Halogen biogeochemistry of invasive perennial pepperweed (Lepidium latifolium) in a peatland pasture

Abstract

[1] Perennial pepperweed (Lepidium latifolium) is a widespread invasive plant in North America. This yearlong field study at a pasture peatland infested with L. latifolium demonstrates that these plants are large emitters, on a per area basis, of methyl chloride (CH 3 Cl) and methyl bromide (CH 3 Br), compounds that contribute to the destruction of stratospheric ozone. Annually averaged net emission rates were 9.0 AE 11.8 mmol m À2 d À1 for CH 3 Cl and 460 AE 430 nmol m À2 d À1 for CH 3 Br, comparable to observed coastal salt marsh emission rates. A stable isotope tracer technique was used to distinguish between simultaneous production and consumption processes for CH 3 Cl and CH 3 Br. Over the course of the year, gross production rates for methyl halides varied widely over different life cycle stages, with the highest fluxes surprisingly occurring at senescence. Maximum emissions of CH 3 Cl and CH 3 Br occurred at midday, the time of highest solar radiation. During the growing season, methyl halide gross production rates were positively correlated with temperature and live biomass, suggesting that the production of methyl halides from L. latifolium at this time was mostly biotic. During plant senescence, the peak emissions of CH 3 Cl and CH 3 Br are unexplained but may be released from L. latifolium associated with transitions in biochemistry. Overall, these flux measurements show expected diel and growing season trends based on plant production for most of the year, overlaid by a high emission peak during senescence, suggesting that both plant biochemical shifts and varying environmental factors need to be considered as important internal and external controls on emissions.

Ecosystem‐Scale Measurements of Methyl Halide Fluxes From a Brackish Tidal Marsh Invaded With Perennial Pepperweed (Lepidium latifolium)

“…In addition to halide concentration variability associated with plant growth stage, there was also interannual variability in the soil water salinity which likely influenced the tissue halide content, with an enrichment in the drought year 2016 as compared to the wet year of 2017 (Figure S3). Absolute values of L. latifolium samples were similar to those from Sherman Island, California (Khan et al, ), with maxima of 51 g Cl − /kg and 1.1 g Br − /kg during senescence. F. salina content in this study was roughly half of reports from the coastal Upper Newport Bay saltmarsh of 200 g Cl − /kg and 2.8 g Br − /kg (Manley et al, ), reflecting the salinity gradient between the brackish marsh (this study) and the salt marshes in Southern California.…”

“…Outside of the tropics, coastal salt marshes are emission hot spots for both compounds, emitting up to an estimated 5% and 17% of global CH 3 Cl and CH 3 Br emissions, respectively, which is also estimated to be up to 12 times larger than the global freshwater wetland source (Carpenter et al, ). Observations of natural salt marshes emissions have been conducted in North America (Khan et al, ; Manley et al, ; Rhew & Mazéas, ; Rhew et al, , ), Europe (Blei et al, ; Dimmer et al, ; Drewer et al, ), and Oceania (Cox et al, ). Reported fluxes, however, have a large variability, spanning over 3 orders of magnitude for CH 3 Cl and 2 orders of magnitude for CH 3 Br.…”

“…In all of these studies, static flux chambers (FCs) were employed to quantify exchange fluxes, enclosing a small surface area (a 0.04 to 1 m 2 flux footprint) of salt marsh vegetation, with measurement sites distributed to either capture different inundation regimes (lower to higher marsh; Blei et al, ; Rhew et al, ) or to study a variety of monospecific vegetation stands (Khan et al, ; Manley et al, ; Rhew et al, ). Observed fluxes were highly variable, even when experiments were paired in time, location, or enclosed vegetation species.…”

Bidirectional exchange of methyl halides between tropical plants and the atmosphere

“…Net CH 3 Cl and CH 3 Br production rates are also correlated ( R 2 = 0.76) and have a molar ratio of 422 ± 96. This ratio is significantly larger than those previously reported in Salt marsh (~4 [ Blei et al ., ]; ~20; [ Rhew et al ., ]), tropical plants (~89 [ Blei et al ., ]; 60–220 [ Saito and Yokouchi , ]), rice fields (~7) [ Redeker et al ., ; Khan et al ., ], and perennial pepperweed (an invasive plant of the Brassicaceae family, ~200) [ Khan et al ., ].…”

Global Methyl Halide Emissions From Rapeseed (Brassica napus) Using Life Cycle Measurements

“…Rapeseed has a usual lifetime of 130–160 days (Buntin et al, 2007; Diepenbrock, 2000) with physiological factors at different life stages varying significantly, such as leaf area index, flower number, pocket number, biomass, plant height, and so forth (Bouttier & Morgan, 1992; Morrison et al, 1992). It is expected that rapeseed, similar to other biological emitters, would reveal varied methyl halide emission levels over the life cycle (Deventer et al, 2018; Khan et al, 2013; Manley et al, 2006; Redeker et al, 2000). Extrapolating a global CH 3 Br budget after incorporating the life cycle variations in emissions may improve estimates.…”