Atmospheric Exchange of Reduced Sulfur Compounds in Natural Ecosystems By

Terrestrial-Atmospheric Exchange of Reduced Sulfur Compounds in Natural Ecosystems By Mary Elizabeth Whelan A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Geography in the Graduate Division of the University of California, Berkeley Committee in charge: Professor Robert C. Rhew, Chair Professor Kurt Cuffey Professor Allen Goldstein Professor Ron Amundsen Fall 2013 Abstract Terrestrial-Atmospheric Exchange of Reduced Sulfur Compounds in Natural Ecosystems by Mary Elizabeth Whelan Doctor of Philosophy in Geography University of California, Berkeley Professor Robert C. Rhew, Chair The sulfur biogeochemical cycle includes biotic and abiotic processes important to global climate, atmospheric chemistry, food security, and the study of related cycles. The largest flux of sulfur on Earth is weathering from the continents into the sulfate-rich oceans; one way in which sulfur can be returned to land is through transport of reduced sulfur gases via the atmosphere. Here I developed a method for quantifying low-level environmental fluxes of several sulfur-containing gases, H2S, COS, CH3SCH3 (DMS), and HSCH3, between terrestrial ecosystems and the atmosphere. COS is the most prevalent reduced sulfur gas in the atmosphere, considered to be inert in the troposphere except for its uptake in plant leaves and to a smaller extent aerobic soils. This dissertation reports two surprising cases that go against conventional thinking about the sulfur cycle. We found that the common salt marsh plant Batis maritima can mediate net COS production to the atmosphere. We also found that an aerobic wheat field soil produces COS abiotically when incubated in the dark at > 25 °C and at lower temperatures under light conditions. 1 We then sought to separately quantify plant and soil sulfur gas fluxes by undertaking a year-long field campaign in a grassland with a Mediterranean climate, where green plants were present only half of the year. We measured in situ soil fluxes of COS and DMS during the non- growing dry season, using water additions to simulate soil fluxes of the growing, wet season. COS and CO2 are consumed in a predictable ratio by enzymes involved in photosynthetic pathways; however, while CO2 is released by back diffusion and autorespiration, COS is usually not generated by plants. Using measurements during the growing season, we were then able to calculate gross primary production by using the special relationship between CO2 and COS. This dissertation has developed a greater understanding of the vagaries of the atmospheric-terrestrial sulfur cycle and explored using that cycle as a tool for studying the carbon cycle. 2 This dissertation is dedicated to Mr. James Chalfant. i Table of Contents 1. Introduction 1 2. Materials and Methods 8 3. Salt marsh vegetation: a carbonyl sulfide (COS) source to the atmosphere 20 4. Carbonyl sulfide produced by abiotic thermal and photo-degradation of 38 soil organic matter from wheat field substrate 5. Exchange of carbonyl sulfide (COS) between a grassland and the 53 atmosphere 6. Conclusion 72 ii 1. Introduction Sulfur is an element essential to all life: proteins usually contain cysteine and methionine, sulfur-containing amino acids with specific structural and biochemical roles (Levine et al., 1996). Sulfur is constantly weathered from the continents and transported to the ocean via river runoff. Many terrestrial primary producers must therefore rely on airborne sulfur sources (Hawkesford and de Kok, 2007). This dissertation contributes to a better understanding of the movement of sulfur gases between terrestrial ecosystems and the atmosphere. Our conception of the sulfur cycle has changed considerably, often informed by technical advances in measuring specific compounds. In the early 20th century, researchers associated with the United States Geological Survey suggested that volcanoes dominated the atmospheric sulfur burden; sulfate was reduced in the ocean to pyrite, returning sulfur to ocean sediments and the rock cycle. In the 1970s, the role of volcanoes was often neglected and a greater importance wrongly assigned to biogenic hydrogen sulfide (H2S) emissions (Brimblecombe, 2003). One of the most prevalent procedures for measuring H2S, the Natusch method (Natusch et al., 1972), inadvertently included interference from carbonyl sulfide (COS), a more abundant and pervasive reduced sulfur gas. This called into question much of the H2S gas phase data collected over marine ecosystems (Cooper and Saltzman, 1987). Similarly, many early chamber measurements of COS fluxes used sulfur-free sweep air which was later shown to lead to COS flux values of the opposite sign than what was actually occurring in many ecosystems (Castro and Galloway, 1991; de Mello and Hines, 1994). There is still a scarcity of methane thiol (CH3SH) related data, perhaps because of difficulties in measuring this reactive compound. In our current understanding, atmospheric sulfur emissions arise from human industry, volcanic activity, formation