PhD Abstract

Greenhouse gas emissions from rewetted peatland forests

Caitlin Rigney, University of Limerick, 2016

Natural peatlands are important sinks of carbon (C) and vital in the global C cycle. Despite covering just 3% of the earth’s land mass, they store as much C as all terrestrial biomass. Drainage for forestry alters the hydrology and chemical reactions in peatlands, converting them from sinks to sources of carbon dioxide (CO2) and nitrous oxide (N2O), while reducing methane (CH4) emissions.

Rewetting is considered an important tool in climate change mitigation and is utilized in addition to other management tools such as Sphagnum introduction to return the C sink function of peatlands and re- establish peat forming conditions in degraded peatlands.  

The first aim of this study was to investigate the controls on CO2, CH4 and N2O dynamics in two rewetted former peatland forest sites in Ireland; one blanket peatland eight years after rewetting (Pollagoona) and one raised peatland three years after rewetting (Scohaboy), produce annual greenhouse gas (GHG) balances for both peatlands and compare them with natural and forested systems.

The second aim was to compare the chemical and physical properties of natural, drained and rewetted peatlands in order to assess the effect of both drainage and subsequent rewetting on peatland properties.  Gas fluxes were measured using the chamber method. Micro sites comprising the dominant vegetation at the study site were established and gas balances produced for one year. Although interannual variablility influences GHG emissions from peatlands, results from this study represent fluxes in a year without extreme climatic episodes.

In this study, both sites acted as CO2 and CH4 sources. Although Pollagoona was an overall CO2-C source (131.6 g CO2-C m-2 yr-1), one microsite acted as a strong C sink (-142.84 g CO2-C m-2 yr-1). Methane emissions were small, totalling 2.94 ± 1.03 g CH4-C m-2 yr-1. Molinia caerulea plots were both the greatest CO2-C (168.4 g CO2-C m-2 yr-1) and CH4-C (2.53±1.01 g CH4-C m-2 yr-1) sources. Pollagoona experienced N2O uptake (-11.78 µg m-2yr-1) due to the behaviour of one microsite during the study period Scohaboy acted as a large CO2-C (585.3 g CO2-C m-2 yr-1) source in all microsites despite re- vegetation of non- brash plots due to the availability of fresh organic matter across the site.

Scohaboy was also a CH4 source emitting 3.25 ± 0.58 g CH4-C m-2 yr-1. Both CO2-C (819.31 g CO2-C m-2 yr-1) and CH4-C (4.76 ±0.98 g CH4-C m-2 yr-1) emissions were highest from the brash plots. Annual nitrous oxide losses were small over the study period (72 µg m-2yr-1). Pollagoona displays little variability in elemental composition between land use type, while significant differences were observed in C and N content between land uses in Scohaboy. Unexpectedly N content of the peat did not increase with depth in four of the six peat cores analysed and this is reflected in their C:N ratio.

All cores contained both labile and recalcitrant OM. Refractory OM was found in forestry samples from both sites and Scohaboy natural. Comparisons between land use types indicate that drainage and subsequent rewetting alter the properties analysed in this study.