Fate of Carbon and Greenhouse Impacts in a Mangrove-Infested Wetland

By Jan TenBruggencate
Board member, Mālama Hulē`ia
March 5, 2020

What is the most environmentally appropriate way to dispose of mangrove wood?
Mālama Hulē`ia has reviewed mulching, composting for aerobic decomposition, burning, biochar, incineration for energy, sinking biomass to anaerobically form a peatland, and leaving existing mangrove to grow and expand.
Each option has positives and negatives, and several differ depending on how they are deployed. Among our findings are that leaving mangrove in place is inappropriate when it is damaging a 600-year-old historic structure. In situations in which fossil fuels would be needed to move material, it may be more appropriate to burn in place, ensuring a hot enough fire. Composting is appropriate if the compost is managed to ensure aerobic digestion, to avoid methane production. Other options are considered in this report.
Mālama Hulē`ia will use the information in this report to address biomass treatment in the most appropriate manner, which may differ depending on specific circumstances.

The questions facing Mālama Hulē`ia as it clears invasive red mangrove along the shores of the historic Alakoko Fishpond include how best, from a climate change perspective, to minimize the climate impacts of the clearing.
We recognize that swampy mangrove forests lock up more carbon than almost any other natural environment, and as much as four times as much as land-based forests. (See https://www.carbonbrief.org/mangrove-deforestation-emits-as-much-co2-as-myanmar-each-year)
An average of global scientific estimates suggests that mangrove forests hold as much as 352 U.S. tons of carbon per acre, most of it in the roots and soil. Only about 18 percent is in the standing trees. (see Appendix 1)
Using a global mean of 64 above-ground U.S. tons of carbon per acre, clearing the 26-acre Alakoko site couple generate 1,664 tons of carbon. (Actual mass collected would of course be far greater, due to the moisture and other compounds in the biomass.)
We have explored many options, including burning for energy production, chipping and composting, chipping and mulching, burning, and simply leaving the trees to grow. All, to one degree or another, are problematic.
Leaving the trees ensures the continued destruction of the historic Alakoko Fishpond wall by the roots and the lever action of trunks that move in strong winds. It also allows the spreading of the invasive mangrove over the coastal area.
Of the other options considered to date, almost all are net producers of greenhouse gases, which has been problematic. We are deciding on the least impactful of a series of bad options. More on this later, but paradoxically, burning in a hot fire may be less climate-damaging than composting, mulching or landfilling. Burning at hotter than 850 degrees centigrade releases primarily carbon dioxide. Lower temperature processes release a larger percentage of methane and nitrous oxide, both of which are dramatically more powerful greenhouse gases than carbon dioxide. Methaneʻs greenhouse warming potential is 30 times CO2; nitrous oxide is 280 times CO2. (See https://www.epa.gov/ghgemissions/understanding-global-warming-potentials)
That said, carbon-dioxide persists longer in the atmosphere.
Burning the woody material in a biomass energy plant like that run by Green Energy Team does replace fossil fuels that might otherwise be burned by our islandʻs electric utility, KIUC. But we note that KIUC is increasingly converting to carbon-free renewables including solar and water power. Furthermore, burning the woody material at the site avoids the use of fossil fuels in transporting the mangrove biomass to the Green Energy plant, a distance of about seven miles. We have not attempted to calculate fossil fuel impacts of the transport.
All this is complicated by the procedures used. For example, extremely well-managed composting, which regularly introduces oxygen to the process can avoid the production of greenhouse gases. The opposite is also true: “Inefficient composting processes can result in anaerobic (rather than aerobic) conditions which produces methane and nitrous oxide.” (See https://www.agric.wa.gov.au/climate-change/composting-avoid-methane-production)
Two other alternatives have arisen, which can create carbon sinks.
One is to bury logs in the same mud in which the mangrove weeds grew. In essence, to store the carbon relatively permanently onsite, creating a carbon-negative wetland. The natural analogue of this is peatlands.
Another is biochar. This involves burning biomass in an oxygen-derived atmosphere, so the carbon is charred but not converted to carbon-dioxide. This material can be used as a soil amendment, and in some examples, it has sequestered that carbon in place for thousands of years. (See http://www.css.cornell.edu/faculty/lehmann/research/terra%20preta/terrapretamain.html)
These two may be the most environmentally sound option available, from a climate perspective.