of sea salt aerosols, aeolian processes, and gas production in natural ecosystems (Watts, 2000). Examining the exchange of different sulfur species over ecosystems gives us information about their origins and fate. There are four sulfur compounds considered in this dissertation: hydrogen sulfide (H2S), carbonyl sulfide (COS), dimethyl sulfide (DMS or CH3SCH3), and carbon disulfide (CS2) (see Table 1). The production and consumption of H2S is related to oxidation-reduction potential, but its atmospheric budget is not well constrained on the global scale (Watts, 2000). DMS and CS2 tend to be from marine and coastal sources and their oxidation yields about half of the COS in the atmosphere (Barnes et al., 1994; L. Wang et al., 2001). COS is the longest lived and most abundant sulfur compound, inert in the troposphere but for a large vegetative sink (Montzka et al., 2007). Investigating what controls these fluxes will help clarify how atmospheric sulfur gases interact with terrestrial ecosystems; in particular how plants and soils could act as biogenic sources of sulfate aerosol precursors. 1 1.1 Human perturbation of the sulfur cycle Natural emissions of sulfur gases will become more important over time as anthropogenic inputs to the sulfur cycle decrease. Humans have perturbed the biogeochemical sulfur cycle, arguably more than any other major element cycle. Some estimates suggest that human industrial emissions of sulfur exceed natural emissions globally by a factor of 2 or 3 (Rodhe, 1999). Overall, the human contribution to the atmospheric sulfur burden are declining because of SO2 regulation. Acid rain formation was first related to human-made SO2 emissions in the 19th century (Smith, 1872), though it was not until 1980 that the United States began to investigate the need for a policy response. SO2 is the most highly soluble of the reduced sulfur compounds (Table 1), reacting with atmospheric water droplets to form sulfuric acid. Water droplets with a pH below 5.6 constituted “acid rain” or snow found downwind of fossil fuel-burning smoke stacks (Likens and Bormann, 1974). Multiple sulfur emissions controls were put in place in the late 1980s and has ameliorated this problem (Lackey and Blair, 1997). Anthropogenic SO2 emissions abatement in Europe and the United States have reduced total atmospheric sulfate by approximately 27% since 1980 (Forster et al., 2007). New developments in emerging economies have caused an overall increase of anthropogenic SO2 in the Southern hemisphere, though the increase is small (2%) for the global budget (Stern, 2005). Diminishing anthropogenic atmospheric sulfur inputs increases the influence of natural emissions on the remaining budget and reduces the total input of sulfur on terrestrial ecosystems. Deposition of sulfate aerosols and SO2 to some vegetated areas is an important source of sulfur to soils and plants (Hawkesford and de Kok, 2007), indicating a potential food security problem as the anthropogenic sulfur source wanes. compound molecular boiling point vapor Henry’s law Redox Lifetime in the weight (°C, pressure constant state atmosphere (g/mol) approximate) (bar at (mol/L at of S 20°C) 25°C) H2S 34.1 -60 18.2 0.086 -II ~3 days (CH3)2S 62.1 37 0.6 0.474 -II ~2 days (or DMS) CS2 76.1 46 0.4 0.054 -II ~7 days SO2 64.1 -10 3.4 1.23 IV 15 min to 4 days COS 60.1 -50 12.5 0.022 -II ~2-7 years Table 1. Most abundant sulfur-containing gases in Earth’s atmosphere. Lifetimes are taken from Brimblecombe 2003, except for COS, taken from Xu et al. 2002. The rest of the table is adapted from Hawkesford and de Kok et al. 2007. 2 1.2 Atmospheric sulfate in the sulfur cycle Much of the sulfur entering the atmosphere starts in the form of SO2 or sulfate -2 (SO4 ). Many reduced sulfur gases are oxidized in the atmosphere. These airborne sulfate particles are the principal component of sulfate aerosols, which are then returned to the ocean or land reservoirs through wet or dry deposition. Dimethyl sulfide (DMS) is thought to be responsible for non-sea-salt sulfate near marine ecosystems. Non-sea salt aerosols can act as cloud condensation nuclei (CCN), changing the lifetime and precipitation patterns of clouds. (Andreae and Crutzen 1997). The theoretical increase in DMS production by phytoplankton in warmer oceans was hypothesized to indirectly counteract global warming. In 1987, Robert Charlson, James Lovelock, Meinrat Andreae, and Stephen Warren wrote a paper suggesting that the climate may be “biologically regulated” by DMS production, popularly known as the CLAW hypothesis. More atmospheric DMS leads to more CCN, more clouds, higher albedo, and a cooler climate (Charlson et al., 1987). The CLAW hypothesis spurred a great deal of scientific research which confirmed many of the links in the proposed feedback mechanism. However, after decades of research, cloud dynamics still possess the largest uncertainty in our understanding of climate change and the CLAW hypothesis has not been proven nor disproven (Ayers and Cainey, 2007).

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