Richard Grayson

The impacts of prescribed moorland burning on water colour and dissolved organic carbon: A critical synthesis

Discolouration of natural surface waters due to the humic component of dissolved organic carbon (DOC) is a costly problem for water supply companies. This paper reviews what is known about the impacts of prescribed moorland vegetation burning on water colour. Relevant research has taken place at three scales: laboratory experiments on peat cores, plot scale sampling of soil waters and catchment scale sampling of stream waters. While laboratory studies suggest burning increases colour production, the evidence from catchment and plot studies is contradictory. Plot studies suggest colour production may decrease or remain unchanged following burning although there is evidence for some transient changes. Catchment studies suggest prescribed moorland burning causes stream water colour to increase, although in most cases the evidence is not clear cut since most studies could not clearly disentangle the effects of burning from those of vegetation cover. The differences in findings between plot and catchment studies may be explained by: i) the short-term nature of some studies which do not measure long-term response and recovery times to burning; ii) the lack of colour measurements from shallow soil depths which contribute more to streamflow than soil water from deeper in the peat; and iii) the possibility of hydrological interactions occurring between different experimental plots at some sites. Additionally, the increase in recent patch burning in some catchments that has been statistically attributed by some authors to increases in stream water colour cannot be reconciled with theoretical calculations. When dilution with waters derived from other parts of the catchment are taken into account, large values of colour have to be theoretically derived from those recently burnt areas that occupy a small proportion of the catchment area in order to balance the change in stream water colour observed in recent years. Therefore, much further process-based work is required to properly investigate whether prescribed vegetation burning is a direct driver of enhanced colour and DOC in upland streams, rivers and lakes.

Variable source and age of different forms of carbon released from natural peatland pipes

We used the carbon isotope composition (14C and δ13C) to measure the source and age of DOC, POC, dissolved CO2 and CH4 (δ13C only) released from three natural peat pipes and the downstream catchment outlet of a small peatland in northern England. Sampling under different hydrological extremes (high flows associated with storm events and low flows before or after storms) was used to explore variability in C sources as flow paths change over short periods of time. The δ13C composition of organic C differed (δ13C-DOC −28.6‰ to −27.6‰; δ13C-POC −28.1‰ to −26.1‰) from that of the dissolved gases (δ13C-CO2 −20.5‰ to +1.1‰; δ13C-CH4 −67.7‰ to −42.0‰) and showed that C leaving the catchment was a mixture of shallow/deep pipe and non-pipe sources. The isotopic composition of the dissolved gases was more variable than DOC and POC, with individual pipes either showing 13C enrichment or depletion during a storm event. The 14C age of DOC was consistently modern at all sites; POC varied from modern to 653 years BP and evasion CO2 from modern to 996 years BP. Differences in the isotopic composition of evasion CO2 at pipe outlets do not explain the variability in δ13C and 14C at the catchment outlet and suggest that overland flow is likely to be an important source of CO2. Our results also show that the sources of CO2 and CH4 are significantly more variable and dynamic than DOC and POC and that natural pipes vent old, deep peat CO2 and POC (but not DOC) to the atmosphere.

Effects of fire on the hydrology, biogeochemistry, and ecology of peatland river systems

Peatlands are found around the world and cover ∼3.4% of the Earth’s surface. In the UK, peatlands cover 17.2% or ∼1.58 Mha of the land surface and occur mainly in upland areas covering the headwaters of most major British rivers. However, large areas are now subject to prescribed vegetation burning despite policy guidance that recommends a strong presumption against burning on deep blanket peat. Wildfires occur sporadically but are forecast to increase in frequency in the future. This paper provides a synthesis of current knowledge about how UK peatland-dominated river catchments respond to fires caused by prescribed vegetation burning and uncontrolled wildfire. We provide insight into the effects of fire on the hydrology, biogeochemistry, and biota of peatland river ecosystems, and the peatland-soil-driven controls on these effects at the catchment scale. Burning increases the depth to water table and water-table variability, although some small-scale studies indicate shallower water table in some places. More work is needed on fire effects on peatland river flow, but recent results suggest a complex response with smaller flow peaks for burned systems associated with most rainfall events, but enhanced peaks compared to unburned systems for the top quintile of rainfall events with the largest total rain. Evidence from biogeochemical studies suggests that fire leads to increased dissolved organic C concentrations in rivers. River biota responses primarily include significant reductions in the density of grazing mayflies but increases among detritivores including Chironomidae and Baetis mayflies. We provide a conceptual synthesis that links the main responses of terrestrial and aquatic systems to fire, and we summarize some major research gaps that should be prioritized to inform future policy around peatland management.