There are many studies about mangrove and carbon sequestration. There is ample evidence that mangrove forests are extremely effective at sequestering carbon. Some sources suggest mangrove may lock up as much as four times more carbon than rainforests. However, other studies show that methane, a powerful greenhouse gas, is released from mangrove soils. This negates approximately 20 percent of the carbon that is stored.
“Mangrove forests are considered some of the most carbon-dense ecosystems in the world with most of the carbon stored in the soil,” says this study in Environmental Research Letters.
In Hawai`i, of course, mangrove is alien, invasive, and destroys native coastal wetland ecosystems. That is one reason Mālama Hulē`ia has embarked on its mission of removing mangrove. We have been replanting the areas with native species. Their carbon sequestration capacity is not yet well understood, but they provide some level of balance—replacing some of the climate services provided by mangrove.
Among our early approaches was to provide cut mangrove biomass for fuelstock in a biomass-to-energy plant that sells energy to our island electric utility. In this way we would create a carbon-neutral balance, replacing mangrove-derived greenhouse gas with greenhouse gas that would have been created by burning fossil fuels. That plan foundered due to issues with the amount of mineral material (dirt and rocks) in our mangrove stockpiles.
Instead, we have chipped mangrove and spread it over eroded slopes along the pond. This had the effect of reducing direct soil erosion from the covered land into the pond during rains. It also blocks or filters erosion from steep slopes above the pond. It acts as a combination of composting and mulching. The chip layer is many inches thick. The deepest layers remain moist and are composting, as evidenced by the presence of microorganism growtth. The sun-exposed surface chips degrade much more slowly since they remain drier.
A question more recently has been the fate of piles of mangrove logs and branches that are alongside the historic Alakoko Fishpond wall, where vehicles/heavy equipment can not be used to haul them out, out of concern for the integrity of the wall. We have discussed burning the piles of woody material, which raised objections from some Mālama Hulē`ia board members, out of concern for the greenhouse gas impact of burning.

Among major greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Water vapor causes a third to two-thirds of all warming. Among the major human-impacted greenhouse gases, the International Panel on Climate Change (2014) estimates that carbon dioxide contributes to warming 76%, methane 16% and nitrous oxide 6% and fluorinated gases 2%.
If mangrove trees are allowed to grow, they continue to sequester carbon in their woody parts, and in the form of fallen leaves and branches in the mud. Some of that carbon returns to the atmosphere as methane (CH₄). Methane is both far more powerful a greenhouse gas than carbon dioxide (28 times more powerful.) It survives in the atmosphere about 9 years. (https://www.esrl.noaa.gov/gmd/education/info_activities/pdfs/CTA_the_methane_cycle.pdf)
Methane is commonly produced in swamps, where carbon-rich materials are worked on by microbes in a moist, oxygen-poor environment. The microbes produce the methane. Landfills are also powerful methane producers, and over a long period of time.
Carbon dioxide (CO₂) is less powerful but long-lasting in the atmosphere, as much as 200 years. And there is so much of it that among human-impacted emissions, it is the most impactful.
Nitrous oxide (or dinitrogen oxide, N₂O), another powerful greenhouse gas, lasts 114 years in the atmosphere. It is 298 times more powerful as a greenhouse gase than carbon dioxide, and also depletes ozone. That combination of longevity and greenhouse potential is powerful. It is largely produced from fertilized lands (farmland), but also by burning of things like fossil fuels, and mangrove. (See https://www.icos-ri.eu/greenhouse-gases/nitrous-oxide)
Water vapor is also a significant greenhouse gas, but has a very short residence time in the atmosphere, and some argue it is more impacted by warming than a cause of it. Furthermore, thereʻs not much we can do about it. (https://www.yaleclimateconnections.org/2008/02/common-climate-misconceptions-the-water-vapor-feedback-2/)
Chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), hydrochlorofluorocarbons (HCFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6) can have thousands of times times the impact of carbon dioxide, but they are present in comparatively small amounts. And they are not a significant player in Mālama Hulē`iaʻs kuleana.

This Australian government site says composting is preferable to landfilling because composting, an aerobic process, does not produce methane while landfilling, an anaerobic process, does produce methane. https://www.agric.wa.gov.au/climate-change/composting-avoid-methane-production
That is a common misconception—that composting prevents the production of methane. In fact, there is plenty of evidence that it does. This 2005 article in Microbiology Ecology, “Thermophilic methane production and oxidation in compost,” by Jackel et al, shows that heat-loving organisms in compost do produce methane. https://academic.oup.com/femsec/article/52/2/175/541445
That said, an actively managed composting system that introduces oxygen into the process can minimize methane production in compost piles. (See https://www.agric.wa.gov.au/climate-change/composting-avoid-methane-production)og9vhkn

