Primary productivity calibration resulted in significant (p<0.05) fits between measured and modelled values (Table 3.4)
graze, no burn graze, 10 yr graze, 20 yr no burn, no graze no graze, 10 yr, no graze, 20 yr α -0.000835 -0.000900 -0.001301 -0.001704 -0.000928 -0.001203 GPmax -0.394330 -0.365593 -0.217564 -0.127970 -0.437923 -0.362194 r2 0.43 0.29 0.20 0.18 0.12 0.19
Table 3.4. α and GPmax values used in modelling and r2 of fit with
measured values.
Taking these values to be representative of the sites during the study period, primary productivity varied between -137.8 and -222.5 gC m-2 yr-1, which is within the range of reported values for upland peat (Worrall et al., 2009a).
3.4.5 Rainfall DOC
Rainfall DOC concentration varied from 0 – 5.4 mgC l-1 over the study period and are similar to values presented in other studies (Worrall et al., 2003a). Inputs from rainwater DOC varied over the three years from -0.9 to -2.1 gC m-2 yr-1 which is a similar to ranges reported elsewhere (Worrall et al.,
2007a; Worrall et al., 2009a). In a study of global precipitation input of DOC, Willey et al. (2000) estimated an input of 0.4 * 109 MgC yr-1, of which 70% fell on land. This is equivalent to an input to land of -1.88 gC m-2 yr-1.
As the soil concentrations of DOC are used in the carbon budget of Hard Hill, rainfall DOC is not needed but is included here for completeness.
3.4.6 Methane
The values of CH4 flux from the plots ranged from 5.25 to 6.86 gC m-2 yr-1.
This range is similar to that reported in Worrall et al. (2009a). In one of the few studies on methane from UK peats, Macdonald et al. (1998) report CH4
3.4.7 Carbon budget
Table 3.5 details the different carbon pathways, measured or estimated during this study, and the estimates for the carbon flux for each year of the study period.
2005 no burn graze, graze, 10 yr graze, 20 yr no graze no burn, no graze, 10 yr, no graze, 20 yr
PP -188.39 -189.85 -173.77 -140.41 -209.25 -219.89 ER 200.44 167.30 136.58 203.95 191.76 209.87 DOC 53.44 50.12 48.17 49.25 50.46 48.21 POC 41.63 27.75 17.56 14.01 22.93 46.19 CH4 5.58 6.51 6.86 5.81 6.18 6.53 DissCO2 2.47 1.85 1.79 2.35 1.99 1.98 Total 115.16 63.68 37.18 134.97 64.06 92.89
2006 no burn graze, graze, 10 yr graze, 20 yr no graze no burn, no graze, 10 yr, no graze, 20 yr
PP -191.81 -192.90 -174.51 -139.82 -213.05 -222.52 ER 188.43 177.76 146.69 176.28 224.43 258.68 DOC 67.16 73.39 66.14 66.23 63.30 75.26 POC 53.95 35.96 22.33 18.15 29.16 61.58 CH4 5.25 6.11 6.49 5.52 5.79 6.17 DissCO2 3.57 3.17 2.91 3.35 2.72 3.02 Total 126.56 103.48 70.06 129.72 112.35 182.18
2007 no burn graze, graze, 10 yr graze, 20 yr no graze no burn, no graze, 10 yr, no graze, 20 yr
PP -186.67 -187.96 -171.15 -137.76 -207.35 -217.33 ER 202.63 172.91 141.40 202.50 200.95 221.27 DOC 63.90 68.16 79.64 66.46 75.95 74.19 POC 53.31 35.54 22.07 17.94 28.82 60.85 CH4 5.46 6.36 6.73 5.71 6.03 6.40 DissCO2 1.28 1.72 1.91 2.17 1.77 1.80 Total 139.92 96.72 80.59 157.02 106.18 147.19 Mean total budget 127.21 87.96 62.61 140.57 94.20 140.75 Standard Deviation 12.39 21.30 22.64 14.49 26.28 44.99
Table 3.5. Summary of each carbon uptake and release pathway for each year (2005-2007) for measured and modelled values.
