Above: Corresponding mean monthly Light Compensation Point (LCP) values based on the Light Response Curves (LRC) models (solving to find the light (PAR) required to obtain a '0' C balance or NEE flux - for daytime period only) during 2012-2014. Note the overall seasonal patterns, inter annual differences, but particularly the generally decline in the order of Nidderdale (green) > Whitendale (purple) > Mossdale (blue). Note: a lower LCP implies greater C uptake.

Below: Interestingly, the differences in LCP seem to be more important in explaining differences in net C uptake (C budgets) than respiration as Reco was quite similar across sites (and was reduced by mowing and burning, see graphs below). Corresponding mean monthly ecosystem respiration (Reco) values based on the Light Response Curves (LRC) models for the three sites during 2012-2015 and for the different treatments (uncut "do nothing" = dn; burning = bt; mowing = mn):

Below Left: Measured ecosystem respiration (Reco) values based on the daytime dark chamber fluxes from NEE plots for all three sites (2012-2014) and for the various treatments (DN="do nothing"; C=burnt; M=mown; Br=brash removal; Sp=Sphagnum addition). DN(new) are newly selected (uncut - tall heather) plots next to the initial DN plots (i.e. at the time of management the first set of heather had been cut in all plots (also including the heather on DN plots) to determine shoot biomass, leaf area and leaf chemical composition).

Below Right: Measured total soil respiration (with roots) values from the uncut SR collars (i.e. including roots) for all three sites during 2012-2014 and for the different treatments (DN="do nothing"; C=burnt; M=mown; Br=brash removal; Sp=Sphagnum addition).

Above: The individual carbon flux components could be combined to provide continuous daily sums of the major carbon fluxes (NEE, GPP, Reco) based on light response curves using climate parameter regressions (see Appendix 6 in the Defra report: Heinemeyer et al., 2019), predicted net primary productivity (NPP) and respiration flux components (above ground: Rab, total soil: Rs, root: Rr and decomposition: Rh) obtained from modelled hourly net ecosystem exchange (NEE) fluxes. Data show the uncut averages together with monthly carbon use efficiencies (CUE) (NPP/GPP) for Nidderdale (left), Mossdale (middle) and Whitendale (right).

Below: The individual carbon flux components for the uncut plots based on the above C flux figures in a summary table showing annual totals and averages.

The light response curve model allowed predicting monthly and thus annual C budgets per site (uncut, below left) and comparing managements (uncut vs burnt & mown, below right). The sites show clear differences as previous (Mossdale > Whitendale > Nidderdale) and, importantly, recovery of net C uptake is faster on burnt than on mown plots, particularly at Nidderdale (the site with minimal heather beetle damage):

The annual carbon flux balance (NEE) values were added up to derive cumulative C budgets, which was combined further with emissions from management (fuel used for burning and mowing management), and for burning biomass combustion and charcoal production (below table). Notably, burning showed a tendency to, over time, become a stronger C sink than mowing, after an initial higher C release. However, whilst Nidderdale (the driest site with fastest heather regrowth) showed a stronger overall C sink than mown plots, losses from biomass combustion still caused a substantial C source at Mossdale and Whitendale. The key seems to be to monitor an entire management rotation to be able to identify a possible optimum rotation length (e.g. for C storage) in relation to environmental site conditions:

Overall net ecosystem carbon balance (NECB) was calculated by combining NEE with fluvial C fluxes (DOC & POC) and methane emissions (below using the median flux). This was done per site (left; combining uncut and managed scenarios including the areas managed) and per individual management (right; assuming the same fluvial export for all as only partly burnt and mown catchment streams were measured). However, there is considerable uncertainty as to the inclusion of fluvial C fluxes in the NECB; notably, a considerable amount of the DOC export has been shown to be many years old whilst the CO2 emissions from rivers can be very old carbon, distorting the meaning of adding DOC export to the annual NECB in relation to the current situation (Billett et al., 2015): "Inclusion of the evasion flux term in the NECB would be justified if evaded CO2 and CH4 were isotopically 'young' and derived from a 'within-ecosystem' source, such as peat or in-stream processing of DOC. Derivation from 'old' biogenic or geogenic sources would indicate a separate origin and age of C fixation, disconnected from the ecosystem accumulation rate that the NECB definition implies". Moreover, it is unclear how degradation at river conditions (light & temperatures) could have altered observed degradation measured at on land under air light and temperature conditions (Moody & Worrall, 2017), considering river temperatures can be different (warmer & cooler at the low and high range, respectively) and light penetration might be very different due to overall particle absorbance in a river from all angles. Therefore, the below tables show NECB values for both, including or excluding fluvial C losses (DOC & POC).

Finally, we did not consider the additional inputs of C via rainfall, as reported by Worrall et al. (2003) these are likely to be about 1.1 g C m-2 yr-1 for DIC and 3.1 g C m-2 yr-2 for DOC inputs. So, in addition to the uncertain losses of DOC and POC via fluvial export, about 4-5 g C should be added to the C sink (or subtracted from a C source).

Finally, the annual C budget data were correlated (below) with key environmental parameters (light, temperature and precipitation; left) and hydrological conditions (water table; right). This identified key parameters such as PAR (light) and precipitation but also a mean annual water table depth thresholds for switching from a net C sink to a net C source (for NEE about -12 cm & for NECB budgets about -8 cm):