What’s in a peat bog?
And, more importantly, how do the communities of plants and microbes you find in a peat bog affect how it functions? This was the overarching question that I sought to answer during my PhD, which I carried out at the Centre for Ecology and Hydrology, Lancaster Environment Centre, and The James Hutton Institute, between Autumn 2008 and Autumn 2012.
Over four years, including three field seasons, I got to know the peat bog at Moor House National Nature Reserve very well indeed. With help from many others, I surveyed, probed, cored and measured many different aspects of the bog, including the plants on the surface, the microbes beneath, and the resulting exchange of greenhouse gases. 419 individual vegetation surveys, 1088 soil samples, and 3700 greenhouse gas samples later, I felt I’d done a reasonable job of mapping out some of the important factors that make a peat bog function as it does.
Peat bogs are areas where the soil consists of a thick layer (greater than 30 cm) of organic material. They exist because wet conditions limit the amount of oxygen, needed by many soil microbes to decompose plant material, available below ground, slowing the decomposition of plant material. As plants die and the amount of dead material outweighs the rate at which it can be decomposed, it begins to accumulate, forming thick deposits over thousands of years. As plants photosynthesise and grow, they remove carbon dioxide (CO2) from the atmosphere, storing most of the carbon in their tissues. Much of this carbon remains locked up in plant tissues after they die and are gradually buried in the peat bog. Globally, peat bogs represent an important store of carbon that, if it were released as CO2 or methane (CH4), could cause climate warming to intensify.
Unfortunately, peat bogs are an ecosystem under threat: the cold bogs of the high latitudes are vulnerable to erosion when plants are removed by burning or overgrazing, whereas tropical peat bogs are deforested and drained to be converted into agricultural land. We need to understand how these pressures might affect peat bogs, and the carbon stored in them, in the future, if we are to work out how peat bogs could affect climate change.
My PhD field site is a blanket bog, a particular type of bog that develops in hilly terrain. Because they develop on slopes, blanket bogs are particularly vulnerable to erosion. The blanket bog at Moor House NNR can be divided into three particular landforms: eroding areas, patches of bare peat within which just a few sedges grow; gullies, sloping features, thickly covered with sedges, grasses, Sphagnum and Polytrichum mosses; and moorland, larger areas dominated by shrubby heather and considered to represent ‘intact’ bog. I aimed to investigate the differences in plant communities, soil microbes, and greenhouse gas emissions between each bog landform, to make some predictions about how the overall functioning of the blanket bog might change if, for example, eroding areas expanded.
What did I find out? Gullies, as suggested by previous research, are important hotspots for CH4 emissions, along with eroding areas. These two landforms are responsible for most of the CH4 emissions from the peat bog, despite only making up a small proportion of its area. CH4 is a more powerful greenhouse gas than CO2 – although it doesn’t stay in the atmosphere as long, its ability to contribute to global warming by trapping solar radiation is 25 times that of CO2. I sampled greenhouse gas emissions from the three landforms over a period of one year and found that, rather than absorbing CO2 from the atmosphere, eroding areas and gullies became sources of carbon during the Winter. Expansion of eroding areas and gullies in blanket peat bogs could result in a loss of carbon and a reduction in their ability to act as carbon stores, potentially contributing to climate change.
Below ground, the microbial communities in the peat suggest that, in gullies, the greater abundance of sedges and grasses contributes to a greater abundance of bacterial decomposers, relative to fungi. Fungi have extra tools to enable them to deal with the complex compounds found in many peat bog plants, while bacteria are more suited to the decomposition of simpler material. Bacteria are also associated with faster decomposition and cycling of compounds, whereas the fungal energy channel tends to be slower. Sedges and grasses are known to produce material that is more enriched with nitrogen than the heather that dominates intact peat bog. The greater abundance of bacteria in gullies is likely to contribute to more rapid cycling of carbon in this landform: the sedges and grasses may absorb CO2 more rapidly than the heather, but the bacterial energy channel could be returning that carbon to the atmosphere just as rapidly, as both CO2 and CH4.
So what? Well, in north-west Europe, it looks as though we’re likely to experience warmer, wetter Winters as the climate warms. These conditions are likely to stimulate the activity of the microbial communities responsible for CH4 emissions from gullies, enhancing the amount of carbon they emit as greenhouse gases during the Winter. Heavier rainfall, combined with existing pressures on blanket peat bogs, could also lead to an expansion of eroding areas and gullies. The above predictions could result in a decline in the ability of blanket bogs to function as carbon stores, causing them to reslease their stored carbon to the atmosphere, contributing to global warming. Understanding how the distribution of blanket peat bogs and their landforms could change in the future will be vital for predicting their impact on the Earth’s climate.