Beneath the rolling green fields of Ireland, a hidden agri-environmental disaster is taking place. Over 300,000 hectares of peata grasslands [1], which includes some of our best pasture lands, are slowly disappearing into thin air. Unfortunately, the drainage that was used to radically improve their agricultural productivity, is also the reason for their rapid decline. This is because when peat is drained, it comes in contact with air and starts oxidizingb and as peat is almost entirely composed of organic matter, this process leaves very little behind.

As a result of oxidation, about 1 to 2 cm of soil is being permanently lost from drained peat fields every year. Moreover, as the oxidation turns carbon (organic matter) to CO2, Irish agricultural peatlands are estimated to release a shockingly high 3-9 million tons annually [2,3]. This amounts to 5-15% of Ireland’s national greenhouse gas budget or equivalent to the entire emissions from the waste, industry or transport sectors. If this situation remains unchanged, then many of these soils will be lost before the turn of the century, but only after having made a considerable contribution to the climate crisis.

While farmers and private landowners own about three-quarters of our peatlands [4], farmers own all of Ireland’s peat pastures. Given the centrality of pastures in our culture and economy it is self-evident that any resolution of this issue must necessarily involve a range of effective options and solutions for farmers and the full variety of farms that they manage.

a Peat is the surface organic layer of a soil that consists of partially decomposed organic matter, derived mostly from plant material, which has accumulated under conditions of waterlogging, oxygen deficiency, high acidity and nutrient deficiency.
b Oxidizing is reacting with oxygen. When peat or organic matter becomes oxidized, carbon dioxide is produced.

Context

To give some context, peat soils in Ireland first started forming in shallow lakes at the end of the last Ice Age, transitioning through nutrient-rich fen to raised bogs, which rose up above the old surface level of the lake to escape the influence of typically nutrient-rich groundwater. Consequently, some peatlands represent an accumulation of 10,000 years of partially-degraded plant material. Historically, these soils were of little agricultural use as the ombrotrophic naturec of raised bogs with their nutrient-poor organic matter meant that nutrient availability was low and grazing poor. In addition, these waterlogged soils are up to 95% water and offer little solid support for livestock therefore, apart from very extensive grazing, the only immediate way to use these bogs was to drain them and cut the peat for fuel (see Fig. 1).
c Ombrotrophic: dependent on rain (or other types of precipitation) for its nutrients, very low in nutrients.
Figure 1. Issues with farming intact peatlands.
Following extensive cutting of the acidic bog peat, the more fertile and less acidic fen peat is exposed at the surface, after having been buried for millennia. When waterlogged, these fen peat soils likewise suffer from low bearing capacity and low nutrient availabilityd, but these properties can be dramatically improved by drainage. As water seeps out from between the fibres of partially decayed organic matter, the soil compacts and becomes firm enough for cattle and machinery (see Fig. 2). Moreover, centuries worth of accumulated plant material, essentially a kind of compost, starts oxidizing and releases its nutrients, fuelling plant growth. Such drained peat soils can perform extremely well, being some of the most productive agricultural soils to be found in Europe. For example, the drained fen peats of East Anglia provide a large proportion of the UK’s salad crops and one-third of England’s fresh vegetables with a value of £3 billion a year [5].
d While fen peat is considered fertile, the nutrients are only freed from the organic matter after it starts oxidizing.
Figure 2. Reclamation of raised bog to peat pasture.

However, the high value afforded by this ‘dryland’ management of natural wetlands comes with a high agricultural and environmental price tag: the peat, a non-renewable resource of ‘fossil’ nutrients and carbon, is wasting away at a rate of 1-2 cm per year. As healthy peatlands only accumulate at about 1 mm of peat per year, this means that it is being lost at 10-20 times the rate of accumulation. Hence, the high agricultural productivity of drained peatlands derives from over decades’ worth of accumulated plant growth being used up each year. The inevitable result of this process can already be seen in some fields, where the once thick and rich peat has been reduced to a thin layer of sticky black earth overlying the impermeable subsoil (see Fig. 3b).

