Sophie M. Green

A mesocosm study of the role of the sedge Eriophorum angustifolium in the efflux of methane—including that due to episodic ebullition—from peatlands

Background & AimVascular plants may reduce episodic ebullition losses of methane (CH4) from peatlands. They transport CH4 to the atmosphere, which may lead to a reduction in pore-water [CH4], bubble formation and release. This effect may be compounded by rhizospheric oxidation and associated methanotrophy. However, any reduction in pore-water [CH4] may be countered by root exudation (substrate for methanogens). The aim of this study was to determine how the presence of sedges affects CH4 emissions from peatlands.MethodsFive pairs of peat cores were collected from a raised bog. One of each pair contained Sphagnum cuspidatum and Eriophorum angustifolium (‘sedge’ cores); the other was dominated by S. cuspidatum (‘no-sedge’). From these the total CH4 efflux—including that due to episodic ebullition—were measured. A partial-shading treatment helped isolate the potential effect of root exudation.ResultsSedge samples had significantly higher CH4 fluxes than no-sedge samples, but episodic-ebullition fluxes were not significantly different. Between full-light and partially-shaded conditions, there was a significant increase in the difference in CH4 fluxes between the sedge and no-sedge cores.ConclusionThe higher rates of CH4 flux from the sedge cores cannot be explained simply by higher rates of CH4 production due to rapid utilisation of exudates.

Does road salting induce or ameliorate DOC mobilisation from roadside soils to surface waters in the long term

Soils down slope of roads have been affected over decades by road salting in the UK uplands. Salt additions to fresh soil facilitate dispersal of organic matter so there is a potential risk of release of DON and DOC to nearby rivers where these run parallel to roads. Over time, however, salting enhances soil pH of naturally acid soils, and thus organic matter degradation through to CO2, thereby, lowering soil organic matter content. In addition any relatively labile organic matter may have already been dispersed. Thus, it is hypothesised that enhanced DOC mobilisation should only be a potential problem if soils not previously exposed to salt become heavily exposed in the future. This paper combines data from field observations and laboratory simulations to elucidate mechanisms controlling organic matter mobilisation processes to determine what controls spatial and temporal trends in DOC concentrations in soil solutions down slope of roads. Organic matter solubilisation is dependent on the degree of road salt exposure soils have had. The laboratory experiment provided evidence that there are two competing effects upon which solubilisation is dependent (a) pH suppression and (b) sodium dispersion. Other organic matter solubility models, if correct, link quite well with the authors “when it’s gone, it’s gone” hypothesis.

Long-term road salting effects on dispersion of organic matter from roadside soils into drainage water

Sodium chloride has been utilised for decades to maintain road safety in winter and some of its detrimental impacts have been well-documented. However, research on the organic fraction of roadside soils has concentrated upon short-term salt-effects. We hypothesise that decades of past leaching and enhanced mineralisation of organic matter have reduced the concentrations of dissolved organic carbon (DOC) flushes currently occurring. We have examined the effects of salt concentration on organic matter mobilisation in soils that have already experienced varying degrees of exposure to road salting in the field over decades. Applications of salt at concentrations experienced in the field have been simulated to quantify the extent that DOC and dissolved organic nitrogen (DON) are still being mobilised for three prior salt-impact scenarios. A balance occurs between the effects on organic matter of long-term soil pH increase (due to continued cation exchange during salt exposure) which enhances its solubility and organic matter mineralisation, short-term pH suppression (due to the mobile anion effect in soil solution) which reduces its solubility, and short- and long-term sodium-induced dispersion. This now determines the influence of road salt on organic matter leaching from roadside soils and into associated drainage waters.

