Tropical-and subtropical-storm surges combined with sea level rise cause saltwater intrusions into low-lying coastal ecosystems along the southeastern coast of the United States, gradually converting freshwater forested wetland into saltmarsh. The transition zone between freshwater and saltwater ecosystems becomes a degraded forested wetland, where the combination of high levels of soil organic matter and elevated concentrations of halide ions creates a dynamic biogeochemical environment that may be a potential hotspot for halocarbon formation such as chloroform, methyl chloride, and methyl bromide. This study conducted field measurements at a transition zone in coastal South Carolina to quantify halocarbon exchange rates and laboratory soil incubations to determine the contributions of biotic versus abiotic processes. The degraded forested wetland showed significant chloroform emission rates (146 ± 129 nmol m −2 d −1 ). The degraded forested wetland remained a net sink for methyl chloride and a negligible source/sink for methyl bromide, unlike the saltmarsh which was a significant source for both. The laboratory incubations strongly suggest that methyl halide consumption in soils at the field site was biotic and that production of methyl halides and chloroform was largely abiotic and temperature-dependent, although additional experiments are required to rule out possible biotic production involving heat-resistant microbes. The results suggest that sea level rise and more frequent storm surges derived from global climate change, in the long term, may increase emissions of chloroform from coastal degraded forested wetlands and of methyl halides if salt marshes expand, with potential impacts for stratospheric ozone depletion.

Natural methyl chloride (CH 3 Cl) and methyl bromide (CH 3 Br) emissions from coastal marsh ecosystems may constitute a significant proportion of stratospheric chlorine and bromine, which catalyze ozone depletion. Current inventories involve substantial uncertainties associated with upscaling plot-scale footprints (i.e., ≤1 m 2 ). Here we present net ecosystem flux measurements of methyl halides from a brackish tidal marsh on the west coast of the United States between April 2016 and June 2017 using the relaxed eddy accumulation method. The measurement footprint encompasses a large part of the studied tidal marsh, including roughly 20 vascular plant species, open water, and soil surfaces. On the annual scale, ecosystem methyl halide emissions showed the strongest relationships to temperature and the growth cycle of halophyte vegetation, whereas on diurnal time scales, fluxes correlated the most with evapotranspiration. The maximum seasonal emissions occurred during the flowering season of Lepidium latifolium (perennial pepperweed), one of the most abundant halophytes on site. The maximum hourly emissions of 111 μg CH 3 Cl · m À2 s· hr À1 and 38 μg CH 3 Br · m À2 · hr À1 were observed during a heat wave in early June. Annually integrated emissions were 135 mg/m 2 for CH 3 Cl and 21 mg/m 2 for CH 3 Br, scaling up to 621 and 96 kg over the entire marsh. We provide a global salt marsh emission inventory that takes into account the spatial distribution of salt marshes in different climate zones, yielding a global salt marsh source of 31 Gg/year CH 3 Cl (range: 10 to 77) and 3 Gg/year CH 3 Br (range: 1 to 8).

Environmental contextNatural haloform emissions contribute to stratospheric ozone depletion but there are major unknown or underestimated sources of these gases. This study demonstrates that soil and water at tidal wetlands are important haloform sources, and emissions peak at the forest–marsh transition zone. The low-lying forested wetlands of the south-eastern United States that are facing sea-level rise and seawater intrusion may become hotspots for haloform emission. AbstractSoil haloform emissions are sources of reactive halogens that catalytically deplete ozone in the stratosphere but there are still unknown or underestimated haloform sources. The >200000ha of low-lying tidal freshwater swamps (forests and marshes) in the south-eastern United States could be haloform (CHX3, X=Cl or Br) sources because sea-level rise and saltwater intrusion bring halides inland where they mix with terrestrial humic substances. To evaluate the spatial variation along the common forest–marsh salinity gradient (freshwater wetland, oligohaline wetland and mesohaline saltmarsh), we measured chloroform emissions from in situ chambers and from laboratory incubations of soil and water samples collected from Winyah Bay, South Carolina. The in situ and soil-core haloform emissions were both highest in the oligohaline wetland, whereas the aqueous production was highest in mesohaline saltmarsh. The predominant source shifted from sediment emission to water emission from freshwater wetland to mesohaline saltmarsh. Spreading out soil samples increased soil haloform emission, suggesting that soil pores can trap high amounts of CHCl3. Soil sterilisation did not suppress CHCl3 emission, indicating the important contribution of abiotic soil CHCl3 formation. Surface wetland water samples from eight locations along a salinity gradient with different management practices (natural v. managed) were subjected to radical-based halogenation by Fenton-like reagents. Halide availability, organic matter source, temperature and light irradiation were all found to affect the radical-based abiotic haloform formation from surface water. This study clearly indicates that soil and water from the studied coastal wetlands are both haloform sources, which however appear to have different formation mechanisms.

