It has been estimated that between 20-30% of the global terrestrial carbon is being held in northern boreal sub-arctic peatlands, storing 455 Pg of C (Gorham, 1991). Since the last glacial period the average rate of peat accumulation was calculated by Gorham (1991) to be 0.096 Pg/yr with the current rate estimated to be 0.076 Pg/yr.
Milne and Brown (1997) calculated that there are 29,209 km2 of peatland in the UK (12%) which is unique in its character compared to the rest of global peatland in that it has been extensively managed to make to useful for agricultural and sporting uses. This peatland contains about 3000 million tonnes of carbon (Cannell et al., 1993). (Stewart and Lance, 1991) estimate that 52% (1.5 million ha out of a total of 2.9 million ha) of the UK's peatland has been drained at some point. The effect of drainage is to lower the water table in order to make the land more productive.
Pristine (that has not been disturbed or altered) peat, in the study by (Hargreaves et al., 2003), has a Net Ecosystem Exchange (NEE) of 25 tonnes C km-2 yr-1 and is therefore a net sink of carbon. As soon as the peat has been drained it then becomes a net source with a NEE of around 300 tonnes C km-2 yr-, however after re-colonisation of the peatland occurs it returns to a net sink with a high NEE. This is down to, in part, by afforestation which accounts for about 9% of all drained peatlands (Cannell et al., 1993). Changes to peat inputs and outputs can also affect Dissolved Organic Carbon (DOC) concentrations(Worrall et al., 2007a) and (Particulate Organic Carbon) POC loss (Worrall et al., 2009) in the fluvial systems of the catchment area.
The rate of carbon accumulation in peatlands is strongly affected by the climate. During the last 200 the rate of accumulation has remained relatively stable at 0.076 Pg/yr (Clymo, 1984). At this current rate the UK peatlands are currently a store of carbon. However, due to the implications of climate change, alterations of weather patterns and human activity they could rapidly become a net source of carbon to the atmosphere.
To understand, model and predict the future carbon budget of UK peat it is important to understand fully its carbon cycle. As in any system it is complicated and extensive but by careful consideration of all its factors it is possible to create a model which can be expanded to a wide area and be used to investigate the effect of different management processes on the ability of the peatland to store carbon.
The model used in this paper is based closely on the Sustainable Uplands RELU project, the Durham Carbon Model and work by Worrall et al (2009). There are various inputs and outputs of the carbon cycle that we need to consider as a whole in the model.
The inputs that the model considers are the amount of CO2 that is introduced to the system by primary productivity and also the amount of CO2 that is introduced by wet and dry deposition either through water vapour or incorporated with particulates. When considering outputs both atmospheric and fluvial outputs must be taken into account. Atmospheric outputs can be assumed to be as the emission of CO2 by respiration of the soil. Methane is also emitted, however, in very small quantities (Hope et al., 2001) but it is a more powerful greenhouse gas (by a power of 20) so must not be completely discounted. Fluvial outputs which are to be calculated in the model are DOC and POC release and dissolved CO2 (Worrall et al., 2009).
To identify each survey site Wales was split up into 1km2 areas. This was then compared to the Hydrology of Soil Types (HOST) classification (Boorman et al., 1995) to identify the 1km2 grid squares which contained significant (10%) quantities of peat. Once the appropriate grid squares had been located and their grid references noted, a model was created to automate altitude look-up (APPENDIX). Following this aerial photos of each of the survey site were viewed (www.getmapping.com) and a survey conducted to assess the following factors: whether burning had occurred, presence of grazing, drainage present, % of bare soil, % of peat burnt and if there was forestry present.
These collected values were then input into the Durham Carbon Model (Worrall et al., 2007b) which compiles many different models in order to try and get as complete a model as possible.
There are several assumptions and models that have to be made and incorporated in order to produce a complete result from our model which is set out below.
Prediction to water table
As this study is based on peat locations throughout the whole of Wales this has several implications when adapting the model used by Worrall et al. (2007b) for use in our model. Worrall's model used depths based on observations at the Environmental Change Network (ECN) site at Moor House, Upper Teesdale using an empirical water balance approach (Worrall et al., 2004) which was the water balance between potential evapotranspiration and rainfall and a depth calculated. This was then calibrated and run for and found to be 70% efficient. Although no method exists of estimating water depth for each 1km2 areas in Wales the only solution was to use the depths for Moor House which has been done for the purpose of this study.
Prediction of catchment runoff
As with prediction of water table depth as Wales is such a large area in comparison compared to the Moor House catchment area. For the purpose of this study I have used the same model as in Worrall et al. (2009) acknowledging the significance of scaling up data from a small area to such a large one. The model that Worrall et al. (2009) uses relates monthly rainfall to monthly runoff and which allowed for seasonal variation, storm effects and lag relationships. It was found to be 68% efficient.
Prediction of primary productivity
To estimate the contribution of primary productivity to the model the approach devised by Bubier et al., (1998) was used. This uses the connection between incident photosynthetically active radiation (PAR) and primary production. It can be shown using this hyperbola:
Soil respiration of CO2
The amount that soil respires can be estimated using the equation by Lloyd and Taylor (1994) which was based on the Arrhenius equation used by Raich and Schlesinger (1992). As in the primary productivity the data has been taken from the Moor House site.
To calculate the amount of dissolved CO2 that would be released the approach used in this model has been adapted from (2005). It uses values from the soil respiration on CO2 (see 2.4), depth to the water table (see 2.1) and soil temperature. The flux is then calculated by using a simple mixing model and the result is input into the model.
Worrall et al. (2003) have show that there is a significant relationship between the water table depth and CH4 flux by using methane measurements from around the UK (Beswick et al., 1998; Chapman and Thurlow, 1998; Choularton et al., 1995; Clymo et al., 1995; Daulat and Clymo, 1998; Fowler et al., 1995; Gallagher et al., 1994; Hargreaves and Fowler, 1998; Hughes et al., 1999; Lloyd et al., 1998; MacDonald et al., 1998; Worrall et al., 2003). For this study the results from Worrall et al. (2009) were used as none were available for the study area.
Dissolved Organic Carbon (DOC)
To calculate the DOC flux for this study the model uses the method adopted by Lumsden et al. (2005) and then modified as precipitation chemistry (which is needed for Lumsden et al.'s approach) is very rarely available. Therefore, the approach that was taken in this instance was to gather an empirical relationship between pH and soil water ionic strength which could then be used to predict the solubility of soil DOC. This could then be calibrated against Bleaklow Plateau and therefore arrive at a soil concentration of DOC. Using runoff data (see 2.2) the flux of DOC could then be calculated. This figure is for the concentration of DOC as it leaves the peat not the concentration that could be found further downstream. This method has shown to be effective in studies (Worrall et al., 2006) as there is no need to include rainfall and no calculations are needed to estimate DOC loss through the river flow process.
Particulate Organic Carbon (POC)
Evans and Warburton (2007) derived an empirical approach to modelling POC flux using the percentage of bare peat on the study area, the surface recession, peat density and the carbon content. This data is only available for the Peak District so the data from Worrall et al. (2009) has been used in this study.
Wet and dry deposition
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