Are calcareous soils in uplands less prone to damage from road salting than acidic soils

Previous studies of upland roadside soils in Cumbria, that would normally be naturally acidic, have highlighted that (a) runoff from roads subjected to long-term road salting can dramatically raise soil pH down slope in upland areas; (b) the soil pH increase dramatically changes N cycling in soils down slope, increasing mineralisation of organic matter, ammonification, ammonium leaching down slope and nitrification and nitrate leaching; (c) the increase in nitrification substantially increases nitrate leaching to down-slope rivers, and this is readily detectable in field studies; and (d) loss of soil organic matter over decades of salting is so great that organic matter is no longer substantially solubilised by high salt concentrations found in soil solution below road drains. This paper tests and supports the hypothesis that such effects are minimal for more calcareous soil ecosystems. It examines the soil and soil solution chemistry on another Cumbrian upland highway, the A686 near Leadgate, Alston. Sodium % of soil CEC values for soil transects affected by spray containing road salt are similar at both the A6 and A686 sites. However, spatial trends in calcium, magnesium, ammonium, and nitrate concentrations as well as pH differ, as a direct result of the higher weathering rate of parent material and possibly also the presence of limestone walls above both spray-affected and control transects at the A686 site.

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.

Ebullition of methane from rice paddies: the importance of furthering understanding

Flooded rice fields and other wetlands (such as peatlands) are the most important source of atmospheric methane (CH4) (Rothfuss and Conrad 1998). Whalen (2005) estimated the total annual CH4 emission at approximately 600 Tg CH4 yr −1 (anthropogenic and natural sources), of which 20 % and 24 % were attributable to rice paddies and natural wetlands, respectively. Until recently, CH4 has not been comprehensively included in C inventories in wetlands because it represents a relatively small proportion of the total C budget (e.g., in peatlands less than 10 % in mass terms of the budget (Baird et al. 2009; Thompson 2008)). The implications of such exclusion can be large in terms of Global Warming Potential (GWP) (Baird et al. 2009), although there is still great uncertainty regarding the formation, size and dynamics of key components of the soil CH4 pool. Compiling a soil methane inventory requires data on free-phase CH4, and dissolved CH4; the latter is limited by the low solubility of CH4, and lack of ionic form such as those associated with CO2 dissolution. The total soil pool is generated and regulated via the balance of methanogenesis (CH4 production), methanotrophy (CH4 oxidation) and atmospheric-soil CH4 interactions (steady/episodic ebullition, diffusion, and plant-mediated transport). Ebullition has only really expanded in research terms in the last decade, and the number of cited studies, although building is limited. Ebullition is the formation and release of bubbles (predominately containing CH4, CO2 and N-related gaseous species) from inundated sub-surface soils or sediments to the water-table and, thereafter, the atmosphere. It remains unclear how bubbles are formed, what processes and conditions are required for their formation and the prerequisites for their release and transport towards the surface. It was originally assumed that the subsurface CH4 existed primarily in the dissolved form, but it has been estimated that 33–88 % of the total CH4 sub-surface store is in the gas-phase (Tokida et al. 2005b; Strack and Waddington 2008). Bubbles are thought to form when the combined partial pressures of the dissolved gases exceed the hydrostatic pressure (Chanton and Whiting 1995). The presence of these bubbles has important biogeochemical effects, including the development of localized CH4 Plant Soil (2013) 370:31–34 DOI 10.1007/s11104-013-1790-1

Nitrogen loss from karst area in China in recent 50 years: An in‐situ simulated rainfall experiment's assessment

Abstract Karst topography covers more than 1/3 of the Peoples Republic of China in area. The porous, fissured, and soluble nature of the underlying karst bedrock (primarily dolomite and limestone) leads to the formation of underground drainage systems. Karst conduit networks dominate this system, and rainfall takes a crucial role on water cycle at China karst area. Nitrogen loss from the karst system is of particular concern, with regard to nutrient use efficiency as well as water quality, as much of the karst system, including steeply sloping terrain, is used for intensive agriculture. We use simulated rainfall experiments to determine the relationship between rainfall and nitrogen loss at typical karst slope land and then estimate nitrogen loss from the karst soil. The results show that both surface runoff and subsurface runoff have a significant linear correlation with rainfall at all studied sites. Subsurface runoff is larger than surface runoff at two karst sites, while the opposite is true at the non‐karst site. Exponential function satisfactorily described the correlation between rainfall and nitrogen concentrations in runoff. Nitrates accounted for 60%–95% of the dissolved nitrogen loss (DN, an index of N‐loss in this research). The estimated annual N‐loss load varies between 1.05 and 1.67 Tg N/year in the whole karst regions of China from 1961 to 2014. Approximately, 90% of the N‐loss load occurred during the wet season, and 90% of that passed through the subsurface. Understanding the processes and estimating N‐loss is highly valuable in determining long‐term soil security and sustainability in karst regions.

