NORMATIVA DE APLICACIÓN Gestión de residuos:

Cumulative CO2 budgets estimated for the three sites with permanent (semi-natural) fen vegetation (EF-LN,

AF-LN and AF-HN) suggest these sites are all functioning as strong net sinks for CO2, with the full uncertainty

range for cumulative NEE falling below zero in all sites and years except for the (merged) 2013-14 year at EF- LN (Figure 4.2, Table 4.1). As noted in Section 2.1.1.3, this period included a period of severe water-table drawdown, during the peak of which (in July 2013) the site switched from a normal pattern of summer net CO2 uptake to net CO2 emission. Although GPP was higher in the drought-affected 2013-14 measurement

year than in 2015, ER was higher by a greater amount (Table 4.1), presumably in part because exposure of a considerable depth (around 70 cm, Figure 2.5) of peat to aerobic conditions caused an increase in peat decomposition rates. As a result, NEE for the 2013-14 measurement year (-55 g C m-2 _yr-1_{) was much lower}

than NEE for the more hydrologically stable 2015 (-183 g C m-2 _yr-1_{). Taking a simple mean of the two one-}

year periods would suggest a mean NEE for EF-LN of -119 g C m-2 _yr-1_{. However, water-table data from the}

project period suggests that the summer of 2013 was hydrologically extreme, and therefore unlikely to be representative of long-term mean uptake. On the other hand, the hydrologically isolated condition of Wicken Fen, which is now elevated above the surrounding subsidence-affected landscape, protected by bunds and supplied with water from adjacent rivers, suggest that it is more vulnerable to water-table drawdown than sites that remain hydrologically connected to the surrounding landscape, including the AF and NB sites. Data from the nearby Bakers Fen suggest that severe water-table drawdown events occurred three times from 2007 to 2015, i.e. one year in three. Weighting the data from EF-LN on this basis suggests a mean NEE of -140 g C m-2 _yr-1_{. In this respect it is also worth noting that previous (incomplete) years of eddy}

covariance measurements at EF-LN in 2009 and 2010 (both relatively dry years) showed evidence of net CO2

release in 2009 and only a weak sink in 2010 (R. Morrison, unpublished data) further supporting the conclusion that the measured CO2 sink in 2015 was larger than average.

Of the two AF fen sites, AF-HN functioned as a strong CO2 sink in both full measurement years (-176 and -139

g C m-2 _yr-1 _{in 2014 and 2015 respectively), giving a site mean NEE of -158 g C m}-2 _yr-1_{. This is very similar to}

the value derived for EF-LN above, although it is worth noting that average GPP for the two full years at AF- HN was approximately 25% lower than at EF-LN, and that ER was lower by a similar amount. This may be due to generally higher temperatures and solar radiation levels in East Anglia compared to Anglesey (Table 2.2). It is possible that productivity at EF-LN is enhanced by inputs of nutrient-enriched river water to the fen, although clear differences in peat nutrient status between the sites were not evident (Table 2.2). Given that the vegetation characteristics of AF-HN and EF-LN (tall Phragmites australis and Cladium mariscus) were rather similar, whereas the vegetation at AF-LN (short fen species) was different, we infer that the vegetation community present may be a better predictor of its CO2 balance than nutrient status, which was

used a priori to classify the natural fen sites.

Compared to the tall fen AF-HN, the short fen AF-LN site functioned as a weaker CO2 sink (-87 g C m-2 yr-1)

during the common 2015 measurement year. Given the very strong similarities in the temporal pattern of both gross and net CO2 fluxes during this period (Figure 4.1) it seems reasonable to conclude that this

represents a genuine contrast in the long-term rates at which the two adjacent sites sequester CO2. Since the

2015 ER values for the two sites were very similar (Table 4.1) differences in CO2 sequestration appear to be

largely attributable to the greater productivity of the tall fen species at AF-HN (the presence of which presumably reflects the nutrient status of the site) compared to the slower-growing short fen species at AF- LN. This difference arose despite AF-LN acting as a net sink for a longer period (161 days in total in 2015) compared to AF-HN (150 days).

Given that the area within the measurement footprint of all three fen flux sites was subject to minimal management during the study period (limited to low-intensity conservation grazing by ponies at the Anglesey Fen site), biomass offtake was assumed to be negligible. Therefore the mean NEE values obtained for each site can be considered to equate to the net ecosystem productivity (NEP) of the site. Over longer periods, however, it is reasonable to assume that biomass removal will occur (at least at the EF-LN site, which is subject to periodic reed cutting) which would have the effect of removing some of the carbon sequestered over the preceding period. On that basis, the C balance estimates obtained for these sites may somewhat over-estimate the true long-term rate of CO2 sequestration by conservation-managed fens.

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Figure 4.2. Cumulative annual CO2 fluxes for eddy covariance flux measurement sites for full calendar years. Note the different y-axis scaling on different subplots. Sites have been grouped in rows by vegetation type.

Note that the upper left panel shows cumulative CO2 at EF-LN from 16th _{July 2013 and 9}th _{July 2014. The 2014}

and 2013 periods have been reversed in the figure in order to show the seasonal development of cumulative fluxes from January to December, for consistency with the other plots. Consequently the plot does now show a ‘true’ year (and has therefore been plotted separately from the continuous 2015 measurement period, upper right panel) although the overall cumulative flux is correct for the July 2013 – July 2014 period.

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