Examining the data shows that all the treatments are net sources of carbon, 37.2 – 182 gC m-2 yr-1, during the study period, though some sites are smaller sources than others (Table 3.5). Over the study period, unburnt sites were, on average, a source of 133.89 gC m-2 yr-1 compared to a source of 91.1 gC m-2 yr-1 and 101.7 gC m-2 yr-1on the 10-year and 20-year plots respectively. Figure 3.5 shows the data in an alternative format with the ranges for carbon flux for each treatment across the study period.
0 20 40 60 80 100 120 140 160 180 200
graze, no burn graze, 10 yr graze, 20 yr no burn, no
graze
no graze, 10 yr, no graze, 20 yr
Treatment Ca rbon b udge t, gC m -2 yr -1 Maximum Mean Minimum
Figure 3.5. Range of carbon budget for each treatment
If only gaseous emissions (ecosystem respiration, primary productivity, and methane) are considered, then grazed and burnt plots are sinks of carbon throughout the period and ungrazed, burnt plots are occasionally sinks of
is included the sites become a source of carbon. The DOC flux in this study is at the higher end of reported values (Worrall et al., 2009a) and this study shows an upward trend over the period (Figure 3.6) suggesting that this may be one possible reason for why the carbon budgets indicate that the plots are carbon sources.
30 40 50 60 70 80 90 2005 2006 2007 Year DO C fl u x, g C m -2 yr -1 graze, no burn graze, 10 yr graze, 20 yr no burn, no graze no graze, 10 yr, no graze, 20 yr
Figure 3.6. DOC flux for each treatment over the study period
To assess any significant differences between treatments, ANOVA was carried out with Year, Burning regime and Grazing regime as factors and post-hoc testing was carried out to investigate where the significant differences lay. Burning and grazing regimes, along with year, were
Factor df p ω 2 Burn 2 0.014 0.23 Grazing 1 0.008 0.19 Year 2 0.017 0.21 Burn*Grazing 2 0.021 0.18 Burn*Year 4 0.226 0.04 Grazing*Year 2 0.642 0.00 Error 4 0.14
Table 3.6. ANOVA of the total carbon budgets. Values of p< 0.05 are
highlighted and ω 2 = proportion of variance explained.
Inter-annual variation accounted for 21% of the variation in the data with 2006 and 2007 being significantly greater sources than in 2005. Grazing explained 19% of the variation with grazed sites having significantly lower sources than ungrazed sites. Finally, burning accounted for the largest proportion of the variance, 23%. Here, the presence of burning rather than a specific regime led to significantly smaller sources. The interaction term between burning and grazing was also significant explaining 18% of the variation in the data.
3.5 Discussion
This study has shown that for some small-scale plots under different
opposite in sign to other studies based on Moor House (Worrall et al., 2003a; Worrall et al., 2009a) which poses the question, why?
It is likely that the management of the plots plays a significant role in contributing to the nature of the carbon budget. This is the first study to calculate total carbon budgets for upland management combinations and previous studies only considered the Trout Beck catchment as a whole. Results from ANOVA show that burning and grazing are significant factors in the carbon budgets. Burnt sites i.e. 10-year and 20-year plots show
significantly lower overall budgets than unburnt plots. The main reason for this difference is the combined effect of higher primary productivity on some burnt sites and lower ecosystem respiration which leads to a negative gaseous carbon balance. This, in turn, reduces the losses seen in the hydrological carbon budget. These higher rates of primary productivity are likely to be due to higher photosynthetic rates found in younger vegetation. Johnson and Knapp (1993) found higher photosynthetic rates along with increased above ground biomass production, inflorescence density and plant height in annually burnt sites. As the vegetation becomes older and
degenerate, its ability to sequester CO2 reduces. Bond-Lamberty et al.
(2004) show, for a fire-prone forest chronosequence in Canada, that net primary productivity in young stands were net sources, middle aged stands were net sinks whilst the oldest stands were carbon neutral. However, it
loss of carbon during combustion of the biomass could outweigh any
benefits to the carbon budgets. In Chapter 5, in a survey of a wildfire, up to 90% of biomass of was lost during combustion and in managed burns this loss may be approximately 60% (Allen, 1964).
