Phenolic Compounds and Black Carbon Feedback Controls on Peat Decomposition and Carbon Accumulation in Southern Peatlands under Regimes of Seasonal Drought, Drainage and Frequent Fire: A New Model for Management of Carbon Sequestration

The Duke University Wetland Center has undertaken a study on phenolic compounds and black carbon feedback controls on peat decomposition and carbon accumulation in southern peatlands. The project is funded by a $995,000 grant from the U.S. Department of Energy's Office of Biological and Environmental Research Terrestrial Ecosystem Science Program.

Peatland ecosystems are among Earth's most efficient carbon sinks. Carbon captured from the atmosphere can be stored in the saturated peat for millennia due to the presence of naturally occurring phenolic compounds which inhibit microbial decomposition. In recent years, however, large areas of peat wetlands worldwide are being burned or drained for agriculture, forestry and to harvest the peat for energy. The organic carbon that is normally stored underwater is exposed to air, dries and decomposes, and emits carbon dioxide back into the atmosphere. Climate change is adding to the problem.

"The main question we are addressing is how southern peatlands continue to store carbon and release lower amounts of greenhouse gases compared to northern peatlands, even under climate-driven increases in temperature and extended droughts," says DUWC Director Curtis Richardson. "The research focuses on the role of phenolics and black carbon, both antibacterial carbon compounds, as biogeochemical controls on peat decomposition along a latitudinal gradient from Minnesota to Peru. What we are learning will provide us with new approaches for managing storage and losses of carbon from millions of acres of peatlands worldwide."

Globally, approximately 1/3 of peat stores are found in subtropical and tropical peatlands (STPs) formed from high-lignin woody biomass. These peatlands have persisted through changing climate and sea level over the last 4000 years and continue to accrete peat along the Atlantic coast from Virginia and North Carolina, to Florida, and to tropical Peru, even under climate driven conditions of drought, warmer temperatures and fire.

Left. DUWC researchers work in a study site at Pocosin Lakes National Wildlife Refuge in eastern North Carolina. Pocosins are nutrient-poor, freshwater, evergreen shrub bogs found in coastal areas of Virginia, North Carolina, and South Carolina. Over the past 4 millennia, such subtropical and tropical peatlands have accumulated and stored over 20% peat of the continental United States. Right. Globally, accidental fires and commercial deforestation and burning account for significant releases of carbon into the atmosphere. The DUWC study will provide much needed information for management of hydrology and fire intensity in natural and degraded shrub/tree peatlands, central to maintaining peat/litter quality (phenol/black carbon) and enhancing long-term carbon accumulation. Photos: DUWC & USFWS

Our questions are: 1) why do these stressed non-sphagnum peatlands accumulate C, and 2) what insights can we gain from studying their natural processes and control mechanisms to provide management and conservation of the vast STPs and boreal peatlands subject to increasing climate forcing.

While most studies focus on northern peatlands, globally important STPs remain woefully underrepresented in Earth System Models. The Wetland Center is conducting a multi-year experimental comparison across STPs to reveal the key process-level mechanisms controlling soil C stabilization, accumulation, and long-term storage. Our work will enable new hydrologic, fire and community management strategies and predictive threshold models to facilitate recovery of disturbed STPs subjected to climate change and lower water tables either by drainage or drought. Elucidation of these mechanisms will also yield perspective towards understanding the effects of global warming and drought on boreal Sphagnum peatlands undergoing climate-induced shifts to wooded plant communities.

Our main hypothesize is that the STP native-fire-adapted shrubs/trees communities produce higher polyphenol litter than Sphagnum/Carex communities. This production difference, in conjunction with climate induced regimes of frequent low-intensity fire, creates recalcitrant decomposition-resistant peat by a dual "latch key mechanism" consisting of high phenol and black carbon (BC, complex aromatics). Together these retardants reduce GHG flux, and C decomposition of STP peats even under altered hydrologic conditions, higher temperatures and drought.

