Abstract
Abstract. Soil uptake of atmospheric hydrogen (H2) and the associated hydrogen isotope effect were studied using soil chambers in a Western Washington second-growth coniferous forest. Chamber studies were conducted during both winter and summer seasons to account for large natural variability in soil moisture content (4–50%) and temperature (6–22 °C). H2 deposition velocities were found to range from 0.01–0.06 cm s−1 with an average of 0.033 ± 0.008 cm s−1 (95% confidence interval). Consistent with prior studies, deposition velocities were correlated with soil moisture below 20% soil moisture content during the summer season. During winter, there was considerable variability observed in deposition velocity that was not closely related to soil moisture. The hydrogen kinetic isotope effect with H2 uptake was found to range from −24‰ to −109‰. Aggregate analysis of experimental data results in an average KIE of −57 ± 5‰ (95% CI). Some of the variability in KIE can be explained by larger isotope effects at lower (<10%) and higher (>30%) soil moisture contents. The measured KIE was also found to be correlated with deposition velocity, with smaller isotope effects occurring at higher deposition velocities. If correct, these findings will have an impact on the interpretation of atmospheric measurements and modeling of δD of H2.
Highlights
Molecular hydrogen (H2) has a globally averaged mixing ratio of approximately 530 ppb and is the second most abundant reduced trace gas after methane (Novelli, 1999)
Soil uptake of H2 is considered to account for ∼ 75% of the global H2 sink (Hauglustaine and Ehhalt, 2002; Price et al, 2007; Rahn et al, 2003; Sanderson et al, 2003) and the reason for the observed ∼ 3% higher H2 concentrations in the southern hemisphere compared to the Northern Hemisphere, which is atypical for an anthropogenically produced gas (Khalil and Rasmussen, 1989)
Over a wide range of temperature, soil moisture content, and deposition velocity, the kinetic isotope effect (KIE) was found to have considerable variability, −24‰ to −109‰. Some of this variability may be associated with soil moisture content, as a smaller KIE was found at intermediate soil moisture contents (10–30%)
Summary
The major sources of H2, accounting for 90% of the global burden, are photochemical oxidation of CH4 and NMHC’s, fossil fuel combustion and biomass burning, whereas ocean degassing, volcanic emissions and production by legumes during N2 fixation make up the remaining 10% (see review by Ehhalt and Rohrer, 2009). HD can serve as a valuable tracer to distinguish between soil uptake and OH oxidation of H2 at the global scale. This approach relies on extensive knowledge of the HD content of atmospheric H2 and its spatial and temporal distribution. Source HD/H2 signature ratios and isotopic fractionation from sinks must be fully characterized in order to provide additional constraint of the H2 budget (Price et al, 2007). We present a series of field chamber experiments conducted over two seasons in a forest ecosystem to determine the magnitude of the KIE during HD uptake by soils and focus the work on characterizing variability across a wide range of soil moisture content
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