Steve Hughes

Export of dissolved organic carbon from peatlands under elevated carbon dioxide levels

Peatlands represent a vast store of global carbon. Observations of rapidly rising dissolved organic carbon concentrations in rivers draining peatlands have created concerns that those stores are beginning to destabilize. Three main factors have been put forward as potential causal mechanisms, but it appears that two alternatives—warming and increased river discharge—cannot offer satisfactory explanations. Here we show that the third proposed mechanism, namely shifting trends in the proportion of annual rainfall arriving in summer, is similarly unable to account for the trend. Instead we infer that a previously unrecognized mechanism—carbon dioxide mediated stimulation of primary productivity—is responsible. Under elevated carbon dioxide levels, the proportion of dissolved organic carbon derived from recently assimilated carbon dioxide was ten times higher than that of the control cases. Concentrations of dissolved organic carbon appear far more sensitive to environmental drivers that affect net primary productivity than those affecting decomposition alone.

Enzymes and biogeochemical cycling in wetlands during a simulated drought

Possible interactions between soil enzymes and thebiogeochemistry of wetlands were investigated duringa field-based drought simulation. Under control(waterlogged) conditions, correlations were foundbetween the activity of the enzyme B-glucosidase andtwo properties associated with carbon cycling, namelyi) CH4 release r = 0.79,p lt 0.01) and ii) dissolvedorganic carbon concentration (r= -0.81, p lt 0.01). In contrast,the transition to drought conditions resulted in correlations betweenB-glucosidase activity and certain mineralisationprocesses, namely the release of mg and Ca(r = 0.72, p lt 0.05). Sulphataseactivity correlated with changes in sulphate concentration during the droughtsimulation (r = 0.73, p lt 0.05).Further support for the suggested enzymic involvement in biogeochemicalprocesses was found in laboratory studies. Theseexperiments indicated that increasing the abundance ofB-glucosidase could stimulate trace gas emissions(p lt 0.001) and increase the concentration ofmagnesium and calcium (p lt 0.05). Increasedsulphatase abundance caused a suppression of methane emissions(p = 0.053).

Contrasted effects of simulated drought on the production and oxidation of methane in a mid-Wales wetland

Abstract Wetlands are a major contributor to the global CH 4 budget. Currently, there is a consensus view that drought restrains CH 4 emissions from wetlands, and that this arises due to a suppression of CH 4 production and stimulation of CH 4 oxidation under the more aerobic conditions that accompany lower water table levels. Our data confirm that under drought conditions, CH 4 production is lower (−73%, P P 4 oxidation during the drought, and in contrast, at the end of the simulation observed significantly less CH 4 oxidation in the drought treated system ( P

The Role of Nitrogen Deposition in Widespread Plant Community Change Across Semi-natural Habitats

Experimental studies have shown that deposition of reactive nitrogen is an important driver of plant community change, however, most of these experiments are of short duration with unrealistic treatments, and conducted in regions with elevated ambient deposition. Studies of spatial gradients of pollution can complement experimental data and indicate whether the potential impacts demonstrated by experiments are actually occurring in the ‘real world’. However, targeted surveys exist for only a very few habitats and are not readily comparable. In a coordinated campaign, we determined the species richness and plant community composition of five widespread, semi-natural habitats across Great Britain in sites stratified along gradients of climate and pollution, and related these ecological parameters to major drivers of biodiversity, including climate, pollution deposition, and local edaphic factors. In every habitat, we found reduced species richness and changed species composition associated with higher nitrogen deposition, with remarkable consistency in relative species loss across ecosystem types. Whereas the diversity of mosses, lichens, forbs, and graminoids declines with N deposition in different habitats, the cover of graminoids generally increases. Considered alongside previous experimental studies and survey work, our results provide a compelling argument that nitrogen deposition is a widespread and pervasive threat to terrestrial ecosystems.

Nitrous oxide emissions from a gully mire in mid-Wales, UK, under simulated summer drought

Certain general circulation models predict that a doubling of atmospheric carbon dioxide concentrations will increase the frequency of summer drought in northern wetlands due to hotter, drier summers. There is currently much uncertainty as to how drought will affect emissions of the greenhouse gas, nitrous oxide, from wetlands. We have demonstrated that an eight centimetre drawdown of the water table in a gully mire does not significantly affect nitrous oxide emissions from this site. However, under a more extreme drought scenario carried out on peat monoliths, nitrous oxide emissions increased exponentially with a linear decrease in water table height. Drought caused a significant increase in nitrous oxide productionbelow the water table but most of the total increase could be attributed to increased emissionsabove the water table. Results from an acetylene block experiment suggested that increased emissions were caused by increased nitrous oxide production from denitrification, rather than by increased production from nitrification. In the laboratory study, drought severity had no effect on peatwater nitrate concentrations below the water table, however, increasing drought severity decreased ammonium concentrations.