The impact of ditch blocking on the hydrological functioning of blanket peatlands

Ditch blocking in blanket peatlands is common as part of peatland restoration. The effects of ditch blocking on flow regimes and nearby water tables were examined in a field trial. After an initial 6-month monitoring period, eight ditches had peat dams installed 10 m apart along their entire length (dammed), four of these ditches were also partially infilled through bank reprofiling (reprofiled). Four ditches were left open with no dams or reprofiling (open). These 12 ditches and the surrounding peat were monitored for 4 more years. An initial five-fold reduction in discharge occurred in the dammed and the reprofiled ditches with the displaced water being diverted to overland flow and pathways away from the ditches. However, there was a gradual change over time in ditch flow regime in subsequent years, with the overall volume of water leaving the dammed and the reprofiled ditches increasing per unit of rainfall to around twice that which occurred in the first year after blocking. Hence, monitoring for greater than one year is important for understanding hydrological impacts of peatland restoration. Overland flow and flow in the upper ~4 cm of peat was common and occurred in the inter-ditch areas for over half of the time after ditch blocking. There was strong evidence that topographic boundaries of small ditch catchments, despite being defined using a high-resolution Light Detection And Ranging-based terrain model, were not always equivalent to actual catchment areas. Hence, caution is needed when upscaling area-based fluxes, such as aquatic carbon fluxes, from smaller scale studies including those using ditches and small streams. The effect of ditch blocking on local water tables was spatially highly variable but small overall (time-weighted mean effect <2 cm). Practitioners seeking to raise water tables through peatland restoration should first be informed either by prior measurement of water tables or by spatial modelling to show whether the peatland already has shallow water tables or whether there are locations that could potentially undergo large water-table recoveries.

Effects of rainfall, overland flow and their interactions on peatland interrill erosion processes: Overland flow and rainfall interactions with peatland erosion

Interrill erosion processes on gentle slopes are affected by mechanisms of raindrop impact, overland flow and their interaction. However, limited experimental work has been conducted to understand how important each of the mechanisms are and how they interact, in particular for peat soil. Laboratory simulation experiments were conducted on peat blocks under two slopes (2.5° and 7.5°) and three treatments: Rainfall, where rainfall with an intensity of 12 mm h−1 was simulated; Inflow, where upslope overland flow at a rate of 12 mm h−1 was applied; and Rainfall + Inflow which combined both Rainfall and Inflow. Overland flow, sediment loss and overland flow velocity data were collected and splash cups were used to measure the mass of sediment detached by raindrops. Raindrop impact was found to reduce overland flow by 10 to 13%, due to increased infiltration, and reduce erosion by 47% on average for both slope gradients. Raindrop impact also reduced flow velocity (80–92%) and increased roughness (72–78%). The interaction between rainfall and flow was found to significantly reduce sediment concentrations (73–85%). Slope gradient had only a minor effect on overland flow and sediment yield. Significantly higher flow velocities and sediment yields were observed under the Rainfall + Inflow treatment compared to the Rainfall treatment. On average, upslope inflow was found to increase erosion by 36%. These results indicate that overland flow and erosion processes on peat hillslopes are affected by upslope inflow. There was no significant relationship between interrill erosion and overland flow, whereas stream power had a strong relationship with erosion. These findings help improve our understanding of the importance of interrill erosion processes on peat.