Several centimetres of peat lost corresponds to an  annual emission rate of 20 to 30 tonnes per hectare resulting in estimated emissions of 3-9 million tonnes of CO2 per year, around 5-15% of Ireland’s total greenhouse gas (GHG) emissions. Furthermore, drained peatlands release a substantial amount of carbon to waters as dissolved organic carbon (DOC), contributing to declines in water quality. Finally, for deep drained peat in particular, more nutrients are released by oxidation than can be used by grass or other crops. These unused nutrients (mainly nitrates) are washed in to streams and rivers, representing an ongoing loss of a valuable, non-renewable resource and substantial pollution burden on our waterways (see Fig. 3a).

Figure 3a. Peat degradation caused by drainage, leading to carbon emissions and and nutrient loss (left panel).
Figure 3b. Thin layers of wasted peat soil overlying lack subsoil on participating Farm Carbon sites (right panel).
The effects of peat drainage are not, of course, just limited to agricultural fields. At a landscape level, in areas with a high proportion of drained peat fields, the water table in non-exploited peatlands is likewise lowered, transforming them from carbon sinks to carbon sources. In these heavily drained areas, the intact peatlands at the centre dry out and lose their critical cover of Sphagnum mosses and other bog plant species, thus allowing for the invasion of trees (birch, pine and willow) from the bog edges. This hugely increases evapotranspiratione, desiccating and degrading the bog even more. As a result, these former havens for wildlife, which regulate water flow into the surrounding lands, lose their capacity not just for carbon storage but for flood attenuation and drought mitigation. Thus, the interplay between agricultural and environmental issues is clear and will only grow more apparent as the impact of climate change increases.

e Evapotranspiration: the process by which water is transferred from the land to the atmosphere by evaporation from the soil and other surfaces and by transpiration from plants.

The Solutions

Fortunately, many of the environmental problems caused by drainage are, to a large extent, reversible. Although it would take thousands of years for most harvested peatlands to regrow to their original thickness, CO2 emissions drop back down to zero almost as soon as the water table is restored to ground level. Unfortunately, restoring the water table up to or just below the surface, also reverses many of the agricultural benefits that drainage has brought, reducing the grazing periodf by re-saturating the peat that cannot support cattle or machinery year-round, and with decreased nutrient availability (see Fig. 4).

f Grazing period: The season and number of days during which a pasture is grazed. Grazing in rewetted peat grassland or natural wet meadows is possible after long dry spells, generally from June to September, when the water table temporarily drops and the soil becomes firm enough.

Figure 4. Pros and cons of raising the water table in peat pastures.

Thus, in all likelihood, full rewetting of peat grasslands, that is, bringing water up to the field surface, will imply a certain decrease in productivity for farmers. If, however, some of these ‘avoided’ greenhouse gas emissions can be calculated and, for example, made available as carbon credits, then financial compensation for the farmer could be obtained for this loss in productivity. This then raises the question as to how credits can be allocated for changes in water table management. As part of its work on the Farm Carbon EIP, Green Restoration Ireland (GRI) has developed a method as the basis for an accurate estimation of greenhouse gas emissions to support the implementation of mitigation measures in a way that rewards farmers both as business people and as stewards of the landscape (see Fig. 5)

Figure 5. Schematic of GRI model for farmer compensation for raising the water table.

For peat grasslands at least, where the vegetation bears no relation to the original peatland habitat of bog or fen, GRI’s approach is aligned with the Dutch ‘Paying for Peat’ standard [6].  This is based on the strong correlation between the mean annual groundwater level and CO2; emissions from peatlands, a position which is supported by scientific research from around the globe [7].

Set against this, the small quantity of annual emissions data from studies on Irish peat grasslands (carried out by gas chambers)g appeared to be an exception to the rule, exhibiting broadly similar emissions for deep and shallow drained sites [8,9]. However, when plotted together with the peat grassland data from the UK [7] and the Netherlands [10], two other countries with similar oceanic climates, the Irish data in fact ‘fits’ the overall trend (see Fig. 6).