Decomposition ‘hotspots’ in a rewetted peatland: implications for water quality and carbon cycling

Restoration of drained peatlands has been promoted to reduce gaseous and aquatic carbon losses; however, there are conflicting reports as to its effectiveness. Here we report “hotspots” of organic matter decomposition as a result of rewetting a drained peatland in Wales, at the field-scale, in the medium/long-term with implications for water quality and greenhouse gas emissions. Low soil moisture levels, that characterise these hotspots before rewetting, regenerate electron acceptors and provide carbon and nutrients which stimulate phenol oxidase-mediated release of phenolic compounds from the peat matrix upon waterlogging. Electron acceptors are then consumed sequentially, eventually favouring CH4 production and rising pH, despite accumulating SO4 levels. The latter two processes promote positive feedback to increased phenol oxidase activities and the release of even more dissolved organic carbon (DOC) and CH4 from the peat matrix. Hotspot formation therefore represents an inextricably linked physico-chemical and biological positive feedback mechanism. Such hotspots account for a large proportion of the mean increase in carbon loss due to rewetting of this naturally drained peatland (e.g. at maximum mean DOC concentrations: with hotspot 997%; without hotspot 102%) and are not “outliers” but important drivers of biogeochemical fluxes that should be included in budgets for carbon and other elements (e.g. sulphur). As such, understanding hotspot formation should allow improved management strategies for restoration, carbon stocks, drinking water quality and even future geo-engineering options in the face of changes in climate and atmospheric chemistry.

Phosphorus in soils and field drainage water in the thame catchment, UK

Field drains were sampled at five farms in a catchment in south-central England. The farms were selected to include the main soil types present in the catchment, stagnogleys and pelosols. The phosphorus content of field drainage water was measured on several occasions from 1999 to 2000, under varying flow conditions. The components measured were soluble reactive phosphorus (SRP), total dissolved phosphorus (TDP) and total phosphorus (TP). SRP concentrations in drainage water were lower than in streams in the catchment which had no apparent point source. This indicated that many streams were receiving unidentified point sources of SRP. Measurements of P in field drainage water samples under high flow conditions showed concentrations of particulate phosphorus and SRP up to 1300 and 300 microg l(-1), respectively, these being associated with high suspended sediment concentrations. Comparison of field drain and soil phosphorus contents at the locations sampled did not provide evidence of an inter-relationship. The equilibrium phosphorus content (EPC0) of surface soil was generally higher than the SRP content of drainage water, at one farm by 1 order of magnitude. The variability in measurements suggested a larger-scale and more focussed survey would be required to characterise catchment-scale phosphorus losses from commercial farms by land use and soil type.

Enzymic involvement in the biogeochemical responses of a Welsh peatland to a rainfall enhancement manipulation

Abstract Microbial enzyme activities were followed during a field-based experimental simulation of the effects of higher rainfall in a Welsh peatland. The treatment did not significantly affect the activities of the carbon cycling enzymes, β-glucosidase, esterase or xylosidase. In contrast, the activity of the enzyme sulphatase decreased by 44% (P<0.001) in response to the wetter conditions. The manipulation suggests that should climate change cause conditions to become wetter in peatlands, then (with the exception of sulphatase) current levels of wetness may be sufficient to limit decomposition processes, and thus any further increase in wetness is unlikely to induce a further decrease in decomposition rates. Correlations were found between the esterase activity and both nitrous oxide flux (r=–0.44, P<0.05), and methane release (r=0.53, P<0.01). Likewise, there was a correlation between xylosidase activity and both carbon dioxide emission (r=0.52, P<0.01) and aluminium concentration (r=0.58, P<0.01). All of the enzymes correlated positively with dissolved organic carbon (range r=0.53, P<0.01 sulphatase to r=0.61, P<0.001 glucosidase). Together, the correlations lend support to recent hypotheses suggesting that enzymes exert an influence over wetland biogeochemical properties.