Specific objectives include:

I: Identify and compare process-level mechanisms controlling peat accretion and C losses from shrub-bogs in North Carolina, subtropical Cladium/shrub peats in the Florida Everglades, and tropical Myrica-Cyrilla bogs in Peru. This never-before-studied latitudinal gradient allows experimental quantification of biotic (plants type) and abiotic (low-intensity fire and drought) contributions to resultant high phenol/low carbon quality litter and specific BC aromatics;

II: Assess the composition and origin of aromatic compounds in peat and porewater at the molecular level and the importance of fire derived aromatic compounds limiting peat decomposition using multiple advanced analytical techniques including Pyrolysis GC-MS, GC-MS, LC-MS, NMR, FTIR, FT-ICR-MS and 3-Dimensional Excitation-Emission Matrix (EEM) fluorescence spectroscopy;

III: Experimentally (field to microcosm scale) determine peat decomposition, GHG fluxes and DOC loss integrated with soil C chemistry, soil bacterial/fungal composition, enzyme activities, and hydrologic properties to enhance our understanding of controls on C storage and fluxes. The field fire study leverages data and research site infrastructure at an ongoing drainage GHG study at the USFWS Pocosin Lakes Wildlife Refuge in North Carolina and with Oak Ridge National Laboratory and USFS investigators working at the Spruce-Peatland Response Under Climate and Environmental Change (SPRUCE) experiment site in Minnesota and with scientists at the Los Amigos Biological Station in Peru. Our research provides the first latitudinal comparative analysis of peatland C chemistry, tests a new dual control model for sustaining C, advanced C chemistry data, and analytical methods, and data for modeling in support of DOE's climate change research program and Earth System Models.

Left The DUWC research sites in Florida's Loxahatchee NWR are accessible only by airboat. Researcher Suzanne Hodgkins from collaborating institution Florida State University prepares porewater samples while DUWC scientist Neal Flanagan looks on. Right. The Spruce-Peatland Response Under Climate and Environmental Change (SPRUCE) Project in northern Minnesota is a prime component of the DOE Terrestrial Climate Change Program. The SPRUCE sphagnum bog forest, like most boreal peatland forests, is considered especially vulnerable to climate change.

An important result of the study is the discovery of a previously unknown dual mechanism that slows peat decay and may help reduce carbon dioxide emissions from subtropical peatlands during times of drought. The naturally occurring mechanism was discovered in the North Carolina pocosins. Preliminary field experiments suggest it may occur in, or be exportable to, peatlands in other regions as well. "When we took peat extracts from the southern peatlands and put them into Canadian peatlands, they slowed down decomposition there, too," said DUWC's Richardson. "The accepted scientific paradigm is that prolonged drought, coupled with global warming and increased drainage of peatlands for agriculture and forestry, will lower water levels. This could cause peatlands to dry out, decay and release massive amounts of carbon back into the atmosphere. Our research supports a less dire scenario. It finds that moderate long-term drought might have less impact on the release of carbon dioxide from peatlands than expected."

The reason lies buried in the peatland soil itself. By comparing the chemistry of soil from pocosin bog peatlands in North Carolina with soil from boreal peatlands in Canada, the DUWC team discovered a significant and previously unrecognized difference between the two Southern wooded peatlands are 5,000 to 8,000 years old and have more complex plant-derived compounds that have allowed them to adapt to drought through a mechanism that regulates the buildup of phenolics and helps slow down decomposition. This natural adaptation, which was not found as abundantly in soil from boreal peatlands of the north, protects stored carbon directly by reducing decay-promoting phenol oxidase activity during short-term drought. The mechanism also indirectly protects stored carbon by spurring a shift in the peatlands' plant cover in response to moderate long-term drought. As water levels drop, plants that contain low levels of phenolics, such as sphagnum moss, ferns and sedges, are replaced by trees and shrubs, which are high in the decay-retarding compounds. "This dual mechanism helps peat resist decay and adapt to climate change," Wetland Center researcher Hongjun Wang said. He believes that high-phenolic shrubs could naturally expand into northern peatlands or be introduced there as water levels drop, offering the hope that scientists might be able to reduce the future risk of large carbon releases. Researchers still need to identify the specific aromatic components or groups of phenolics that are responsible for the decay-retarding mechanism. Plants produce and contain thousands of compounds, so this may take time. But it will be worth the effort, providing new approaches for managing storage and losses of carbon from millions of acres of peatlands worldwide.

Left. DUWC Research Scientist Hongjun Wang checks a set instrument that measures peat accretion at a DUWC research site in eastern North Carolina's Pocosin Lakes NWR. Right. The Los Amigos Biological Station, part of the Amazon Conservation Association, is in lowland Amazonian forest at the base of Peru's southern Andes. DUWC researchers collected peat samples from palm swamp locations there.

Selected Publications

Wang, H., C.J. Richardson, M. Ho. 2015. Dual-controls on carbon loss during drought in peatlands. Nature Climate Change 5:584- 587. https://www.nature.com/articles/nclimate2643

Winton, R.S., N. Flanagan, and C.J. Richardson. 2017. Neotropical peatland methane emissions along a vegetation and biogeochemical gradient. PLoS One 12(10):e0187019. https://doi.org/10.1371/journal.pone.0187019