Statistical analysis of the correlation further shows that the Irish data fall well within the standard deviation of the correlation. Analysis of the combined dataset (UK, Dutch and Irish data) indicates that for every 10 cm increase in the water table, annual CO2 emissions are reduced by approximately 5 tonnes per hectare (see Fig 7).

g Gas chambers are considered to be subject to large inaccuracies of up to 150-200%.
Figure 6. The correlation between ground water table and CO2 emissions for peat pastures.
Sources: Evans et al. (2021) [7], Renou-Wilson et al. (2014, 2016) [8,9], Schothorst (1982) [10] .
Figure 7. Quantified avoided CO2 emissions achieved by raising the water table in 10 cm increments.

Conversely, while increasing the ground water level decreases CO2 emissions, they also lead to the production of small amounts of methane, a greenhouse gas that is 27 times more potent than CO2 [11]. However, scientific studies unequivocally show that the large reduction in CO2 emissions achieved by raising the water table in drained peat soils far outweighs the small increase in methane emissions.

For the current Farm Carbon EIP, GRI uses the above correlation (see Fig. 6), in combination with a correlation for methane (see Appendix 1), as a fair basis for estimation of potential carbon credits. This system has important advantages when compared to the broad drainage categories used in the IPCC guidelines [12] or the Greenhouse-Gas-Emissions-Site-Types (GEST)h employed either by the UK Peatland Code [13] or the Germany’s MoorFutures Standard [14].

For example, raising the water table from 60 cm to 40 cm below the surface would not change a field’s drainage status, according to the standard ‘deep vs shallow-drained’ categories used in the IPCC guidelines. A water table of -60 cm and -40 cm are both considered to be ‘deep-drained’ so the same emission factor applies and no carbon credits would be awarded under the IPCC rules.

In reality of course, raising the water table by 20 cm, whether it be from −60 cm to −40 cm or from −30 cm to −10 cm, reduces annual CO2 emissions by almost 10 tonnes per hectare. When extrapolated to the full acreage of Irish peat grassland, a modest 20 cm increase in water level would equate to between 1 and 3 million tons of CO2 mitigation, or between 2 and 5% of Irelands’ total current annual GHG emissions!

Hence, an inherent problem of broad drainage categories is that certain small management changes, which can nevertheless yield considerable environmental benefits, may be disregarded. In contrast, any change in drainage status could qualify for compensation using a straightforward correlation of groundwater level to GHG emissions and potential carbon credits. The latter thus allows for a much higher level of flexibility in water table management, giving farmers the freedom to regulate drainage levels according to their own views on what is appropriate for their farms and livelihood. This steady-paced process also demonstrates first-hand to farmers how a regenerative environmental process can slow the loss of their peat soils.

h Greenhouse Gas Emission Site Types is an indirect technique to quantifying GHG emissions. According to this approach, it is possible to assess and monitor GHG fluxes from peatlands by measuring water table levels together with the vegetation forms identified on the sites. The GEST approach provides estimates of the balance of greenhouse gases based on the forecasts of vegetation dynamics and of water conditions.
Figure 8. Factors to be considered by farmer when raising the water level.

Initial results of this approach have been very promising, and our active collaboration with participating farmers has demonstrated its value. While the word ‘rewetting’ is used by some to falsely conjure up images of productive fields turned to lakes, farmers are understandably reluctant to undertake actions that will have a profound effect on farm management, with or without financial compensation.

In contrast, farmers in the Farm Carbon EIP have the option to incrementally raise the water tables in their fields by means of adjustable dams, giving them full agency in terms of management of their water tables. This has been instrumental in achieving the high level of farmer collaboration in the project’s central aim of rewetting, namely a stepwise approach to compensation for avoided GHG emissions that clears the way for a stepwise approach to raising water tables. Thus, with full engagement and inclusion in the decision-making process, the project has taken the ‘fear factor’ out of the rewetting ‘boogie man’. Perhaps the next key objective needs to be replacement of the term ‘rewetting’ with the more accurate one of ‘water table management’!

