Water quality
We will measure soil water in peat and streams at the flow weirs to determine basic quality indicators such as pH, DOC, POC. The peat and stream water colour will be monitored and will reveal linkages between management, vegetation, weather conditions and observed export of carbon via DOC and POC. We are using rhizon samplers to collect peat soil water (see Clark et al., 2012) and sample stream water at the flow weirs (which are located at the sub-catchment outlets).
Importantly, we did not find any difference in peat DOC between burnt mown and uncut plots. In fact, DOC declined over time, which could be linked partly to an increase in both pH and temperature. However, we did find increase DOC in mown catchments over time during the post-management period, clearly linking to carbon released during decomposition from the brash layer.
Different water coloration reflecting different export rates of DOC and POC
Fire causes air pollution but can also impact on water quality
However, many other plant and soil processes impact on stream water quality
Water sample DOC values (ppm or mg per L) across the three sites before vs. after treatment started (March 2013) for PLOTS
Water sample DOC values (ppm or mg per L) across the three sites before vs. after treatment started (March 2013) for SLOPES
Stream Carbon: average (mean) DOC concentration during pre (2012/13) and post (2013-2020) period for the three sites for each sub-catchment (burnt vs mown)
Stream Carbon: average (mean) POC concentration during pre (2012/13) and post (2013-2020) period for the three sites for each sub-catchment (burnt vs mown)
Total annual flow C export: as DOC & POC per site and for each sub-catchment (burnt vs mown) during the pre- (2012/13) period and the post-management period (2013-2020). Important to note is that whilst the burnt catchments show overall higher DOC stream C export than mown catchments, a similar difference was already observed during the pre-management period. However, at Mossdale mowing seems to have reduced the overall DOC C export (compared to the difference in the pre-management year 2012/13.) For POC there was no real change between pre- and post-management periods. Moreover, what is also important is the concentration of both DOC and POC in the water - both showed an increase for mown compared to burnt streams, particularly when considering pre-management differences. - which will feed into the reservoirs and relates to possible eutrophication and water treatment costs. Coincidentally, this increase in concentration of DOC and POC also relates to the reduced flow volume in mown compared to burnt catchments (see water balance).
Monthly POC export (top) and DOC export (bottom) in stream flow based on flow weir samples (per ha) at all three sites during pre-treatment and post treatment periods (ie after mowing and burning early in 2013) - missing values due to drought (ie no flow) or pending analyses. There was no significant difference between burnt and mown catchments, but between sites.
Table outlining the total annual POC and DOC carbon export in streams at each site (average of burnt & mown as there was no difference) for either the overall average (AVG) or the pre- and post-management periods.
Below: Water sample pH values across the three sites in either flow or peat pore water samples increased over time (left) in both, flow (top) and plot (bottom) samples, with considerably higher values over the post- than during the pre-management period, which could partly be linked to an overall increase in temperature (with a very warm year in 2014; right):
Above: Monthly water quality indicators in stream flow at all three sites per treatment (C = burnt; T = mown) during pre- and post-management periods (i.e. after mowing and burning in early 2013) - missing values due to drought or pending analyses. Indicators are (from left to right): Au/m (true colour at UV 400nm); Hazen (Au/m *12); E4/E6 (ratio of UV 465nm/665nm).
Below: The overall time series (left) and the seasonal averages (right) for the flow samples (including UV254 and corresponding SUVA values; UV254/DOC) revealed unusually low UV254 (and correspondingly SUVA) values in 2013 and overall showed highest values in Hazen and UV254 in summer and for E4/E6 in winter months.
Finally, the present study (when using sub-catchment averages) also showed no significant difference for DOC but indicated a possible opposite effect, with a (so far non-significant) negative relationship of DOC concentrations from the plots (but not on slopes) with heather and a positive relationship with sedge (see below):
Above: Water quality averages (per sub-catchment) in 2016 (post-management) for plots (left) and slope locations (right) at the three sites measured as DOC (top), Hazen, (middle) and SUVA (bottom) versus the average cover (%) of either heather (Calluna vulgaris; purple) or sedge (Eriophorum spp.; green) at the locations. Note the slope locations were not managed as part of the experiment and were a mix of heather regrowth stages on previously burnt areas within the managed sub-catchments. Linear regression equations (non-significant) are provided for each graph (same colour code as for vegetation). Linear regressions for the individual plot-level management showed a significantly positive SUVA correlation with Sphagnum cover (p< 0.01) and a negative one with Calluna cover (p< 0.05) and on individual slope locations a significant (p< 0.001) positive correlation with Sphagnum and Eriophorum (sedge) cover, but correlation coefficients were very low.
Above: Water sample SUVA values in 2016 (post-management) as the mean per management for plots (left) and the individual slope locations (right) at the three sites against heather (purple), Sphagnum (orange) or sedge (green) species cover at the sample locations. Note different axes scales, and slope locations were not experimentally managed and were a mix of heather re-growth stages on previously burnt areas within the managed sub-catchments. Linear regression equations (same colour code as for vegetation) are significant (p< 0.001).