Chris D. Evans

Dissolved organic carbon trends resulting from changes in atmospheric deposition chemistry.

Several hypotheses have been proposed to explain recent, widespread increases in concentrations of dissolved organic carbon (DOC) in the surface waters of glaciated landscapes across eastern North America and northern and central Europe. Some invoke anthropogenic forcing through mechanisms related to climate change, nitrogen deposition or changes in land use, and by implication suggest that current concentrations and fluxes are without precedent. All of these hypotheses imply that DOC levels will continue to rise, with unpredictable consequences for the global carbon cycle. Alternatively, it has been proposed that DOC concentrations are returning toward pre-industrial levels as a result of a gradual decline in the sulphate content of atmospheric deposition. Here we show, through the assessment of time series data from 522 remote lakes and streams in North America and northern Europe, that rising trends in DOC between 1990 and 2004 can be concisely explained by a simple model based solely on changes in deposition chemistry and catchment acid-sensitivity. We demonstrate that DOC concentrations have increased in proportion to the rates at which atmospherically deposited anthropogenic sulphur and sea salt have declined. We conclude that acid deposition to these ecosystems has been partially buffered by changes in organic acidity and that the rise in DOC is integral to recovery from acidification. Over recent decades, deposition-driven increases in organic matter solubility may have increased the export of DOC to the oceans, a potentially important component of regional carbon balances. The increase in DOC concentrations in these regions appears unrelated to other climatic factors.

Export of organic carbon from peat soils

Warmer conditions may be to blame for the exodus of peatland carbon to the oceans.

Causes of concentration/discharge hysteresis and its potential as a tool for analysis of episode hydrochemistry

Episodic variations in dissolved solutes are frequently complicated by a cyclical relationship between concentration and stream discharge. Established three-component models of runoff generation are used to explain this hysteresis effect and to illustrate how different component concentrations produce different hysteresis forms. It is demonstrated that a two-component model cannot reproduce all the hysteresis forms commonly observed. A method, based on the three-component system, is derived by which C/Q hysteresis can be used to predict relative component concentrations. This may provide a qualitative chemical description of sources supplying runoff for locations where these have not yet been directly established or a validation test where possible components have been sampled. The method has been tested using data collected at streams in the Adirondacks, New York, and the Northern Appalachian Plateau, Pennsylvania, during the Episodic Response Project of the U.S. Environmental Protection Agency. Predicted component compositions were in good agreement with measurements made during other studies and with those subsequently obtained from one of the Pennsylvania watersheds.

The importance of the relationship between scale and process in understanding long-term DOC dynamics

Concentrations of dissolved organic carbon have increased in many, but not all, surface waters across acid impacted areas of Europe and North America over the last two decades. Over the last eight years several hypotheses have been put forward to explain these increases, but none are yet accepted universally. Research in this area appears to have reached a stalemate between those favouring declining atmospheric deposition, climate change or land management as the key driver of long-term DOC trends. While it is clear that many of these factors influence DOC dynamics in soil and stream waters, their effect varies over different temporal and spatial scales. We argue that regional differences in acid deposition loading may account for the apparent discrepancies between studies. DOC has shown strong monotonic increases in areas which have experienced strong downward trends in pollutant sulphur and/or seasalt deposition. Elsewhere climatic factors, that strongly influence seasonality, have also dominated inter-annual variability, and here long-term monotonic DOC trends are often difficult to detect. Furthermore, in areas receiving similar acid loadings, different catchment characteristics could have affected the site specific sensitivity to changes in acidity and therefore the magnitude of DOC release in response to changes in sulphur deposition. We suggest that confusion over these temporal and spatial scales of investigation has contributed unnecessarily to the disagreement over the main regional driver(s) of DOC trends, and that the data behind the majority of these studies is more compatible than is often conveyed.

Deep instability of deforested tropical peatlands revealed by fluvial organic carbon fluxes.