Of course, there are still issues to be work out. Groundwater tables and drain levels are not the same and thus ground water level monitoring is necessary to justify payments. Currently in our project, this is done via dip-wells installed and monitored in collaboration with the REWET project [15], but in future, financing must be found to pay for dip-wells or other monitoring systems as well as for data verification. Moreover, the effect of, for example, drain blocking on the ground water level across a field is not straightforward and will depend on various factors including historical use, vegetation, weather, soil, subsoil and the hydrology of neighbouring fields. Raising the water table all the way may not always be possible, and the ground water table is expected to vary with distance from ditches and as a result of unevenness of the surface. Nevertheless, all evidence suggests that every centimetre increase in groundwater level counts so any water level increase in peat soils, however small, should be incentivised.

Future Potential

If we turn our eyes to the Netherlands, we can find some insight, both to see what we could be dealing with, and how we might deal with it. There, drainage and the associated shrinkage and degradation of peat has already resulted in large areas of highly productive pastures lying well below sea level.

With their first-hand experience of the ground disappearing from beneath their feet, Dutch farmers of peat pastures are understandably worried about the reality of soil subsidence and they have been trying to strike a balance between drainage and productivity for decades, if not centuries. Their long experience has demonstrated that when a water table of 40 cm below the surface is maintained, it is ‘business as usual‘, as farm management is barely affected and high yields can be maintained which is of particular importance for the Netherlands as one of the world’s biggest food producers. 

Many Irish peat grasslands are drained to a depth of −60 cm and deeper, so ‘going Dutch’ (see Fig. 9) and raising the water table to −40 cm would represent a major step forward in terms of climate mitigation, which could be taken right now without affecting current farming practices in any significant way.

In contrast to the use of drainage status categories [11], the GRI Farm Carbon approach allows farmers to be compensated for any improvements. As such, we believe that this will be an important tool to help Ireland achieve national targets for climate action, restoration of biodiversity and water quality.

Figure 9. Climate benefits for 'business-as-usual' rewetting of Ireland's peat grasslands.
Dr. Bastiaan Molleman
– Science Officer & Research Analyst, Farm Carbon EIP
BSc, MSc, PhD
Dr. Douglas McMillan
– Project Manager, Farm Carbon EIP
BSc, CEnv, MIEMA, MSB, MIBI, MAEE, Grad IOSH, Fellow IITD, FSL
 

Appendix - I. Methane Emissions

The anoxic conditions associated with a high water table lead to the production of small amounts of methane (CH4), a greenhouse gas that has a global warming potential that is 27 times stronger than CO2 [16]. Thus, although methane emissions are relatively low, the greenhouse potential can be significant. Methane emissions for drained and rewetted peat grasslands are plotted in Figure A1 against the mean annual water table, showing a strong correlation. A sigmoid function is used to describe the data, with a coefficient of correlation of R2 = 0.93.
Figure 10. Correlation between methane emissions and mean annual ground water table for drained and rewetted peat grassland. NB: Dutch methane emissions are much higher than Irish and UK emissions, possibly as a result of artificially high seasonal fluctuation of water levels or long-term intensive cultivation, and are therefore not included in the correlation. Data from: Renou-Wilson et al. (2014 [9] and 2016 [10]); Evans et al. (2016)[16] , Brown (2017) [17], Van den Pol-Van Dasselaar. (1999) [18], and Schrier-Uijl et al. (2010) [19].
Additional factors have been reported to increase methane emissions, most notably the presence of aerenchymatous plantsi, which are thought to transport methane directly form the anoxic soil (where it’s produced) to the atmosphere, bypassing the oxic layer (where it may be broken down). Changes in vegetation upon raising the water table may thus change methane emissions. This is an unknown factor that will need to be closely monitored in rewetted fields. To calculate the full GHG emissions from a field based on the average annual water table, an estimate for emissions of methane and CO2 is calculated using the correlations presented here. Methane emissions, expressed as CO2 equivalents, are added to CO2 emission to find the combined GHG emissions at a given water table depth (see Figure A2). Using water table before and after, the greenhouse effect and the corresponding carbon credits is found for any change in water table management.
i Plants with an aerenchyma, soft porous tissue on the inside of the stems, which are used by plants growing in waterlogged conditions to transport oxygen the roots.
Figure 11. Combined greenhouse effect of water table management.

References

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