Full site carbon and GHG budgets were constructed based on the best available data for each location, as follows:
1) CO2 fluxes were based on eddy covariance NEE data wherever these were available, using the mean
of full year fluxes as shown in Table 4.1 (for EF-LN a weighted mean was used, as described in Section 4.1). Any lateral carbon removal in harvested biomass at agricultural sites was included in the CO2 balance (Table 4.1).
2) Where eddy covariance data were unavailable, CO2 fluxes were taken directly from modelled
chamber GPP and ER fluxes for conservation-managed sites (MM-RW, NB-LN, NB-HN, TM-RW), and from ER only for bare peat extraction sites (MM-EX, TM-EX). For the remaining agriculturally managed sites (MM-DA and SL-IG) a more complex approach was required (see below).
3) Terrestrial CH4 fluxes, and ditch CO2 and CH4 fluxes where available, were taken from static chamber
measurements where available (Table 4.2). At the re-wetted sites (MM-RW and TM-RW) fluxes from any remaining ditches were assumed to be the same as those for the terrestrial area. For the two extraction sites (MM-EX and TM-EX) and for the remaining arable site (MM-DA) we assumed that ditch fluxes were equal to the average measured flux at other arable sites.
4) Aquatic DOC fluxes were taken directly from the values given in Table 4.5, for all sites except those at TM. At these sites, fluxes were estimated (based on similar measured DOC concentrations at the TM and MM sites) by dividing measured DOC fluxes from the corresponding MM site by the water fluxes for that site (Table 4.4), then multiplying by the estimated annual runoff at Thorne Moors (see Section 2.6.1.3).
5) Dissolved CO2 fluxes were taken from Table 4.5 where available, or from the most analogous sites
where unavailable (MM-RW for TM-RW, MM-EX for TM-EX, mean of EF-DA and EF-SA for MM-DA). 6) Dissolved CH4 and POC fluxes were taken from Table 4.5 where available, and assumed to be zero
otherwise. DIC fluxes were not included in the calculation for the reasons given in Section 4.3.2.3. For the two agriculturally managed sites without flux towers, it was not possible to take data from the static chambers directly, because GPP could not be reliably modelled from environmental variables alone. We therefore used data from comparable sites with flux towers to derive a best estimate of the net ecosystem CO2 balance including biomass offtake (net ecosystem productivity, NEP).
MM-DA: To estimate the NEE of this site, we first adjusted the chamber-based estimate of ER (1151 g C m-2
yr-1) based on the ratio of eddy covariance to chamber-derived ER at the East Anglian arable sites. This was
necessary because, although the chamber- and eddy covariance-based models of ER were very similar for most of the measurement period, the chamber-based models failed to capture the increase in autotrophic respiration that occurred during periods of rapid crop growth. At EF-DA and EF-SA the ratios of EC to chamber-derived ER were almost identical at 1.21 and 1.23 respectively, therefore a ratio of 1.22 was applied to the data from MM-DA, giving an adjusted ER of 1405 g C m-2 yr-1. Secondly, we calculated the
ratios of NEP to ER for each measurement of the six flux measurement years at EF-DA and EF-SA, which gave a mean of 46% and a range of 30% to 69%. This is effectively an estimate of the ratio of heterotrophic
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respiration (i.e. CO2 loss) to total respiration (incorporating autotrophic plant respiration), comparable to the
approach used in the ‘gain-loss’ method of CO2 accounting for organic soils (IPCC, 2014). Applying this ratio
to the adjusted ER value for MM-DA gives an estimated NEP of 647 g C m-2 yr-1. Assuming a similar rate of
biomass offtake in the wheat crop at MM-DA to that at EF-SA (635 g C m-2 yr-1), would suggest that NEE at
MM-DA is close to zero.
