This project was directed toward the quantification of fluxes of gaseous biogenic sulfur compounds from freshwater wetlands. These compounds (primarily hydrogen sulfide (H2S), dimethyl sulfide (DMS), and carbonyl sulfide (OCS)) have been implicated in the regulation of planetary albedo by the formation of microscopic atmospheric aerosols when they oxidize, and the further role of these aerosols as cloud condensation nuclei (CCN). The role of continental sources and sinks for these compounds is poorly understood. The present study was undertaken to quantify the source and sink strength of high latitude wetlands, and to delineate factors that regulate this flux

Hines, Mark E.

English

NASA Technical Reports Server

ASA-CR-196286
/ /J :
FINAL TECHNICAL REPORT
NASA Grant NAGW-2771
Gas Exchange in Wetlands: Controls and Remote Sensing
Principal Investigator
Mark E. Hines
Institute for the Study of Earth, Oceans and Space
University of New Hampshire
Durham, NH 03824
Project Period
October 1, 1991 - September 30, 1992
(NASA-CR-19_286) dAS EXCHANGE IN
_ETLANDS: CONTROLS ANO _EMOTE
SENSING Final Technical Report, 1
_ct. 1991 - 30 Seo. 1992 (New
H_mnshire Oniv.) 7 p
N95-I0169
Unclas
G3/45 0016791
https://ntrs.nasa.gov/search.jsp?R=19950003757 2020-06-16T11:51:59+00:00Z
Introduction
This project was directed toward the quantification of fluxes of gaseous biogenic sulfur
compounds from freshwater wetlands. These compounds (primarily hydrogen sulfide (H2S),
dimethyl sulfide (DMS), and carbonyl sulfide (OCS)) have been implicated in the regulation of
planetary albedo by the formation of microscopic atmospheric aerosols when they oxidize, and
the further role of these aerosols as cloud condensation nuclei (CCN). The role of continental
sources and sinks for these compounds is poorly understood. The present study was undertaken
to quantify the source and sink strength of high latitude wetlands, and to delineate factors that
regulate this flux.
Sampling Sites and Methods
The majority of research during the one-year project was conducted in Sallie's Fen in New
Hampshire, which is a poor fen dominated by Sphagnum mosses (de Mello and Hines, in press).
Preliminary work in this fen revealed it as an unusually strong source of DMS when compared
to marine sites that are rich in sulfur (Hines, in press). Preliminary work also suggested that
these types of wetlands emit more DMS from ombrotrophic areas than from mineral-rich ones
which is in contrast to other gases like methane. Therefore, the goals of the project were to test
the hypotheses that DMS emissions are high in these systems, and that they are highest from
nutrient poor areas. The data were put into a seasonal framework. In addition, we tested that
hypothesis that increased inputs of inorganic S would stimulate the emissions of gaseous S.
Emissions of biogenic S gases were determined using dynamic (use of a sweep gas) flux
chambers as described in Morrison and Hines (1990). Experiments comparing static chambers
and dynamic ones demonstrated that both chambers were suitable for DMS measurements, but
that only the static chamber technique could be used for OCS which is consumed by the wetland
(de Mello and Hines, in press). Gases were collected from chambers using a mass flow
controller system with volume totalizing, and samples were trapped in Teflon loops immersed
in liquid argon. Gases were analyzed in the laboratory by gas chromatography with flame
photometric detection.
2
An experimentwasconductedto examinetheeffectof S additionson theproductionand
emissionof Sgases.Plotswerefertilizedwith aninorganicScompoundandthedepth
distributionof Sgases,andtherateatwhichtheywereemittedfrom thesurface,were
monitored. In oneplot, thesurficialvegetation(primarily Sphagnum) was removed prior to the
amendment with S.
Another set of flux measurements was made in three wetlands in the Experimental Lakes
Area (ELA) in western Ontario, Canada in July, 1992. This work was conducted as part of a
larger experiment to determine the effects of flooding on the production and release of biogenic
gases, and on the methylation and demethylation of mercury. The work conducted in 1992 was
during a pre-flood year. Post-flood measurements will be made in 1994 with funding from
another source. During the pre-flood year we determined S gas emission rates along wetland
gradients from the central pond to the surrounding upland. We also measured the
concentrations of S gases in the ponds to determine the relative importance of wetlands and