Soluble reactive phosphorus levels in rainfall, cloud water, throughfall, stemflow, soil waters, stream waters and groundwaters for the Upper River Severn area, Plynlimon, mid Wales

Soluble reactive phosphorus (SRP) data are presented for rainfall, cloud water, soil waters, stream waters and groundwaters at the Plynlimon catchments in mid Wales to examine the hydrochemical functioning of inorganic phosphorus for an acidic and acid sensitive area characteristic of much of the UK uplands. In general, stream water concentrations are low compared to lowland areas. Average concentrations of SRP in rainfall and cloud water (0.3 and 0.9 microM l(-1), respectively) are higher than in stream water with wider ranges in concentration (0-19.3 and 0-20.9 microM l(-1), respectively). Throughfall and stemflow is enriched in SRP compared to rain and cloud water by a factor of approximately twofold and sixfold, respectively: the average concentrations and ranges are 0.73 and 0-6.61 microM l(-1) for throughfall and 2.12 and 0-18.61 microM l(-1) for stemflow. Soil water SRP concentrations measured in the surface layers of representative areas of podzol and gley soils, are further enriched with respect to inputs. Average concentrations and ranges for the L/F and Oh horizons in the podzols are 3.1 microM l(-1) (range: 0.03-17.2 microM l(-1)) and 0.75 microM l(-1) (range: 0.03-2.64 microM l(-1)), respectively. Correspondingly, the average values and ranges for the L/F and Oh horizons in the gley are 2 microM l(-1) (range: 0.03-16.65 microM l(-1)) and 0.4 microM l(-1) (range: 0.03-8.61 microM l(-1)). SRP concentrations in stream and ground water are lower than in atmospheric inputs and surface soil waters and show marked spatial variability. This variability is linked to three catchment features. (1) For streams draining podzolic soils, most of the SRP is retained by the catchment. For this situation, stream and ground waters have average concentrations of approximately 0.05 microM l(-1) with a range of 0-1.47 microM l(-1). There is no clear stream or groundwater SRP response to felling despite a large release of SRP from felling debris (brash) and the forest floor (L/F horizon) with average post-felling concentrations of 11.02 microM l(-1) (0.40-155.0 microM l(-1)) and 23.60 microM l(-1) (0.26-172.23 microM l(-1)), respectively. (2) For forested catchments with gley soils, stream water SRP concentrations are more variable with, in one case, much higher concentrations than for the podzol counterparts (range in average 0.05-0.46 microM l(-1)). (3) For the streams draining gley soils, felling results in a mixed SRP response. At the local scale (ditch drainage), there is a marked enrichment in SRP concentration (average concentrations increase from 0.05 to 1.31 microM l(-1), with a peak concentration of 4.0 microM l(-1)). This response is consistent with the observed mobilisation of SRP from brash and forest floor material (post-felling mean concentrations of 9.39 and 11.94 microM l(-1), respectively). However, stream water concentrations are an order of magnitude lower than observed in the soil waters implying considerable immobilisation of SRP between the soils and the stream. At the larger catchment scale, no discernable enrichment in SRP is observed following felling. The results are related to input-output budgets and the findings interpreted in terms of the dominant hydrogeochemical processes operative and environmental management issues.

The phosphorus budget of the Thame catchment, Oxfordshire: 2. Modelling.

The phosphorus budget of the River Thame was modelled at a daily time scale, using estimates of diffuse and point source contributions of discharge. The model simulated suspended sediment (SS), soluble unreactive phosphorus (SUP), soluble reactive phosphorus (SRP) and particulate phosphorus (PP) concentrations within the main river and major tributaries. Diffuse source estimates of phosphorus loads were based on characteristic losses from identified main landscape classes, with hydrology described by a simple conceptual storage model. In-stream flow was modelled using a kinematic wave equation. Transfer of suspended sediment and phosphorus components was approximated by advection. In-stream sources and sinks included uptake and release of soluble reactive phosphorus by bed sediment, instant equilibration between SRP and the PP concentration on suspended sediment, and flow-related entrainment and deposition of suspended sediment. Simulations at sites within the catchment were compared with measurements made in 1998-1999. Results showed the P budget is dominated by mixing of diffuse and point source water, but some within-river processes have been shown to be capable of significantly influencing SRP concentrations. The development of a sediment entrainment and deposition component of the model has proved particularly valuable in emulating the hysteretic relationship between discharge and suspended sediment concentration in the river. It also provides a measure of available bed sediment.