On the whole, the largest sources in this study were the unburnt sites, in particular the unburnt, ungrazed, sites; however ungrazed, 20 year plots are also large source due in part to large POC concentrations from these sites. On these sites there are very high respiration values and the lowest primary productivity values. The low primary productivity could be driven by the lower rates of CO2 uptake in older vegetation as described previously. The
higher ecosystem respiration could be due in part to the position of the water table on these plots. The deepest water tables are found on the unburnt plots (Chapter 2) allowing a greater depth of aerobic decomposition leading to greater respiration values (Moore et al., 1998).
Grazing also plays a significant role in the carbon budgets of these sites. The effect of grazing is similar to burning in that new vegetation growth is encouraged leading to negative NEE and lower sources. Increased CO2
exchange efficiency has been observed on grazed prairie grasslands and has been linked to the presence of young, highly photosynthetic leaves (Owensby et al., 2006).
Another possibility for the source-sink discrepancy is that the hydrological export of carbon has changed since the study period of Worrall et al. (2009a). If considering only gaseous exchange, then the site is net sink of carbon but when other pathways are included the site is a net source. This is a similar situation to Worrall et al. (2007a) that while the site overall was estimated to be a net source of 11.2 to 20.9 gC m-2 yr-1 the gaseous
components were a net sink.
Previous studies (Worrall et al., 2007a; Worrall et al., 2009a) have scaled results from Moor House to the larger UK scale. By taking the extent of peat in the UK to be between 14,790 km2 (Tallis and Meade, 1998) and 29,209
km2 (Milne and Brown, 1997), estimates of the area of UK peat were randomly selected from this range and combined with randomly a selected carbon budget from the ranges in this study. This is repeated 100 times for each treatment. This results in an estimate of the carbon budget of the UK under each management regime of between 1.27 ± 0.38 Tg yr-1 and 3.21 ± 0.6 Tg yr-1 (Figure 3.7). These ranges are larger than results presented in Worrall et al. (2007a).
0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50
graze, no burn graze, 10 yr graze, 20 yr no burn, no graze no graze, 10 yr, no graze, 20 yr
Management U K C ar bon B udge t, Tg yr -1
Figure 3.7. UK carbon budgets based on management regimes from this study
3.6 Conclusion
By using the best combination of experimental and modelling approaches, the treatments at Hard Hill are shown to be net sources of carbon of between 37.2 and 182 gC m-2 yr-1. However, the presence of burning and grazing appears to limit the magnitude of this source by reducing ecosystem respiration and enhancing net ecosystem exchange. When considering the carbon budgets of upland peat managed by fire, the loss of biomass and carbon through combustion must be considered in order to assess where carbon benefits lie.
exchange (137 to 259 gC m-2 yr-1). Hydrological export of carbon via DOC is
the next largest component of the carbon budget (48 to 80 gC m-2 yr-1) and turns the sites, which are gaseous sinks of carbon, into an overall source of carbon. By extrapolating these ranges across the UK, the carbon budget of UK peat soils would be a net source of up to 3.2 TgC yr-1
Chapter 4:
Loss and transformation of carbon from vegetation
4.1 Introduction
It is estimated that the burning of biomass releases between 2 and 6 x 1015 gC yr-1 (Crutzen and Andreae, 1990; Wittenberg et al., 1998). This burning of biomass leads to production of black carbon (or char) on the scale of 270 x 1012 gC yr-1 (Kuhlbusch and Crutzen, 1995). Black carbon has been considered refractory in the environment and thus represents a input of carbon (Kuhlbusch, 1998). These additional inputs of carbon have been ignored in the carbon budgets of fire-affected peatlands; neither Garnett et al. (2000) nor Harden et al. (2000) consider char inputs, and both show that fire limits the magnitude of sink.