SL-IG: For this managed grassland site, the same procedure as above was applied, using the nearby SL-EG as the reference flux tower site (EF-EG was not used because, in addition to being much more distant, it is also dissimilar in terms of both peat properties and the absence of biomass harvesting). Based on the SL-EG data, the ratio of eddy covariance to chamber-derived ER was 1.33, and the mean ratio of NEP to ER for the three years of flux measurements was 12% (range 9% to 14%), suggesting that autotrophic rather than heterotrophic respiration dominates the total flux at this site. Applying these values to the chamber-based ER estimate of 2717 g C m-2 yr-1 gives an estimated NEP of 433 g C m-2 yr-1 for this intensive grassland site. If
we assume the same rate of biomass offtake at SL-IG as at SL-EG (203 g C m-2 yr-1) this would give an NEE for
the site of +230 g C m-2 yr-1. In reality it is possible that the grass harvest at SL-IG is higher than at SL-EG,
given the intensive management and fertilisation of the site, giving a smaller NEE. However, different assumptions about biomass removal rates would not affect the calculation of NEP, or therefore of the overall C or GHG balance of the site.
Final estimates of the carbon and GHG balances of all 15 study sites are shown in Tables 4.6 and 4.7. The results suggest that the net ecosystem carbon balance (NECB) of the sites ranges from -281 g C m-2 yr-1 to
+773 g C m-2 yr-1. The most important influence on NECB is clearly the combination of NEE and (where
present at agricultural sites) biomass offtake. However other fluxes have a significant influence on NECB at a subset of sites, notably CH4 emissions at conservation-managed fens and raised bogs, and DOC export at
raised bogs. The climate forcing impact of the sites also ranges from negative to positive, from a net GHG sink of 3.6 t CO2-eq ha-1 yr-1 to a net source of 28.5 t CO2-eq ha-1 yr-1 (Table 4.7) The GHG balance of the sites
is dominated by the sum of NEE and biomass removal at drained agricultural sites, and the balance of CO2
uptake versus CH4 emission at the conservation-managed and re-wetted sites.
Note that the GHG balances presented in Table 4.7 do not include N2O fluxes, because these were not
measured at all sites. However, the data collected as part of N2O study described in Section 3 gave tentative
annual fluxes from EF-DA and SL-IG of 5.72 and 8.49 kg N2O-N ha-1 yr-1 respectively which (based on a 100
year GWP of 298) would equate to an additional GHG emission of 5.36 and 7.95 t CO2-eq ha-1 yr-1
respectively. If included in the GHG balance below, this would increase emissions from SL-IG by 48%, and those from EF-DA by 18%. It seems reasonable to assume that N2O emissions from EF-SA would be similar to
those from EF-DA. However, at the other arable site included in the N2O study, MM-DA, emissions remained
very low even after fertiliser addition. At the remaining unfertilised sites, we assumed that N2O emissions
were negligible. Whilst we were not currently able to fully test the validity of this assumption, the ditch dissolved N2O data did provide some evidence of the potential for N2O production. At fertilised agricultural
sites (EF-SA, EF-DA, SL-IG) ditch N2O concentrations were periodically high, consistent with measured N2O
emissions from the peat surface at EF-DA and SL-IG. At conservation-managed sites with no evidence of agricultural nitrogen enrichment (e.g. based on peat core data), dissolved N2O concentrations were
consistently below ambient atmospheric concentrations, which suggests that (if anything) these sites could be acting as net N2O sinks. However at the two Wicken Fen sites (EF-LN and EF-EG), both of which are
periodically irrigated with nutrient-enriched river water, ditch N2O concentrations were periodically
elevated, which suggests that these sites could be acting as emission sources. Ditch fluxes at these sites have been investigated in further detail in Peacock et al. (2017).
The environmental factors controlling variations in GHG fluxes between sites are considered in Section 4.5, and the implications for UK lowland peat emission factors in Section 6.
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Table 4.6. Full estimated carbon balance of all study sites (all data in g C m-2 yr-1). Sites are listed in approximate order of land-use intensity.
Note that biomass offtakes in the form of extracted peat were not quantified for the active MM-EX extraction site (although no harvesting took place at the measurement location itself). Active extraction has ceased at TM-EX.
Table 4.7. Greenhouse gas balance of all study sites (all data in t CO2-eq ha-1 yr-1). Calculations are based on a 100 year global warming potential of 25 for CH4, a DOC to CO2 conversion rate of 0.9 and a POC to CO2 conversion rate of 0.7. Sites are listed in approximate order of land-use intensity. N2O fluxes were not measured at all sites, and are therefore omitted from the table.