Brian Reynolds

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.

Fluxes of CO2, CH4 and N2O from a Welsh peatland following simulation of water table draw-down: Potential feedback to climatic change

A potential effect of climatic change was simulated by manipulating the water table height within intact peat monoliths. The treatment decreased methane flux (maximum −80%) and increased both carbon dioxide flux (maximum 146%) and nitrous oxide flux maximum 936%). Returning the water table height to its original level caused both nitrous oxide and carbon dioxide flux to rapidly return to control levels. However, methane flux remained at its experimentally induced low levels.

Microbial activity and enzymic decomposition processes following peatland water table drawdown

Microbial activity and enzymic decomposition processes were followed during a field-based experimental lowering of the water table in a Welsh peatland. Respiration was not significantly affected by the treatment. However, the enzymes sulphatase, β-glucosidase and phosphatase were stimulated by between 31 and 67% upon water table drawdown. A further enzyme, phenol oxidase, was not significantly affected. The observation of elevated enzyme activities without an associated increase in microbial respiratory activity suggests that drought conditions influence peatland mineralisation rates through a direct stimulation of existing enzymes, rather than through a generalised stimulation of microbial metabolism (with associated de-novo enzyme synthesis). Hydrochemical data suggest that the stimulation may have been caused by a reduction in the inhibitory action of iron and phenolics in the peat pore waters. Overall, the findings support the recent hypothesis that drier conditions associated with climate change could stimulate mineralisation within wetlands. ei]R Merckx

Are temporal variations in the nitrate content of UK upland freshwaters linked to the North Atlantic Oscillation

Synchron ous patterns of variation in NO3 concentration, observed in upland lakes and streams across gradients of climate, altitude, land use and nitrogen deposition over the last decade, show a strong negative correlation with winter values for the North Atlantic Oscillation Index and mean winter temperature. Variations in a longer term stream NO3 record validate this relationship, and although mechanisms are yet unclear, may be linked to the length of time the soil profile remains frozen during the winter. These findings are of particular significance to our understanding of the relationship between atmospheric N deposition and concentrations and fluxes of NO3 in runoff, and the potential response to future climatic changes. Copyright (C) 2000 John Wiley and Sons, Ltd.

Baseflow buffering of streamwater acidity in five mid-Wales catchments

Streamwater pH and concentrations of calcium, bicarbonate and aluminium have been monitored in two forest and three grassland catchments at the Plynlimon area of upland mid-Wales. Baseflow in all the streams is characterised by waters bearing calcium and bicarbonate and which are low in hydrogen ions and aluminium. Stormflow shows more complex chemical variation. Stormwaters are acidic, low in calcium and bicarbonate, and contain aluminium; except in the stream draining the mineralised grassland catchment. At this latter site, streamwater pH is relatively high and constant, even at high flows, and aluminium is not detected. Limited buffering of streamwater acidity is also observed in the streams draining the mineralised forest catchment and the agriculturally improved grassland catchment. There is little difference in the means and ranges of acidity between the stream draining the unmineralised part of the forest and the stream flowing from the unimproved unmineralised semi-natural grassland catchment. Mean concentrations of aluminium are almost three times higher in the forest stream. The streamwater data are discussed in relation to the chemistry of soil waters and vegetation canopy throughfall in the forest and grassland systems. The significance, to water quality, of the hydrological pathways operating in the two systems is also examined. The study shows that the presence of CaCO3 in the bedrock, even on a small scale, and liming of the land in the catchment can both have a marked effect on stream water acidity. This result is important when remedial action in areas subject to catchment acidification is considered and demonstrates that it is difficult to generalise about catchment responses to acidic atmospheric inputs. The results also show that for catchments with similar soils and bedrock geology, those under coniferous forest do not necessarily have more acid streamwater than those under semi-natural grassland vegetation. Aluminium concentrations in the stream are not simply determined by Al(OH)3 solubility controls as streamwater concentrations of aluminium for the semi-natural grassland catchment are only one third those for the forest catchment, even though soil types are the same.

The consequences of chronic nitrogen additions on N cycling and soilwater chemistry in a Sitka spruce stand, North Wales

Abstract Major changes in dissolved nitrogen (N) leaching losses occurred during 5 yr of weekly applications of N to the forest floor of a 30-yr-old Sitka spruce ( Picea sitchensis (Bong.) Carr.) stand in Aber forest, N. Wales. N was applied as sodium nitrate (NaNO 3 ) at rates of 35 and 75 kg N ha −1 yr −1 and ammonium nitrate (NH 4 NO 3 ) at 35 kg N ha −1 yr −1 , representing either a 125% or 270% increase in total N deposition. There was a close relationship between NO 3 inputs and NO 3 leaching losses from below the rooting zone over the 5-yr period indicating this stand was NO 3 -saturated pretreatment. In contrast, NH 4 was retained within the soil profile. Some evidence for mobilisation of reduced N from the forest floor N, in excess of inputs, was observed in all treatments in the initial years of N applications. However, this disappeared in later years and all mobilised N from the forest floor was retained in lower mineral horizons. In the mineral soil, both forms of N application resulted in elevated H + and/or total Al concentrations. These effects were transitory in the high-dosage NaNO 3 treatment because a new equilibrium was established between concentrations of Na in soil solution and amounts of Na on the soil exchange complex. However, continuing reductions in base cation to Al ratios were observed in the NH 4 NO 3 35-kg-N-ha −1 -yr −1 treatment due to acidification associated with NH 4 retention. No significant changes in dissolved organic carbon or base cation concentrations were observed in organic or mineral horizon soil solutions in any N treatment. The results demonstrated that any future increases in oxidised N deposition to similarly mature Sitka spruce stands will result in immediate increases in NO 3 leaching. In acid-sensitive areas, this will result in both eutrophication and acidification of stream water due to the link between NO 3 leaching and Al concentrations. The speed of response to increased deposition of reduced N will depend on the N status of the ecosystem. These results are discussed within the context of responses observed at other NITREX sites, and provide information on the rate and sequence of responses to chronic N deposition by forest ecosystems.

