Riparian shading controls instream spring phytoplankton and benthic algal growth.

Dissolved oxygen (DO) concentrations showed a striking pattern in a multi-year study of the River Enborne, a small river in SE England. In each of three years (2010-2012), maximum DO concentrations were attained in mid-April, preceded by a period of steadily increasing diurnal amplitudes, followed by a steady reduction in both amplitude and concentration. Flow events during the reduction period reduce DO to low concentrations until the following spring. Evidence is presented that this pattern is mainly due to benthic algal growth which is eventually suppressed by the growth of the riparian tree canopy. Nitrate and silicate concentrations are too high to inhibit the growth of either benthic algae or phytoplankton, but phosphate concentrations might have started to reduce growth if the tree canopy development had been delayed. This interpretation is supported by evidence from weekly flow cytometry measurements and analysis of the diurnal, seasonal and annual patterns of nutrient concentrations. As the tree canopy develops, the river switches from an autotrophic to a heterotrophic state. The results support the use of riparian shading to help control algal growth, and highlight the risks of reducing riparian shade.

Characterising phosphorus and nitrate inputs to a rural river using high-frequency concentration-flow relationships.

The total reactive phosphorus (TRP) and nitrate concentrations of the River Enborne, southern England, were monitored at hourly interval between January 2010 and December 2011. The relationships between these high-frequency nutrient concentration signals and flow were used to infer changes in nutrient source and dynamics through the annual cycle and each individual storm event, by studying hysteresis patterns. TRP concentrations exhibited strong dilution patterns with increasing flow, and predominantly clockwise hysteresis through storm events. Despite the Enborne catchment being relatively rural for southern England, TRP inputs were dominated by constant, non-rain-related inputs from sewage treatment works (STW) for the majority of the year, producing the highest phosphorus concentrations through the spring-summer growing season. At higher river flows, the majority of the TRP load was derived from within-channel remobilisation of phosphorus from the bed sediment, much of which was also derived from STW inputs. Therefore, future phosphorus mitigation measures should focus on STW improvements. Agricultural diffuse TRP inputs were only evident during storms in the May of each year, probably relating to manure application to land. The nitrate concentration-flow relationship produced a series of dilution curves, indicating major inputs from groundwater and to a lesser extent STW. Significant diffuse agricultural inputs with anticlockwise hysteresis trajectories were observed during the first major storms of the winter period. The simultaneous investigation of high-frequency time series data, concentration-flow relationships and hysteresis behaviour through multiple storms for both phosphorus and nitrate offers a simple and innovative approach for providing new insights into nutrient sources and dynamics.

Climate change and coupling of macronutrient cycles along the atmospheric, terrestrial, freshwater and estuarine continuum.

This paper provides an introduction to the Special Issue on "Climate Change and Coupling of Macronutrient Cycles along the Atmospheric, Terrestrial, Freshwater and Estuarine Continuum", dedicated to Colin Neal on his retirement. It is not intended to be a review of this vast subject, but an attempt to synthesize some of the major findings from the 22 contributions to the Special Issue in the context of what is already known. The major research challenges involved in understanding coupled macronutrient cycles in these environmental media are highlighted, and the difficulties of making credible predictions of the effects of climate change are discussed. Of particular concern is the possibility of interactions which will enhance greenhouse gas concentrations and provide positive feedback to global warming.

The effects of a changing pollution climate on throughfall deposition and cycling in a forested area in southern England.

This study compares two sets of measurements of the composition of bulk precipitation and throughfall at a site in southern England with a 20-year gap between them. During this time, SO(2) emissions from the UK fell by 82%, NO(x) emissions by 35% and NH(3) emissions by 7%. These reductions were partly reflected in bulk precipitation, with deposition reductions of 56% in SO(4)(2-), 38% in NO(3)(-), 32% in NH(4)(+), and 73% in H(+). In throughfall under Scots pine, the effects were more dramatic, with an 89% reduction in SO(4)(2-) deposition and a 98% reduction in H(+) deposition. The mean pH under these trees increased from 2.85 to 4.30. Nitrate and ammonium deposition in throughfall increased slightly, however. In the earlier period, the Scots pines were unable to neutralise the high flux of acidity associated with sulphur deposition, even though this was not a highly polluted part of the UK, and deciduous trees (oak and birch) were only able to neutralise it in summer when the leaves were present. In the later period, the sulphur flux had reduced to the point where the acidity could be neutralised by all species - the neutralisation mechanism is thus likely to be largely leaching of base cations and buffering substances from the foliage. The high fluxes are partly due to the fact that these are 60-80 year old trees growing in an open forest structure. The increase in NO(3)(-) and NH(4)(+) in throughfall in spite of decreased deposition seems likely due to a decrease in foliar uptake, perhaps due to the increasing nitrogen saturation of the catchment soils. These changes may increase the rate of soil microbial activity as nitrogen increases and acidity declines, with consequent effects on water quality of the catchment drainage stream.