open water as sources of biogenic S gases to the atmosphere.
Results and Discussion
Much of this work is presented in documents that have been either accepted for
publication, are in review for publication, or are being prepared for publication from graduate
theses that have been completed. The following is a summary.
The study comparing the static with the dynamic chamber technique for measuring S gas
exchange yielded information which was extremely useful. Virtually all of the data available on
fluxes of S gases from continental habitats was made using dynamic chamber approaches.
Earlier studies reported that dynamic chambers that utilize S-free sweep gas will underestimate
flux rates for OCS and carbon disulfide (CS2) from forest soils. Our work showed that this is
true for wetlands as well (de Mello and Hines, in press). However, our work demonstrated that
the dynamic chambers are adequate for use with gases that emitted from a habitat, and that
previous DMS flux data are probably correct in most cases. Our data also showed that wetlands
are net sinks for OCS, which is in contrast to all other published studies which employed S-free
sweep air and dynamic chamber techniques. Our data also revealed that the anaerobic peats in
these wetlands are sources of OCS, but that the surficial peat and living vegetation consume
sufficient OCS to make the ecosystem a net sink (de Mello and Hines, in press).
Emissions of DMS greatly dominated S gas fluxes in all of the Sphagnum-dominated
wetlands examined. This differed from marine wetlands where methane thiol (MSH) and other
S gases were a significant portion of the total (Morrison and Hines, 1990; Hines, in press).
Fluxes of DMS varied by nearly two orders of magnitude throughout the wetlands (Hines, in
press). Fluxes at individual sites were controlled strongly by temperature. We were unable to
measure emissions of DMS when temperatures were below -5 ° C. During 24 hr studies, DMS
emissions varied by several fold as temperatures changed. However, temperature changes did
not explain two order of magnitude changes in DMS flux from one site to the next on the same
day. Fluxes in the minerotrophic sections of Sallie's Fen were less than those in the nutrient-
poor (also low pH) areas as proposed. The high fluxes in the ombrotrophic areas were faster
than those measured in many salt marshes despite the fact that the latter contain >100 fold more
S (de Mello et al., submitted).
Additions of inorganic S to experimental plots in Sallie's Fen caused a rapid increase in S
gases in peat pore waters, particularly DMS and MSH. In a site in which all green vegetation
had been removed, these gases increased to nearly millimolar levels in pore waters within three
days. In undisturbed sites, the dissolved gases increased for one day and then decreased again
to low levels (de Mello et al., submitted). However, the maximum concentrations at the
undisturbed sites were 10-fold lower than those at the "bare" site. Although the concentrations
of dissolved gases increased in response to S additions, the fluxes did not increase, at least for 3-
4 days following the S amendment. These results indicated that the system is capable of
producing large quantities of S gases when additional S is available. However, the presence of
plants prevents the accumulation of S gases. Interestingly, sites with plants always emitted
more S gases than unvegetated sites, and the addition of Sphagnum to bare sites enhanced S gas
emissions greatly. Sites with no Sphagnum emitted very small quantities of S gases. Although
4
MSH concentrationsincreasedgreatlyin responseto Sadditions,thisgaswasrarely measured
within flux chambersevenwhenDMS fluxeswerehigh. It appearsthat MSH is selectively
oxidizedprior to transport,probablyduringits capillarymovementin Sphagnum (de Mello et
al., submitted).
All of the experimental data indicated that DMS (and MSH) was produced from the
methylation of inorganic S (H2S) (i.e., H2S methylated to MSH, followed by the methylation of
MSH to DMS). This differs greatly from DMS in marine environments where the bulk of the
DMS produced is derived from the hydrolytic cleavage of an organic S compound which serves
as an osmoregulant in vascular plants and algae.
Emissions of DMS from the ELA wetlands were quite high in some cases even though the
atmospheric input of S to this part of Canada is very low relative to the New Hampshire sites.
The wetlands emitted much more S gases than did the central ponds. This is important since