Global budgets of methyl halides are not balanced between currently identified sources and sinks. Among biological sources, rapeseed is regarded as the second largest terrestrial source of CH 3 Br, extrapolated from laboratory-based incubations and limited field measurements. This study analyzes the CH 3 Br budget from rapeseed (Brassica napus "Empire"), using field-based life cycle measurements, yielding a globally scaled emission rate of 2.8 ± 0.7 Gg year −1. Though this verifies that rapeseed is a significant global source, it is just half of the previous estimation, even after accounting for the doubling of global annual rapeseed production since then. The ozone-depleting potential of rapeseed is further sustained through CH 3 Cl and CH 3 I emissions, which were measured for the first time and scaled to 5.3 ± 1.3 and 4.0 ± 0.8 Gg year −1 globally. Plain Language Summary Stratospheric ozone absorbs incoming solar UV radiation, attenuating the harmful radiation exposure for life on Earth's surface. Halogen atoms transported via halocarbons, including methyl halides, can catalyze ozone destruction efficiently in the stratosphere. Anthropogenic sources of halocarbons have been decreasing consistently since the implementation of the 1987 Montreal Protocol and its amendments. However, some natural sources, especially those influenced by anthropogenic activities, may offset some of the achievement of reduced halocarbon emissions. This study quantifies methyl halide emissions from cultivated rapeseed (Brassica napus, cultivar: Empire), based on life cycle measurements and normalized to seed production. This yields a global crop contribution of 2.8 ± 0.7 Gg of methyl bromide (CH 3 Br) annually, which is smaller than previous estimates (5.1-6.6 Gg), supporting the conventional view that there must be other unidentified or underestimated sources for CH 3 Br. This study also quantifies for the first time that rapeseed emits 5.3 ± 1.3 Gg of methyl chloride (CH 3 Cl) and 4.0 ± 0.8 Gg of methyl iodide (CH 3 I) each year. Due to the increasing demand on rapeseed products such as canola oil, its global methyl halide emissions are expected to grow in the future.

Methyl bromide (CH3Br) and methyl chloride (CH3Cl) are major carriers of atmospheric bromine and chlorine, respectively, which can catalyze stratospheric ozone depletion. However, in our current understanding, there are missing sources associated with these two species. Here we investigate the effect of copper(II) on CH3Br and CH3Cl production from soil, seawater and model organic compounds: catechol (benzene-1,2-diol) and guaiacol (2-methoxyphenol). We show that copper sulfate (CuSO4) enhances CH3Br and CH3Cl production from soil and seawater, and it may be further amplified in conjunction with hydrogen peroxide (H2O2) or solar radiation. This represents an abiotic production pathway of CH3Br and CH3Cl perturbed by anthropogenic application of copper(II)-based chemicals. Hence, we suggest that the widespread application of copper(II) pesticides in agriculture and the discharge of anthropogenic copper(II) to the oceans may account for part of the missing sources of CH3Br and CH3Cl, and thereby contribute to stratospheric halogen load.

Methyl chloride (CH 3 Cl) and methyl bromide (CH 3 Br) are the predominant carriers of natural chlorine and bromine from the troposphere to the stratosphere, which can catalyze the destruction of stratospheric ozone. Here, penguin colony soils (PCS) and the adjacent tundra soils (i.e., penguin-lacking colony soils, PLS), seal colony soils (SCS), tundra marsh soils (TMS), and normal upland tundra soils (UTS) in coastal Antarctica were collected and incubated for the first time to confirm that these soils were CH 3 Cl and CH 3 Br sources or sinks. Overall, tundra soil acted as a net sink for CH 3 Cl and CH 3 Br with potential flux ranges from −18.1 to −2.8 pmol g −1 d −1 and −1.32 to −0.24 pmol g −1 d −1 , respectively. The deposition of penguin guano or seal excrement into tundra soils facilitated the simultaneous production of CH 3 Cl and CH 3 Br and resulted in a smaller sink in PCS, SCS, and PLS. Laboratory-based thermal treatments and anaerobic incubation experiments suggested that the consumption of CH 3 Cl and CH 3 Br was predominantly mediated by microbes while the production was abiotic and O 2 independent. Temperature gradient incubations revealed that increasing soil temperature promoted the consumption of CH 3 Cl and CH 3 Br in UTS, suggesting that the regional sink may increase with Antarctic warming, depending on changes in soil moisture and abiotic production rates.

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<p>Volatile organic compounds (VOCs) is a group of highly reactive gaseous species in the atmosphere with significant environmental implications, such as influencing air quality and Earth’s radiation balance. Natural ecosystems constitutes a large part of VOCs inventory with vegetation as well-known sources and soils as potential unidirectional interface yet relatively less studied. Here, we collected soil samples from two representative temperate ecosystems: beech forest and heather heath, and incubated them under manipulated conditions, such as at different temperatures,  and/or exposed to different ambient VOC levels, using a dynamic flow-through system coupled with a PTR-ToF-MS, from which production and/or uptake rates of some selected VOCs were measured and calculated. Results showed that these soils were natural sources of a variety of VOCs, and their emission strength and profile were influenced by soil biogeochemical properties (e.g., soil organic matter, moisture) and temperature. These soils were switched to natural sinks of most VOCs when supplying VOC substrates to the headspace of the enclosed soils at parts per billions level, and the sink size positively responded to the amount of VOCs available in the ambient air. Further analysis indicated that the observed VOC uptake by soils were likely driven by microbial metabolism plus a minor contribution from physical adsorption to soil particles. Overall, our study suggests that soil uptake of VOCs may conceal the simultaneous production and turn it into VOC sinks when ambient VOCs become readily available, such as significant VOC sources existing near surface, thereby regulating the net performance of ecosystem exchange of these environmentally important trace gases.</p>