Estimating uncertainty in terrestrial critical loads and their exceedances at four sites in the UK.

Critical loads are the basis for policies controlling emissions of acidic substances in Europe and elsewhere. They are assessed by several elaborate and ingenious models, each of which requires many parameters, and have to be applied on a spatially-distributed basis. Often the values of the input parameters are poorly known, calling into question the validity of the calculated critical loads. This paper attempts to quantify the uncertainty in the critical loads due to this "parameter uncertainty", using examples from the UK. Models used for calculating critical loads for deposition of acidity and nitrogen in forest and heathland ecosystems were tested at four contrasting sites. Uncertainty was assessed by Monte Carlo methods. Each input parameter or variable was assigned a value, range and distribution in an objective a fashion as possible. Each model was run 5000 times at each site using parameters sampled from these input distributions. Output distributions of various critical load parameters were calculated. The results were surprising. Confidence limits of the calculated critical loads were typically considerably narrower than those of most of the input parameters. This may be due to a "compensation of errors" mechanism. The range of possible critical load values at a given site is however rather wide, and the tails of the distributions are typically long. The deposition reductions required for a high level of confidence that the critical load is not exceeded are thus likely to be large. The implication for pollutant regulation is that requiring a high probability of non-exceedance is likely to carry high costs. The relative contribution of the input variables to critical load uncertainty varied from site to site: any input variable could be important, and thus it was not possible to identify variables as likely targets for research into narrowing uncertainties. Sites where a number of good measurements of input parameters were available had lower uncertainties, so use of in situ measurement could be a valuable way of reducing critical load uncertainty at particularly valuable or disputed sites. From a restricted number of samples, uncertainties in heathland critical loads appear comparable to those of coniferous forest, and nutrient nitrogen critical loads to those of acidity. It was important to include correlations between input variables in the Monte Carlo analysis, but choice of statistical distribution type was of lesser importance. Overall, the analysis provided objective support for the continued use of critical loads in policy development.

Recovery from acidification in the Tillingbourne catchment, southern England: catchment description and preliminary results.

Measurements of acid deposition and streamwater chemistry made in 1979-1982 and 1999-2000 are compared for a small, acid-sensitive catchment in Southeast England. The location, geology, soils, vegetation and hydrology of the catchment are described. The catchment is located on an acidic cretaceous sandstone with a low permeability clay sub-stratum. Soils are predominantly podzol and gley, with some mesotrophic peat. The catchment is forested. Mean volume-weighted concentrations in precipitation have changed approximately in proportion to emission changes. SO4(2-) has declined by 61%, H+ by 75%, both NO- and NH4+ by 37% and Cl- by 26%. Changes in wet deposition are greater, sulfate deposition declined by 69%, non-marine SO4(2-) by 73%, H+ deposition by 75%, NO3- and NH4+ by 50% and Cl- by 41%. Sulfate deposition in throughfall, a surrogate for total deposition measurement, has declined by 82% and non-marine SO4(2-) by 86%. Some of these changes are due to alterations in the tree cover and location of the collectors. In 1979-1982, the flux of NO3- and NH4+ in throughfall was less than in rainfall, 7.5 compared with 11.3 kg N ha(-1) year(-1), showing that N uptake by the canopy was greater than dry deposition of these species. However, in 1999-2000, the throughfall flux of N was greater than rainfall, 19.6 compared to 5.7 kg N ha year(-1), indicating that canopy uptake is not occurring to the same extent. Surface water was sampled at the same locations in the catchment during the two periods. At the catchment exit, mean pH increased, from 3.93 to 4.21 mg l(-1), and SO4(2-) declined from 20.2 to 16.7 mg l(-1) (18%). The decrease in SO4(2-) is much less than the reduction in deposition, suggesting that the predicted recovery is being delayed by release of sulfur from the soil. In contrast, NO3- concentrations in the catchment waters increased from 0.22 to 0.52 mg N l(-1) (133%) despite the reduction in N deposition. NH4+ concentrations were low during both study periods. It is concluded that recovery from acidification is probably occurring, but is possibly being delayed by desorption of soil S. The catchment is also showing signs of increasing N saturation, despite a reduction in N inputs.