Impact of afforestation on the soil solution chemistry of stagnopodzols in mid-Wales

Chemical data for stagnopodzol soils and soil solutions are presented for grassland and plantation coniferous forest systems in upland mid-Wales. The results show that the introduction of the forest, with consequent increase in anion concentrations, has increased concentrations of Al in the soil solution by 1.5 to 3 times. Hydrogen ion concentrations have only increased by a small amount. Simple cation exchange relationships have been proposed to explain some of these changes.

An analysis of long-term trends, seasonality and short-term dynamics in water quality data from Plynlimon, Wales

This paper examines two hydrochemical time-series derived from stream samples taken in the Upper Hafren catchment, Plynlimon, Wales. One time-series comprises data collected at 7-hour intervals over 22 months (Neal et al., 2012-this issue), while the other is based on weekly sampling over 20 years. A subset of determinands: aluminium, calcium, chloride, conductivity, dissolved organic carbon, iron, nitrate, pH, silicon and sulphate are examined within a framework of non-stationary time-series analysis to identify determinand trends, seasonality and short-term dynamics. The results demonstrate that both long-term and high-frequency monitoring provide valuable and unique insights into the hydrochemistry of a catchment. The long-term data allowed analysis of long-term trends, demonstrating continued increases in DOC concentrations accompanied by declining SO(4) concentrations within the stream, and provided new insights into the changing amplitude and phase of the seasonality of the determinands such as DOC and Al. Additionally, these data proved invaluable for placing the short-term variability demonstrated within the high-frequency data within context. The 7-hour data highlighted complex diurnal cycles for NO(3), Ca and Fe with cycles displaying changes in phase and amplitude on a seasonal basis. The high-frequency data also demonstrated the need to consider the impact that the time of sample collection can have on the summary statistics of the data and also that sampling during the hours of darkness provides additional hydrochemical information for determinands which exhibit pronounced diurnal variability. Moving forward, this research demonstrates the need for both long-term and high-frequency monitoring to facilitate a full and accurate understanding of catchment hydrochemical dynamics.

Evidence that soil carbon pool determines susceptibility of semi-natural ecosystems to elevated nitrogen leaching

Deposition of reactive nitrogen (N) compounds has the potential to cause severe damage to sensitive soils and waters, but the process of ‘nitrogen saturation’ is difficult to demonstrate or predict. This study compares outputs from a simple carbon–nitrogen model with observations of (1) regional- and catchment-scale relationships between surface water nitrate and dissolved organic carbon (DOC), as an indicator of catchment carbon (C) pool; (2) inter-regional variations in soil C/N ratios; and (3) plot scale soil and leachate response to long-term N additions, for a range of UK moorlands. Results suggest that the simple model applied can effectively reproduce observed patterns, and that organic soil C stores provide a critical control on catchment susceptibility to enhanced N leaching, leading to high spatial variability in the extent and severity of current damage within regions of relatively uniform deposition. Results also support the hypothesis that the N richness of organic soils, expressed as C/N ratio, provides an effective indicator of soil susceptibility to enhanced N leaching. The extent to which current C/N is influenced by N deposition, as opposed to factors such as climate and vegetation type, cannot be unequivocally determined on the basis of spatial data. However, N addition experiments at moorland sites have shown a reduction in organic soil C/N. A full understanding of the mechanisms of N-enrichment of soils and waters is essential to the assessment of current sensitivity to, and prediction of future damage from, globally increasing reactive nitrogen deposition.

Hydrogeochemical controls for inorganic aluminium in acidic stream and soil waters at two upland catchments in Wales

Results of two studies of acidic and acid sensitive catchments typical of upland Wales are presented. The streams draining these catchments are less acidic and contain lower inorganic aluminium concentrations than their soil counterparts. Within each soil horizon there are distinct linear or curvilinear relationships with H1 and inorganic aluminium reactions. The stream water H1 and inorganic aluminium chemistries are determined by mixing of these soil waters with a groundwater component not sampled during these studies except in a partially CO2 degassed form in the stream at extreme baseflow conditions. While during the mixing processes within the soil/groundwater zones, inorganic aluminium may be regulated by simple solubility controls, no such controls are operative within the streams. The classical cubic relationship between the activities of Al3+ and H+ for the ion exchange reactions does not hold; a power term, which can vary from site to site and horizon to horizon in the range 1–3 is required. The implications for modelling studies are discussed.