There is a great deal of information, often contradictory, about burning. One apparent factor in the numbers is time. Burning produces a lot of carbon dioxide at once. Composting, mulching and landfilling produce a lot of methane, but over decades. You need to look at long-term data to be comparing greenhouse apples with apples.
This paper argues that in short-term analyses of third-world agricultural uses, burning is bad: The global warming potential CO2‐equivalent emissions calculated for the entire crop cycles were at least five times lower in chop‐and‐mulch compared with slash‐and‐burn.From “An integrated greenhouse gas assessment of an alternative to slash‐and‐burn agriculture in eastern Amazonia,” by Davidson et al in 2008 in Global Change Biology. https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1365-2486.2008.01542.x
A 14% to 18% reduction in CO2‐eq emissions in almond orchards from putting biomass into soil versus burning. “Life Cycle–based Assessment of Energy Use and Greenhouse Gas Emissions in Almond Production, Part II: Uncertainty Analysis through Sensitivity Analysis and Scenario Testing” in 2015 by Marvinney et al in Journal of Industrial Ecology.
In Spanish fruit tree orchards, a 39-56% reduction in greenhouse gas emissions from incorporating pruning materials in soil compared to burning. Greenhouse gas emissions from conventional and organic cropping systems in Spain. II. Fruit tree orchards. Eduardo Aguilera et al, 2015, Agronomy for Sustainable Development.
In Brazilian sugar cane fields, stopping the burning of cane before harvest saves dramatically in carbon dioxide equivalent releases: “Our estimates indicate that conversion from burned to green plot could save from 310.7 (not considering soil carbon sequestration) to 1484.0 kg CO2equiv. ha−1 y−1 (considering soil carbon sequestration).” From the 2011 paper by Barretto De Figueire et al, in Agriculture, Ecosystems and Environment, Greenhouse gas balance due to the conversion of sugarcane areas from burned to green harvest in Brazil.
This counerintuitive Australian study found that if you extend calculations of emissions over 30 years, burning is more greenhouse gas-friendly than either composting or landfilling, and that small-scale inefficient composting is worse than large-scale well-managed composting. From a paper delivered to ISWA Conference in Portugal 2009, by Hutton et al, Waste management options to control greenhouse gas emissions – Landfill, compost or incineration? https://www.iswa.org/uploads/tx_iswaknowledgebase/10-302_FP.pdf
This study used calculations with data from the International Panel on Climate Change on municipal solid waste. It reported that composting releases high concentrations of greenhouse gas early in its use, and then tapers off. Landfilling produces high amounts of methane, although its impact can be reduced if the methane is captured. (Methane can then be burned for energy, which makes greenhouse sense if it reduces carbon dioxide production from fossil fuel use.) Incineration (burning) releases carbon dioxide, but minimizes release of much more powerful greenhouse gases methane and nitrous oxide.
This 2006 study by Svoboda et al, Nitrous Oxide Emission from Waste Incineration, in Chemical Papers-Slovak Academy of Sciences, says that incineration (of municipal solid waste) is a good way to manage nitrous oxide release, if the temperature of burning is high enough. It suggests minimal nitrous oxide release when the temperature is higher than 850 degrees Centigrade. https://www.researchgate.net/publication/226651405_Nitrous_Oxide_Emissions_from_Waste_Incineration
Methane is produced in incomplete combustion of wood. Hot dry fires produce little methane, according to a study: Biomass Burning and the Production of Methane, by Levine, Cofer and Pinto, of NASA Atmospheric Sciences and the U.S. EPA atmospheric research lab.
How to tell whether temperature is hot enough:
Red glow 932F/500C.
Red flames 1,112F/600C to 1,832F/1000C.
Orange flames 1,832F/1000C and 2,192F/1200C.
Yellow flames 2,192F/1200C and 2,552/1400C.
Hotter than that flames go to blue and white, but blue and white are rarer temperatures in an open wood fire.

Storing carbon underwater is a significant, long-term means of storing carbon. In a water-rich environment, with limited access to oxygen, carbon can remain entombed for long periods. Witness shipwreck timbers, long-sunken logs in lakes and rivers, human bodies in peat bogs.
Drained peat bogs release large amounts of methane. Rewetting drained bogs can stop the process. Here are some articles on that.
Peat and Repeat: Can major carbon sinks be restored by rewetting the worldʻs drained bogs? https://www.scientificamerican.com/article/peat-and-repeat-rewetting-carbon-sinks/
Carbon sequestration via wood burial https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2266747/
1800s-era sunken logs are now treasure; here are the men who find them https://www.latimes.com/nation/la-na-sinker-wood-20140713-story.html
Peatlands store twice as much carbon as all the worldʻs forests https://www.unenvironment.org/news-and-stories/story/peatlands-store-twice-much-carbon-all-worlds-forests
Storing biomass in wet ground can build a peatland, which has the potential to be significantly carbon negative, according to this 2018 paper by Windham-Myers et al. Potential for negative emissions of greenhouse gases (CO2, CH4 and N2O) through coastal peatland re-establishment: Novel insights from high frequency flux data at meter and kilometer scales. https://iopscience.iop.org/article/10.1088/1748-9326/aaae74/meta
The Windham-Myers study actually measured releases and uptake of greenhouse gases in a California peatland along the Sacramento River—a place that is water-saturated with large amounts of organic material in the ground. It found, paradoxically, that greenhouse gas uptake was greater than greenhouse gas release in specific cases, and it concluded that peatlands can actually be major carbon sinks. It found that these peatlands released more carbon dioxide by day and absorbed it at night. They released methane in all conditions. They took up nitrous oxide by daylight and were neutral at night. On average, the greenhouse gas equivalent emission in peatlands was negative.
There has even been a patent issued on underground wood storage to sequester carbon: https://patents.google.com/patent/US20100145716
There are certainly lots of complexities, but Another option open to Mālama Hulē`ia is to do what nature does in wetlands, bury the carbon in the mud.