An experimental study on the response of blanket bog vegetation and water tables to ditch blocking

We studied the effect of ditch blocking on vegetation composition and water-table depths in a blanket peatland. Measurements were made for a period of four years (water tables) and five years (vegetation) in the inter-ditch areas of three experimental treatments: (i) open ditches, (ii) ditches blocked with closely-spaced dams and (iii) ditches partially infilled with peat and blocked with dams. It is often assumed that ditch blocking will lead to an increase in the abundance of Sphagnum and, potentially, a reduction in the abundance of sedges, particularly the cotton grasses. However, our data show no treatment effects on the abundance of either group. We did find an effect of time, with the abundance of both sedges and Sphagnum spp. varying significantly between some years. For the sedges there was no systematic change over time, while for the Sphagnum spp. abundance tended to increase through the study period. This systematic change was not related to a measure of the vigour of the sedges, although vigour was lower towards the end of the study compared to the beginning. Our vegetation data are consistent with our water-table data. As with plant type abundance, we did not find any statistically significant differences in water-table depths between treatments, both for annual averages and summer averages. We comment on why ditch blocking does not seem to have affected water tables and vegetation composition at our study site.

Methane and carbon dioxide fluxes from open and blocked ditches in a blanket bog

Background and aimsThere is growing interest in how the rewetting of drained peatlands can restart their carbon (C) sink function. However, there are few studies on the effect of ditch blocking on the within-ditch C balance. For a UK blanket bog we assessed how methane (CH4) emissions, net ecosystem exchange (NEE), and the overall greenhouse gas (GHG) balance expressed as carbon dioxide equivalents (CO2-e) responded to ditch blocking.MethodsWe conducted a fully replicated field trial on a blanket bog in the Upper Conwy catchment, North Wales, UK. Twelve parallel ditches, that ran approximately downslope, were investigated. Four were left open, four had peat dams installed at intervals of a few metres along their length, and four were partially infilled with peat (reprofiled) and dammed. For a period of four years after blocking, we measured peatland-atmosphere fluxes of CH4 and CO2 within the ditches.ResultsCH4 fluxes, NEE and overall GHG balance (expressed in terms of CO2-e) in the experimental area showed no evidence of varying systematically between the different types of ditch treatment (open, dammed, and reprofiled). In addition, there was little evidence that CH4 fluxes or CO2-e balance changed systematically with time since blocking.ConclusionsWe found no evidence of consistent differences between blocking treatments in terms of CH4 emissions or overall CO2-e balance. There was high spatial and temporal variability in CO2 and CH4 fluxes within each treatment. We did not observe a post-blocking ‘spike’ in CH4 fluxes.

Peatland ditch blocking has no effect on dissolved organic matter (DOM) quality

Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences, Uppsala, Sweden Bangor Wetlands Group, School of Biological Sciences, Bangor University, Gwynedd, UK Centre for Ecology and Hydrology, Environment Centre Wales, Bangor, Gwynedd, UK water@leeds, School of Geography, University of Leeds, Leeds, UK Geography, College of Life and Environmental Sciences, University of Exeter, Exeter, UK Correspondence Mike Peacock, Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences, Uppsala, Sweden. Email: michael.peacock@slu.se; mikepeacocknin@yahoo.co.uk Funding information Department for Environment, Food and Rural Affairs, Grant/Award Number: SP1202