Collaborative Research: Does tree encroachment with altered hydrology in peatlands accelerate or suppress decomposition?

Peatlands are important ecosystems in the global carbon (C) cycle owing to their ability to take up and store vast quantities of C under cool and wet conditions. While these conditions have tended to be stable for long periods of time, vast changes are now taking place. Some peatlands have been drained for agriculture or forestry.

Others will become warmer and drier in a warming climate. What happens to all the stored C when peatlands are drained or dry up? The answer to this complex question requires understanding the impacts of changes in vegetation, tree growth and peat decomposition below ground through interactions with fungi and other microorganisms.

This project will use a large-scale experimental set-up to study the changes that take place underground as peatland soils are drained and trees begin to colonize. Experiments will test how changes in the below ground fungal community accompanying tree roots will impact retention and decomposition of stored C and its potential release into the atmosphere as greenhouse gases like carbon dioxide and methane.

Increased retention of peat C would potentially help mitigate global warming while release of greenhouse gases could result in even greater, faster warming. In the course of conducting the research this project will also afford training opportunities for undergraduate and graduate students and outreach to high school teachers and students.

Peatlands store about 1/3 of soil C globally in 1/30th of the Earth's land area. Peatland C is vulnerable to oxidation as a result of climate change or water drainage. Peatland C stocks are generally protected under saturated conditions.

However many peatlands will become drier in a warming climate. While it is generally assumed that drier conditions will increase decomposition, there are potential feedbacks that lead to major uncertainty in how long-term drying will alter the trajectory of decomposition. For example, drier conditions have been shown to favor the encroachment of woody plant communities in peatlands, which may result in changes in decomposition rates.

Changes in the fungal community associated with different plant functional groups (ectomycorrhizal trees, Ericaceae, sedges) are particularly important in mediating changes in decomposition, yet our understanding of how these different fungal groups influence decomposition in situ is rudimentary. The overarching goal of this project is to understand the countervailing effects of woody plant encroachment and long-term drainage on aerobic and anaerobic decomposition in peatlands.

Experiments will test two key hypotheses: i) tree encroachment will increase decomposition in drained peatlands as a function of the extracellular enzyme suite of the ectomycorrhizal fungal (EcMF) community; and ii) divergent fungal decomposer pathways and drainage histories will generate peat with differing capacity for donating and accepting electrons under anaerobic conditions. A three-way full factorial experiment will use large, intact peat pedons in a climate controlled mesocosm facility, manipulating peat drainage history, water table position, and tree presence.

This will be paralleled by a field experiment in which tree root access will be manipulated over a drainage gradient. The mesocosm approach is the key to this study, so that drainage history and water tables can be manipulated, thus disentangling short- and long-term impacts of changing hydrology. The field experiment will anchor results of the mesocosm manipulations with conditions in the natural environment.

Through this and detailed characterization of fungal community functional changes and consequent effects on oxidative enzymes, decomposition, peat chemistry, and "redox pumping" in peat, the project will provide mechanistic insight into the long-term stability of peat in response to altered hydrology. Undergraduate and graduate student training will be integral to the research goals, and the students will help communicate results to high school students and teachers.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.