation changes were investigated in a laboratory incubation experiment, in which field-moist subsamples were held in contact with ambient air at 20° C for different time periods, ranging from 1 minute to 21 days under controlled boundary conditions. In different experimental variants (with/without Na azide treatment), abiotic changes were distinguished from biotic changes. We strictly maintained anoxic conditions throughout the entire process of sampling, transport, sample pre-treatment, and analysis by synchrotron-based K-edge X-ray absorption near-edge spectroscopy (XANES).

The contribution of reduced S species to total S at the beginning of the experiment increased in the order: Thai soil (30%) < sandy German marsh soil (48%) < loamy German marsh soil (54%) < clayey German marsh soil (63%). The oxidation state changes followed a zero-order kinetics. For all soils, aeration times up to 60 minutes resulted only in minor oxidation of reduced S forms. For the clayey and the loamy soils, also within 12 h (one low-tide/high-tide sequence), only about 10% of the initial reduced S pool was oxidized, whereas the changes were larger for the sandy soils. Within 21 days, between 40% (sandy German marsh soil) and 73% (clayey German marsh soil, Thai intertidal soil) of the initial reduced S pool was oxidized. The relative contribution of abiotic and biotic reactions to the oxidation of reduced S was 40% and 60%, respectively (exception: sandy German marsh soil with an abiotic contribution of 70%). In summary, S oxidation was much faster for the intertidal soil from Thailand compared to the German intertidal soils. For the latter, the S oxidation rates decrease with increasing mean grain size and decreasing initial reduced S pools.