A common management technique used on peat soils of the UK uplands is the burning of vegetation on a regular cycle. The burning is undertaken to promote new vigorous vegetation growth that provides improved forage for sheep grazing. Furthermore, the rotational burning practice over cycles from five to 25 years means that a patchwork of vegetation is created that
provides both forage and cover for the ground-nesting red grouse. Presently the UK Government Department of Environment, Food and Rural Affairs (DEFRA) restricts managed burns in terms of timing, frequency and size of burnt area (DEFRA, 2007a). The timing restrictions on burning are to ensure
it takes place while the ground and vegetation are sufficiently moist to ensure a “cool burn” and thus reduce damage to the underlying peat. The size of burnt areas is restricted to no more than 30m wide by 150 m long in order to limit the possibility of runaway wildfire, but this width of burn strips coincides with preferred foraging distance of the red grouse.
In a recent review of the consequences of heather and grassland burning, including that on peat (DEFRA, 2005; Tucker, 2003), there were very few studies reported that examined the consequences of burning for carbon storage. Worrall et al. (2007d) has shown significant differences between burning regimes in soil water conductivity, pH and the depth to the water table. Chapter 2 found no significant difference in dissolved organic carbon (DOC) concentrations for a year either side of a managed burn and for the same site showed a significantly higher water table with increased frequency of burning. Ward et al. (2007)have shown significant increases in gross ecosystem CO2 fluxes in burned and grazed treatments relative to the
control plots. Intentional and catastrophic burns have been linked to
increased peat erosion and therefore mass losses of carbon and particulate organic carbon (POC) (Mackay and Tallis, 1996).
Garnett et al. (2000) examined peat accumulation of carbon under three treatments (grazed/unburnt, grazed/burnt, and ungrazed/unburnt) using
production of soot particles with changes in industrialisation in the region. Peat accumulation and carbon accumulation were calculated above this common horizon. Garnett et al. (2000) report a mean difference between burnt and unburnt treatments of 2.3 kg m-2 . This difference represents a reduced carbon accumulation with burning of 73 Mg C km-2 yr-1. This study only covers a 10-year burning frequency. Other studies have considered the role of wildfire rather than managed burning on the accumulation of peat. Kuhry (1994) suggested reduced peat accumulation in Boreal forests due to natural wildfires. However, these studies are not carbon accumulation studies; rather they are peat accumulation studies, i.e. they do not consider the presence of different carbon types.
In managed burning, the biomass is turned into fumes, smoke and char. Fumes and smoke represent the airborne fraction of the combustible
biomass with the former representing the vapours from burning and the latter representing the airborne solid and liquid particulates; char is the solid
material that remains following burning. Some biomass remains as unburnt but possibly dead material and therefore represents an additional litter input. The fumes and smoke represent a loss of carbon from the ecosystem and the loss of live vegetation also means the loss of litter production in years subsequent to the burn until there is full recovery of the vegetation. In opposition to the losses, the production of char and of dead biomass litter represents an input of carbon into the peat. This means that at the time of
the fire litter input is substituted for char inputs. Litter is a high-volume, low- carbon content, labile organic matter relative to char that is a low-volume, high-carbon content material. Char has mean residence times of up to 10,000 years in soils (Swift, 2001) while typical turnover times of soil organic matter in soil surface layers is between 6 and 20 years (Torn et al., 2005). The question becomes whether sufficient refractory carbon and/or sufficient dead biomass can be produced during a fire that can offset the loss of biomass by burning and the subsequent loss of litter production.
This chapter seeks to understand the impact of burning of vegetation on loss of carbon to the atmosphere and the production of char products.
Laboratory studies of soil carbon loss and transformation have been
conducted by a number of workers (e.g. Almendros et al., 2003), and studies of experimental burns have occurred in the field (e.g. Fearnside et al., 2001). However, no study has measured the loss and conversion of biomass in the laboratory in order to inform the estimation of the carbon budgets of fires.
4.2 Chapter Objectives
The objectives of this chapter are to:
• Conduct experimental burning in laboratory conditions to investigating the loss of biomass and production of char;
• Investigate the carbon storage of peat under different burning regimes.