Improved automation of dissolved organic carbon sampling for organic-rich surface waters

In-situ UV-Vis spectrophotometers offer the potential for improved estimates of dissolved organic carbon (DOC) fluxes for organic-rich systems such as peatlands because they are able to sample and log DOC proxies automatically through time at low cost. In turn, this could enable improved total carbon budget estimates for peatlands. The ability of such instruments to accurately measure DOC depends on a number of factors, not least of which is how absorbance measurements relate to DOC and the environmental conditions. Here we test the ability of a S::can Spectro::lyser™ for measuring DOC in peatland streams with routinely high DOC concentrations. Through analysis of the spectral response data collected by the instrument we have been able to accurately measure DOC up to 66 mg L(-1), which is more than double the original upper calibration limit for this particular instrument. A linear regression modelling approach resulted in an accuracy >95%. The greatest accuracy was achieved when absorbance values for several different wavelengths were used at the same time in the model. However, an accuracy >90% was achieved using absorbance values for a single wavelength to predict DOC concentration. Our calculations indicated that, for organic-rich systems, in-situ measurement with a scanning spectrophotometer can improve fluvial DOC flux estimates by 6 to 8% compared with traditional sampling methods. Thus, our techniques pave the way for improved long-term carbon budget calculations from organic-rich systems such as peatlands.

Effects of Needle Ice on Peat Erosion Processes During Overland Flow Events

Freeze‐thaw processes play a role in increasing erosion potential in upland areas, but their impact on overland flow hydraulics and fluvial erosion processes are not clearly established. We provide the first quantitative analysis demonstrating that needle ice production is a primary process contributing to upland peat erosion by enhancing peat erodibility during runoff events following thaw. To quantify the effects of needle ice on peat physical properties, overland flow hydraulics, and erosion processes, physical overland flow simulation experiments were conducted on highly frost‐susceptible blanket peat with and without needle ice processes. For each treatment, overland flow rates of 0.5, 1.0, and 2.0 L/min and slopes of 2.5° and 7.5° were applied. Peat erodibility, sediment concentration, and sediment yield were significantly increased in treatments subjected to needle ice processes. Median peat losses were nearly 6 times higher in peat blocks subject to needle ice processes than in peat blocks not subject to needle ice processes. Needle ice processes decreased mean overland flow velocities by 32–44% via increased hydraulic roughness and changes to surface microtopographic features, with microrills and headcut development. Needle ice processes increased the hydrodynamic force of shear stress by 55–85%. Erosion rates under needle ice processes exhibited a significant linear relationship with stream power. Our findings indicate that models of overland flow‐induced peat erosion would benefit from a winter component that properly accounts for the effects of needle ice processes on peat erodibility and erosion.

Water-level dynamics in natural and artificial pools in blanket peatlands

Perennial pools are common natural features of peatlands, and their hydrological functioning and turnover may be important for carbon fluxes, aquatic ecology, and downstream water quality. Peatland restoration methods such as ditch blocking result in many new pools. However, little is known about the hydrological function of either pool type. We monitored six natural and six artificial pools on a Scottish blanket peatland. Pool water levels were more variable in all seasons in artificial pools having greater water level increases and faster recession responses to storms than natural pools. Pools overflowed by a median of 9 and 54 times pool volume per year for natural and artificial pools, respectively, but this varied widely because some large pools had small upslope catchments and vice versa. Mean peat water-table depths were similar between natural and artificial pool sites but much more variable over time at the artificial pool site, possibly due to a lower bulk specific yield across this site. Pool levels and pool-level fluctuations were not the same as those of local water tables in the adjacent peat. Pool-level time series were much smoother, with more damped rainfall or recession responses than those for peat water tables. There were strong hydraulic gradients between the peat and pools, with absolute water tables often being 20–30 cm higher or lower than water levels in pools only 1–4 m away. However, as peat hydraulic conductivity was very low (median of 1.5 × 10−5 and 1.4 × 10−6 cm s−1 at 30 and 50 cm depths at the natural pool site), there was little deep subsurface flow interaction. We conclude that (a) for peat restoration projects, a larger total pool surface area is likely to result in smaller flood peaks downstream, at least during summer months, because peatland bulk specific yield will be greater; and (b) surface and near-surface connectivity during storm events and topographic context, rather than pool size alone, must be taken into account in future peatland pool and stream chemistry studies.