Both the Goyt and Bleaklow localities included a range of sites. Each site consisted of a collection of replicate monitoring apparatus (plots); with six plots per site, divided equality into a duplicate set (three nested plots) (30 secs - minutes walking distance). The plots within a set were positioned ~2-3 meters apart (Figure 2.5). Plots were composed of: a gas collar and dip well (at minimum). Additional water sampling traps were installed in the Goyt study.
2.3.1.1 Water table depth and water sampling
Each plot was instrumented with a dipwell (Figure 2.6.a) made of 1m long, 5cm diameter PVC pipes. The pipes had ~0.5 cm diameter holes drilled at ~10cm intervals running down the length of the pipes. The top and base of the pipes were left open. The dipwells were inserted perpendicularly to the ground, leaving ~20cm of pipe above ground. The holes allowed soil pore water passage along a pressure gradient from the saturated peat vertically and horizontally into the dipwell. The dipwells enabled the measurement of the water table depth (WTD) and the collection of a soil pore water sample at WTD. The WTD is the level between the inside of the dipwell and peat soil once the pressure is at equilibrium. A tape meter was used to measure depth the WTD. When the water table was deep a conductivity probe was used as the visibility in the dipwell was poor.
53 Figure 2.5: Site monitoring plots layout: a) a set/ nest of triplicate plots (half of a study sites set of plots); b) second set (Bleaklow 2011).
Figure 2.6: i) cross section diagram of sets of equipment diplaying instumentaion relative to the peat surface and the water table. ii) Example an experimental plot. The Lettering on both i) and ii) represent the following: a) dipwell b) runoff trap (present at the Goyt Valley study sites) c) gas collar d) gas chamber e) Infra-red gas analyser (IRGA).
b
c
e
a
d
i)
ii)
a)
~10 m
b)
54 The conductivity probe was comprised of a hollow 1.5 m pole (< 5 cm diameter), with a 1 m tape measure fixed lengthways and a simple open electrical circuit. At the top of the pole is a compartment containing batteries connected to resistors and an LED. The connecting wires are extended through the pole to create anodes at its base. When the conductivity pole is placed into the dipwell the anodes make contact with the soil pore water, the conductive soil pore water allows the completion of the circuit, thus lighting the LED and allowing the detection of the water table, and a reading is noted. The offset (height of dipwell from the soil surface) is deducted from the measurement to give the WTD (Equation 2.1).
𝑊𝑇𝐷 = 𝐼𝑛𝑠𝑖𝑑𝑒 𝑑𝑖𝑝𝑤𝑒𝑙𝑙 𝑊𝑇 − 𝐷𝑖𝑝𝑤𝑒𝑙𝑙 𝑜𝑓𝑓𝑠𝑒𝑡 Equation 2.1
A dip-probe was used to collect soil pore water samples from the dipwell after WTD was measured. The same depth is therefore variable as it is determined by the WTD. The sampling probe is composed of a bamboo cane and a 30ml steralin (sealable, polycarbonate tubes) attached at one end. Once collected, samples were poured into, and stored in, 30ml steralin. The samples were labelled with the date of collection, site name, plot number and sample type.
2.3.1.2 Gaseous carbon fluxes: (CO2)
To measure the effects of treatment on CO2 gas flux, monitoring must be conducted infield.
Measurements can be made in either a steady state (open) or non-steady state (closed) chamber (Kutzbach et al. 2007). The closed chamber method is frequently used to measure
the net CO2 exchange between the atmosphere and low saturated canopies typical of
peatlands (Nykänen et al. 2003, Rowson et al. 2013, Sottocornola et al. 2007, Worrall et al. 2011). This method was chosen for the research as it allows the assessment to be conducted
55 over short time intervals (minutes long) (Kim and Henry 2013), and so it is both time and cost effective. Importantly, the method is simple to operate in remote, difficult to reach areas such as upland peatlands (Kutzbach et al. 2007), and enables measurement relative to a range of covariates (e.g. Water table depth and photosynthetically active radiation (PAR).
At each of the plots (six per site) plastic collars were installed (Figure 2.6.C). At the
collars, measurements were taken of CO2 gas concentrations in order to calculate the flux of C
to and from peat surface. Measurements were made using a dynamic, closed chamber method, with an infrared gas analyser (IRGA) (EGM-4, PP-Systems, Hitchin, UK) connected to a 20 cm tall by 15 cm diameter acrylic closed chamber (CPY-2, PP-Systems, Hitchin, UK) as per Rowson et al. (2010) and Dixon et al. (2013). The chamber was placed onto the collar where
the IRGA measures the concentrations of CO2 (in ppm) within the chamber. Over a period of
two minutes the IRGA took measurements at intervals of 4 seconds over a period of 2 minutes.
A gas flux was then calculated using a linear regression of CO2 concentration over time (g CO2
m-2 h-1) (Rowson 2007). Gas observations were made prior to any sample collection to
minimise the impact the observers’ presence on the flux.
Separate readings were taken from each collar, per site, per monthly visit. These
include: ecosystem respiration (Reco) (measured in the dark), and a net ecosystem exchange
(NEE) (measured in light). Absence of light is simulated by using a close-fitting u-PVC sleeve,
which prevents the passage of all PAR into the chamber. The Reco was measured first as it
reduced the greenhouse warming effect on the ecosystem within the collar, minimising the impact of the presence of the chamber upon the following reading taken on that collar for NEE. The difference between these two readings (Equation 2.2) was used to derive gross
photosynthesis (Pg) as it is not directly measured. By convention a flux into the peat is given a
56
𝑃𝑔 = 𝑁𝐸𝐸 − 𝑅𝑒𝑐𝑜
Equation 2.2
Using sensor probes inside the chamber, environmental variables of air temperature (K) and
PAR (μmol m-2 s-1) were recorded concurrently with the measurement of CO2.