Temperature and moisture dependence of soil H_2 uptake measured in the laboratory

The soil sink of molecular hydrogen is the largest and most uncertain term in the global atmospheric H_2 budget. Lack of information about the mechanisms regulating this sink limits our ability to predict how atmospheric H_2 may respond to future changes in climate or anthropogenic emissions. Here we present the results from a series of laboratory experiments designed to systematically evaluate and describe the temperature and soil moisture dependence of H_2 uptake by soils from boreal forest and desert ecosystems. We observed substantial H2 uptake between −4°C and 0°C, a broad temperature optimum between 20°C and 30°C, a soil moisture optimum at approximately 20% saturation, and inhibition of uptake at both low and high soil moisture. A sigmoidal function described the temperature response of H_2 uptake by soils between −15°C and 40°C. Based on our results, we present a framework for a model of the soil H_2 sink

Molecular hydrogen uptake by soils in forest, desert, and marsh ecosystems in California

The mechanism and environmental controls on soil hydrogen (H_2) uptake are not well understood but are essential for understanding the atmospheric H_2 budget. Field observations of soil H_2 uptake are limited, and here we present the results from a series of measurements in forest, desert, and marsh ecosystems in southern California. We measured soil H_2 fluxes using flux chambers from September 2004 to July 2005. Mean H2 flux rates and standard deviations were −7.9 + −4.2, −7.6 + −5.3 and −7.5 + −3.4 nmol m^(−2) s^(−1) for the forest, desert, and marsh, respectively (corresponding to deposition velocities of 0.063 + −0.029, 0.051 + −0.036, 0.035 + −0.013 cm s^(−1)). Soil profile measurements showed that H_2 mixing ratios were between 3% and 51% of atmospheric levels at 10 cm and that the penetration of H_2 into deeper soil layers increased with soil drying. Soil removal experiments in the forest demonstrated that the litter layer did not actively consume H_2, the removal of this layer increased uptake by deeper soil layers, and the exposure of subsurface soil layers to ambient atmospheric H_2 levels substantially increased their rate of uptake. Similar soil removal experiments at the desert site showed that extremely dry surface soils did not consume H2 and that fluxes at the surface increased when these inactive layers were removed. We present a model of soil H_2 fluxes and show that the diffusivity of soils, along with the vertical distribution of layers that actively consume H_2 regulate surface fluxes. We found that soil organic matter, CO_2 fluxes, and ecosystem type were not strong controllers of H_2 uptake. Our experiments highlight H_2 diffusion into soils as an important limit on fluxes and that minimum moisture level is needed to initiate microbial uptake

Anthropogenic Impacts on Global Storage and Emissions of Mercury from Terrestrial Soils: Insights from a New Global Model

We develop a mechanistic global model of soil mercury storage and emissions that ties the lifetime of mercury in soils to the lifetime of the organic carbon pools it is associated with. We explore the implications of considering terrestrial mercury cycling in the framework of soil carbon cycling and suggest possible avenues of future research to test our assumptions and constrain this type of model. In our simulation, input of mercury to soil is by atmospheric deposition, in part through leaf uptake and subsequent litter fall, and is moderated by surface photoreduction and revolatilization. Once bound to organic carbon, mercury is transferred along a succession of short-lived to long-lived carbon pools and is ultimately reemitted by respiration of these pools. We examine the legacy of anthropogenic influence on global mercury storage and emissions and estimate that storage of mercury in organic soils has increased by $\sim20\%$ since preindustrial times, while soil emissions have increased by a factor of 3 $(2900 Mg yr^{−1}$ versus $1000 Mg yr^{−1})$. At steady state, mercury accumulates in the most recalcitrant soil carbon pools and has an overall lifetime against respiration of 630 years. However, the impact of anthropogenic emissions since preindustrial times has been concentrated in more labile pools, so that the mean lifetime of present-day anthropogenic mercury in all pools is $\sim80$ years. Our analysis suggests that reductions in anthropogenic emissions would lead to immediate and large reductions in secondary soil mercury emissions.Engineering and Applied Science