Tropical peatlands contain one of the largest pools of terrestrial organic carbon, amounting to about 89,000 teragrams (1 Tg is a billion kilograms). Approximately 65 per cent of this carbon store is in Indonesia, where extensive anthropogenic degradation in the form of deforestation, drainage and fire are converting it into a globally significant source of atmospheric carbon dioxide. Here we quantify the annual export of fluvial organic carbon from both intact peat swamp forest and peat swamp forest subject to past anthropogenic disturbance. We find that the total fluvial organic carbon flux from disturbed peat swamp forest is about 50 per cent larger than that from intact peat swamp forest. By carbon-14 dating of dissolved organic carbon (which makes up over 91 per cent of total organic carbon), we find that leaching of dissolved organic carbon from intact peat swamp forest is derived mainly from recent primary production (plant growth). In contrast, dissolved organic carbon from disturbed peat swamp forest consists mostly of much older (centuries to millennia) carbon from deep within the peat column. When we include the fluvial carbon loss term, which is often ignored, in the peatland carbon budget, we find that it increases the estimate of total carbon lost from the disturbed peatlands in our study by 22 per cent. We further estimate that since 1990 peatland disturbance has resulted in a 32 per cent increase in fluvial organic carbon flux from southeast Asia—an increase that is more than half of the entire annual fluvial organic carbon flux from all European peatlands. Our findings emphasize the need to quantify fluvial carbon losses in order to improve estimates of the impact of deforestation and drainage on tropical peatland carbon balances.

Spectroscopic confirmation of a galaxy at redshift z = 8.6

Galaxies had their most significant impact on the Universe when they assembled their first generations of stars. Energetic photons emitted by young, massive stars in primeval galaxies ionized the intergalactic medium surrounding their host galaxies, cleared sightlines along which the light of the young galaxies could escape, and fundamentally altered the physical state of the intergalactic gas in the Universe continuously until the present day. Observations of the cosmic microwave background, and of galaxies and quasars at the highest redshifts, suggest that the Universe was reionized through a complex process that was completed about a billion years after the Big Bang, by redshift z ≈ 6. Detecting ionizing Lyman-α photons from increasingly distant galaxies places important constraints on the timing, location and nature of the sources responsible for reionization. Here we report the detection of Lyα photons emitted less than 600 million years after the Big Bang. UDFy-38135539 (ref. 5) is at a redshift of z = 8.5549 ± 0.0002, which is greater than those of the previously known most distant objects, at z = 8.2 (refs 6 and 7) and z = 6.96 (ref. 8). We find that this single source is unlikely to provide enough photons to ionize the volume necessary for the emission line to escape, requiring a significant contribution from other, probably fainter galaxies nearby.

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.

Recovery from Acidification in European Surface Waters: A View to the Future

Abstract There is now overwhelming documentation of large-scale chemical recovery from surface water acidification in Europe, but to date there has been little documentation of biological recovery. Modelling studies based on current emission reduction plans in Europe indicate that there will be further chemical recovery. The uncertainties in these scenarios mainly relate to the future behavior of nitrogen in the ecosystem and the effects of climate change. Four major climate-related confounding factors that may influence the chemical and biological recovery process are: i) increased frequency and severity of sea-salt episodes; ii) increased frequency and severity of drought; iii) in-creased turnover of organic carbon; iv) increased mineralization of nitrogen. International cooperative work to abate acidification has so far been very successful, but there is still a long way to go, and many potential setbacks. It is essential that future development of water chemistry and aquatic biota in acidified waterbodies continue to be monitored in relation to further emission reductions of S and N and future effects of climate change.

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.

Use of dynamic soil-vegetation models to assess impacts of nitrogen deposition on plant species composition: an overview

Field observations and experimental data of effects of nitrogen (N) deposition on plant species diversity have been used to derive empirical critical N loads for various ecosystems. The great advantage of such an approach is the inclusion of field evidence, but there are also restrictions, such as the absence of explicit criteria regarding significant effects on the vegetation, and the impossibility to predict future impacts when N deposition changes. Model approaches can account for this. In this paper, we review the possibilities of static and dynamic multispecies models in combination with dynamic soil-vegetation models to (1) predict plant species composition as a function of atmospheric N deposition and (2) calculate critical N loads in relation to a prescribed protection level of the species composition. The similarities between the models are presented, but also several important differences, including the use of different indicators for N and acidity and the prediction of individual plant species vs. plant communities. A summary of the strengths and weaknesses of the various models, including their validation status, is given. Furthermore, examples are given of critical load calculations with the model chains and their comparison with empirical critical N loads. We show that linked biogeochemistry-biodiversity models for N have potential for applications to support European policy to reduce N input, but the definition of damage thresholds for terrestrial biodiversity represents a major challenge. There is also a clear need for further testing and validation of the models against long-term monitoring or long-term experimental data sets and against large-scale survey data. This requires a focused data collection in Europe, combing vegetation descriptions with variables affecting the species diversity, such as soil acidity, nutrient status and water availability. Finally, there is a need for adaptation and upscaling of the models beyond the regions for which dose-response relationships have been parameterized, to make them generally applicable.