estimates of wetland emissions of S gases have been based on data from samples of standing
water or from small ponds, indicating that previous estimates of flux have been grossly
underestimated. In fact, S gas emissions become very slow when an active site becomes
flooded after a prolonged rain event.
We noted a pronounced spatial trend in the magnitude and speciation of the S gas flux in
the wetlands at the ELA as one moves from the pond toward the upland. The floating
Sphagnum mat emitted primarily H2S with very little DMS. The central part of the wetland
emitted all DMS at the highest rates measured. Near the upland, DMS continued to dominate
flux, but rates were quite slow. This trend agrees with our hypothesis that the ombrotrophic
areas of wetlands are the primary sources of DMS.
Biogenic S gas emissions are unusually high in the northern wetland environments.
Because of the relatively small global cover of these wetlands and the short growing season,
they are not large contributors to the global atmospheric S burden. However, since S gases are
short lived in the atmosphere, these wetlands can be important contributors on a regional basis
in remote areas.
5
Theses and Publications:
de Mello, W. Z. 1992. Factors controlling fluxes of volatile sulfur compounds in Sphagnum
peatlands. Ph.D. thesis. University of New Hampshire.
Murray, G. 1994. Rates of sulfate reduction and methanogenesis in an artificially acidified
northern wetland. M.S. thesis. University of New Hampshire.
Hines, M.E. in press. Emissions of sulfur gases from wetlands. In: D.D. Adams, S.P.
Seitzinger and P.M. Crill (eds.), Cycling of Reduced Gases in the Hydrosphere.
de Mello, W.Z. and M.E. Hines. in press. Application of static and dynamic enclosures in
determining DMS and OCS fluxes in Sphagnm-dominated peatlands. Implications for the
direction and magnitude of flux. J. Geoophys. Res.
de Mello, W.Z., M.E. Hines and S.E. Bayley. submitted. Northern freshwater wetlands as
sources of dimethyl sulfide to the atmopshere. Global Biogeochemical Cycles,
Presentations at Scientific Meetings :
de Mello, W.Z., M.E. Hines and S.E. Bayley. 1992. Factors controlling sulfur gas exchange in
Sphagnum-dominated wetlands. American Society of Limnology and Oceanography
Annual Meeting. Santa Fe, New Mexico.
Hines, M.E., W.Z. de Mello and C. A. Porter. 1993. Sulfur gas exchange in Sphagnum-
dominated wetlands. Biogeochemistry of Wetlands Symposium, Baton Rouge, Louisiana.
6
In Press: Journal of Geophysical Research
Application of static and dynamic enclosures for determining
dimethyl sulfide and carbonyl sulfide exchange in Sphagnum
peatlands: Implications for the magnitude and direction of flux
William Z. de Mello t and Mark E. Hines
Institute for the Study of Earth, Oceans and Space, University of New Hampshire, Durham
Abstract. A static enclosure method was applied to determine the exchange of dimethyl sulfide
(DMS) and carbonyl sulfide (OCS) between the surface of Sphagnum peatlands and the atmo-
sphere. Measurements were performed concurrently with dynamic (fow through) enclosure
meas-urements with sulfur-free air used as sweep gas. This latter technique has been used to
acquire the majority of available data on the exchange of S gases between the atmosphere and
the continental surfaces and has been criticized because it is thought to overestimate the true
flux of gases by disrupting natural S gas gradients. DMS emission rates determined by both
methods were not statis-tically different between 4 and >400 nmol m-: h -_, indicating that
previous data on emissions of at least DMS are probably valid. However, the increase in DMS
in static enclosures was not linear, indicating the potential for a negative feedback of enclosure
DMS concentrations on efflux. The dynamic enclosure method measured positive OCS flux
rates (emission) at all sites, while data using static enclosures indicated that OCS was consumed
from the atmosphere at these same sites at rates of 3.7 to 55 nmol In-: h t . Measurements using
both enclosure techniques at a site devoid of vegetation showed that peat was a source of both
DMS and OCS. However, the rate of OCS efflux from decomposing peat was more than
counterbalanced by OCS consumption by veg-etation, including Sphagnum mosses, and net
OCS uptake occurred at all sites. We propose that all wetlands are net sinks for OCS.


Gas exchange in wetlands: Controls and remote sensing

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