Soil Uptake of Molecular Hydrogen and Remote Sensing of Soil Freeze and Thaw

Soils play a large role in the cycling of atmospheric trace gases and are an important component of the climate system. The bulk of my thesis was directed at the role of soils in the global molecular hydrogen (H₂) cycle. I conducted field measurements of H₂ uptake in three Southern California ecosystems, and found that both the diffusion of H₂ into soils and the distribution of biological activity with depth controlled uptake rates at the surface. I then moved into the laboratory, where I mapped out the temperature and moisture controls on the biological uptake of H₂ in both desert and boreal forest soils. These experiments yielded simple relationships between moisture, temperature, and uptake rate, which I then used to constrain H₂ uptake by soils in a mechanistic model. The model is based on the 1D diffusion equation with a sink term, and is driven by a combination of remote sensing products and land surface modeling output. I calculated a mean annual soil H₂ sink of 67.3 ± 5.5 Tg. The model was able to reproduce the seasonal cycle at high northern latitudes, and implies that seasonal variability in snow cover is a key process controlling H₂ uptake. I found that snow cover and soil moisture control the uptake of H₂ globally, which may have important implications for the hydrogen budget in future climate change scenarios. My second thesis topic involved the development of a remote sensing technique using passive microwave brightness temperatures to identify the freeze-thaw status of soils, which I applied to areas north of 45°N. I found a significant increase in the growing season length in North America by 3.8 days/decade, driven by both an earlier spring thaw and later fall freeze. The lengthening of the growing season may affect the carbon and hydrogen cycles at high northern latitudes, and is a new metric of global change.</p

Molecular hydrogen uptake by soils in forest, desert, and marsh ecosystems in California

The mechanism and environmental controls on soil hydrogen (H2) uptake are not well understood but are essential for understanding the atmospheric H2 budget. Field observations of soil H2 uptake are limited, and here we present the results from a series of measurements in forest, desert, and marsh ecosystems in southern California. We measured soil H2 fluxes using flux chambers from September 2004 to July 2005. Mean H2 flux rates and standard deviations were −7.9 + −4.2, −7.6 + −5.3 and −7.5 + −3.4 nmol m−2 s−1 for the forest, desert, and marsh, respectively (corresponding to deposition velocities of 0.063 + −0.029, 0.051 + −0.036, 0.035 + −0.013 cm s−1). Soil profile measurements showed that H2 mixing ratios were between 3% and 51% of atmospheric levels at 10 cm and that the penetration of H2 into deeper soil layers increased with soil drying. Soil removal experiments in the forest demonstrated that the litter layer did not actively consume H2, the removal of this layer increased uptake by deeper soil layers, and the exposure of subsurface soil layers to ambient atmospheric H2 levels substantially increased their rate of uptake. Similar soil removal experiments at the desert site showed that extremely dry surface soils did not consume H2 and that fluxes at the surface increased when these inactive layers were removed. We present a model of soil H2 fluxes and show that the diffusivity of soils, along with the vertical distribution of layers that actively consume H2 regulate surface fluxes. We found that soil organic matter, CO2 fluxes, and ecosystem type were not strong controllers of H2 uptake. Our experiments highlight H2 diffusion into soils as an important limit on fluxes and that minimum moisture level is needed to initiate microbial uptake

Temperature and moisture dependence of soil H 2 uptake measured in the laboratory

The soil sink of molecular hydrogen is the largest and most uncertain term in the global atmospheric H2 budget. Lack of information about the mechanisms regulating this sink limits our ability to predict how atmospheric H2 may respond to future changes in climate or anthropogenic emissions. Here we present the results from a series of laboratory experiments designed to systematically evaluate and describe the temperature and soil moisture dependence of H2 uptake by soils from boreal forest and desert ecosystems. We observed substantial H2 uptake between −4°C and 0°C, a broad temperature optimum between 20°C and 30°C, a soil moisture optimum at approximately 20% saturation, and inhibition of uptake at both low and high soil moisture. A sigmoidal function described the temperature response of H2 uptake by soils between −15°C and 40°C. Based on our results, we present a framework for a model of the soil H2 sink