Greenhouse Gas Emissions from a Created Brackish Marsh in Eastern North Carolina

Introduction: The Potential of the Created Marsh

This study was done on a created brackish marsh in North Carolina. The purpose of the study was to determine what environmental factors influenced greenhouse gas (GHG) emissions, and to see how the emissions of a created marsh differ from those of a natural marsh. One important feature of tidal wetlands is that they are good areas for carbon sequestration in soils – if the marsh is built and maintained correctly, this can lead to the marsh becoming a carbon sink. Wetlands are beneficial in many other ways as well; they improve water quality, provide habitats and mitigate flood waters. However, it is important to note that GHG emissions from the marsh could end up removing any benefit from sequestration – marshes are sites of varied anaerobic and biogeochemical conditions, spatially and temporally, which can make them huge GHG producers. Created marshes can also help with carbon sequestration, but their emissions have not been extensively studied.

Three zones, based on vegetation type and elevation, divided the created marsh.  Fluxes were measured in these zones using a static flux chamber method. The majority of fluxes were from CO2, but even these values were lower than in other wetlands (and could be erased by photosynthetic uptake). The hope is that with proper planning, created marshes could become net sinks for GHG’s. One discovery of this study is that CH4 flux rates decrease with an increase in salinity – placing created marshes in areas of high salinity may be helpful. This study also found that long/frequent periods of saturation can minimize nitrification, thereby allowing more complete denitrification (which reduces N2O emissions).  Therefore, an overall higher salinity and frequent tidal flush in created marshes can balance GHG emissions.

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The Research Site:

The site where this experiment was done was a new 14 hectare brackish marsh, whose construction was completed in 2007 (the land where the marsh was built had previously been used as row-crop agriculture). When the marsh was nearly complete, three plant species were added based on the distribution of these same plants in a natural marsh nearby. When finished, this particular marsh had two low tides and two high tides per day.

Then the research itself began and gas measurements were taken four to six years after construction (from 2011-2013), by adding three experimental blocks along the tidal creek.  Two were placed in the created marsh and one in the natural marsh which was used as the reference marsh and was situated just downstream; this allowed for easy comparison. Researchers made sure that the soil properties between the created and natural marshes were very similar, so that the data could be compared with little error. Relative to the created marsh, the natural marsh had lower soil density and a higher average carbon content. However, the two marshes had similar plant density, the same tidal cycle, and the same water chemistry.
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The greenhouse gas samples extracted at this site were taken using a replicated static flux chamber method. Researchers were aware that gas fluxes can be affected by many factors, so they made sure there was a good spread of chambers: a total of 18 chambers in the created marsh, and 6 chambers in the reference marsh. The collection of samples was done at the same time every day for consistency, and was done when the tide was low enough that the soil was exposed (so that no water blocked direct gas diffusion).

At the same time as the measurements of the gases were being taken, other environmental factors were being tested for their effect on emissions; these included pore water salinity, temperature of soil, conductivity, and the soil redox potential (availability of electrons in the system, which are necessary to all organic and inorganic chemical reactions).

Other testing was also done to measure specific possible factors affecting gas output. This included a litter bag study, where 10g of each of the three species of marsh plants were placed in fiberglass screen mesh bags, both above ground and under in the three separate plant zones, to measure their decomposition rates (as possible carbon sources). Scientists also measured the soils microbial denitrification enzyme activity (in this case, related to the capability to produce N2O). When all these results had been compiled, the scientists used linear regression analysis to compare the GHG measurements with the environmental factors, the final list of which included soil temperature, groundwater level, plant type, site, soil redox potential, salinity, tide direction, sampling month, and sampling year. They also looked at how these factors differed in the created and natural marshes.

 

Results and Conclusion: Better than Nature?

From the measurements and analysis of the emissions and related environmental factors, the results of the study were as follows: the flux of CO2 in soil was significantly affected by the soil temperature (positive correlation), the redox potential of the soil (negative correlation), the plant type present, the intertidal site, and the sampling time (including both month and year). CH4 flux was significantly affected by the type of plant present and the time of the sampling. The study also found that N2O was unaffected by any of the environmental factors measured.

Different elevations also had different flux levels. Rates were significantly higher upstream in the created marsh as compared to the reference marsh, and the same down stream in both the created and reference marshes. The CO2 mean flux rate was lower in the mid marsh area than the low and high marsh areas where the plant species Juncus roemerianus was dominant. In the reference marsh the CO2 mean flux was lower in mid marsh than in the low area. However, in the downstream area there were no significant changes in CO2 fluxes based on elevation.

The CH4 flux by comparison, was lowest in the mid marsh area where the plant species Juncus roemerianus dominated. The CH4 flux rates were also approximately 100 times higher in the low marsh, and 50 times higher in the high marsh (compared across the three sites). Finally, both CO2 and CH4 fluxes had a slight decrease over the three year period of the study, as the marsh aged.

In the created marsh, the emissions of CO2 and CH4 were within the ranges of previous studies. The soil CO2 flux was significantly correlated to soil temperature; the flux was lowest late in the winter, and highest late in the summer. The reasoning behind this is that temperature affects microbial enzyme activity – when the temperature is low, the decomposition of organic matter is slowed down, which can decrease GHG emissions; the opposite is true in low temperatures. However, soil temperature only explained 16% of the variation in CO2 flux. CH4, on the other hand, was not affected by soil temperature; it was, however, affected by plant life, as the flux was lower in the zone where Juncus roemerianus was dominant

In reference to N2O, the low flux was most likely due to the low concentrations of NO3; the low soil redox potential also prevented nitrification but favoured the denitrification of available NO3, meaning it’s even less likely to facilitate flux of N2O.

80-90% of the global warming potential was contributed by the CO2 flux from the marsh; however, the uptake of CO2 during photosynthesis was not included in the global warming potential balance (photosynthetic uptake of CO2 would help with soil emissions, since these marshes sequester carbon). GHG emissions in the created marsh were low relative to natural marshes in freshwater systems – the researchers suggest that further research is needed for tracking emissions through time and in other locations, to see how trends compare over spatial variations (if this is localized or global, or has a relation to the techniques used). The marsh wasn’t an important source of CH4 or N2O.

In conclusion researchers found that the created marsh was not a significant emitter of CH4 or N2O. The majority of the emissions from the marsh were CO2, but overall in comparison to other measurements from previous research in natural marshes, these results were low. Created marshes, in future, should be studied for carbon sequestration potential – especially in light of the low emissions from this marsh – and could eventually become a way to reduce the amount of greenhouse gases entering our atmosphere.