More than a thousand years ago, Central Americans were incorporating charred carbon into the soil, creating dark soils called terra preta. (See https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/terra-preta)
Biochar is created by heating biomass in the absence of oxygen, a process called pyrolysis. It creates black, carbon-rich product similar to charcoal, and which has been touted as a way to improve soils and sequester carbon. (See https://e360.yale.edu/features/refilling_the_carbon_sink_biochars_potential_and_pitfalls)
Some of the processes and problems are reviewed in this paper from the University of Massachusetts. https://ag.umass.edu/sites/ag.umass.edu/files/reports/timmons_-_biochar_report_10-16-17.pdf

From this analysis, among the ways to handle the greenhouse implications of our actions, from least impact on warming to most, here are our options:
1. Leave the trees growing along the Hulē`ia shore. So they continue sequestering carbon and dropping that material into the muddy ground below, although some of that mud produces methane, and some carbon materials are lost to the river, where they have an uncalculated potential for release into the atmosphere.
2. Cut and bury the woody material in the swamp mud, which in a period of rising sea levels will remain wetted for, likely, centuries.
3. Create biochar with the mangrove biomass and deploy it as a soil amendment.
4. Mulch the wood in very dry conditions, to limit rotting and methane production. (Difficult in our mesic (moderate moisture) environment.)
5. Burn the wood, ensuring a very hot fire to limit methane and nitrous oxide production, although this does support conversion of woody carbon to carbon dioxide.
6. Compost the wood (which effectively is what is happening in the moist soil below the surface of our chip fields, but inefficiently, which promotes methane production.)
7. Landfill, which also has the potential to select for methane production over years to decades.

ACKNOWLEDGEMENTS: Mālama Hulē`ia carbon comittee. Members: Jan TenBruggencate, Ruby Pap, Bill Evslin, Luke Evslin, Sara Bowen, Peleke Flores.

Summary: A global mean of 352 U.S. tons of carbon per acre, 18 percent of which is above ground.
a. 937 tC ha-1 (418 us TONS/ACRE) Carbon Sequestration in Mangrove Forests: https://www.tandfonline.com/doi/abs/10.4155/cmt.12.20?journalCode=tcmt20
b. Global average 152 Mg/ha; Hawaii:120-160 Mg/ha above ground Mangrove Forests as Incredible Carbon Stores: https://blog.nature.org/science/2013/10/11/new-science-mangrove-forests-carbon-store-map/
c. 374.7 US Tons/Acre; 1,375 tons CO2. Mangrove Forest Carbon Sequestration https://raidboxes.io/wp-content/uploads/2019/05/Carbon-Sequestration-in-Mangroves.pdf
d. 2-200 kg/m2 in southern Gulf of California. Gulf Mangroves Store Carbon by the Ton http://datamares.ucsd.edu/stories/gulf-mangroves-store-carbon-by-the-ton/
e. 1,023 MgC/ha Mangroves among the most carbon-rich forests in the tropics. https://www.nature.com/articles/ngeo1123
f. 43 MgC/Ha Low Carbon Sink Capacity of Red Sea Mangroves https://www.researchgate.net/publication/319337625_Low_Carbon_sink_capacity_of_Red_Sea_mangroves
g. 565-1259 Mgc/ha for various mangrove species, Protocols for the measurement, monitoring and reporting of structure, biomass and carbon stocks in mangrove forests http://citeseerx.ist.psu.edu/viewdoc/download?doi=
Converted to U.S. tons per acre:
a. 418 U.S. Tons/Acre
b. Global 68 U.S. Tons/Acre (above ground only); Hawaii 53-71 U.S. Tons/Acre (Mean 64)
c. 374.7 U.S. Tons/Acre
d. 9-888 U.S. Tons/Acre (mean 440)
e. 455 U.S. Tons/Acre
f. 19 U.S. Tons/Acre
g. 251-560 Tons/Acre (mean 405)

Mean of a,c,d,e,f,g is 352 U.S. Tons/Acre
Based on item b., the amount above ground (thus the amount Malama Hule`ia is clearing, 64 tons per acre, or 18 percent of the total.