The effects of excess nitrogen deposition on young Norway spruce trees. Part I the soil.

The effects of wet-deposited nitrogen on soil acidification and the health of Norway spruce were investigated in a pot experiment using an open-air spray/drip system. Nitrogen was applied as ammonium ((NH(4))(2)SO(4)) or nitrate (HNO(3)/NaNO(3)) in simulated rain to either the soil or the foliage for a period of two years five months. Symptoms of forest decline were not reproduced. Adverse effects relating to soil acidification and N saturation were observed and depended on the chemical form of N. The plant-soil system absorbed most of the soil-applied NH(+)(4) at doses of up to 65 kgN ha(-1) year(-1) but only 54% at a dose of 125 kgN ha(-1) year(-1). About 60% of soil-applied NO(-)(3) was absorbed in all treatments. Ammonium treatments acidified the soil, increased base cation leaching, and mobilised acidic cations. Nitrification was not the major source of acidity, however. Nitrate inputs increased soil pH. Critical loads calculated using current criteria were 60-120 and 30-60 kgN ha(-1) year(-1) for ammonium and nitrate, respectively. Ammonium is more likely to damage forest ecosystems, however, illustrating the need for care in the definition of critical loads.

The effects of excess nitrogen deposition on young Norway spruce trees. Part II the vegetation.

The effects of wet-deposited nitrogen on soil acidification and the health of Norway spruce were investigated in a pot experiment using an open-air spray/drip system. Nitrogen was applied as ammonium ((NH(4))(2)SO(4)) or nitrate (HNO(3)/NaNO(3)) in simulated rain to either the soil or the foliage. Symptoms of forest decline as observed in the field were not reproduced, and there was no evidence of direct toxicity. Treatments did, however, have significant effects on tree nutrition. Both NH(+)(4) and NO(-)(3) treatment applied to the foliage lowered foliar K concentrations. NH(+)(4) to a greater extent. Soil-applied NH(+)(4) reduced foliar Mg concentrations and increased foliar Al and Fe. Soil-applied NO(-)(3) significantly reduced foliar P concentrations, and at high doses prevented the alleviation of P deficiency by fertiliser. These effects could be important in some field situations. Ammonium deposition is predicted to be more damaging than nitrate deposition, although the latter may be critical for forests where P status is marginal, such as in parts of the British uplands.

Timescales of recovery from acidification: Implications of current knowledge for aquatic organisms.

Current understanding of the mechanisms of recovery of surface waters from acidification leads to several conclusions which must be considered when devising emission control strategies. Recovery can be regarded as being in two stages: an initial phase in which the waters respond to the reduced SO4(2-) deposition, and a second phase which depends on recovery of the soil base status. If an acceptable water quality is not produced in the first phase, recovery will be extremely slow, taking centuries. This may be true of very sensitive areas such as S Norway. Faster and deeper emission reductions will not significantly speed recovery in these situations: liming is then the only practicable method. Areas where the weathering rate is higher will recover faster, and here recovery may be delayed (by decades) by two poorly understood processes-release of SO4(2-) and release of organic acids from soils. Research on the control of these processes and on the extent of lake resource in each category is urgently needed.

Excess nitrogen deposition: issues for consideration.

This paper briefly reviews some major mechanisms by which deposition of inorganic N compounds from the atmosphere could be damaging forest and natural ecosystems. Twelve issues which needed further discussion or research were identified. These were: has N deposition increased; what is a N-saturated ecosystem; can the time of onset of N saturation be predicted; can fertiliser experiments simulate the effects of atmospheric deposition; are there relationships between N input and N leaching; is N deposition leading to acidification; does high N input lead to toxicity symptoms in trees; does N input increase tree susceptibility to stress; does N input induce nutrient deficiency; does increasing N affect natural plant communities; what are the effects on aquatic ecosystems; can a 'critical load' for protection of ecosystems be defined? There is a brief critical discussion of each issue. It is concluded that there is not enough understanding of ecosystem function to define a critical load objectively, but that limits can be defined for some ecosystems.

Rapporteurs' report on discussions at the workshop on excess nitrogen deposition, Leatherhead, September 1987.

This paper reports major points from the discussions which took place during the Workshop on Excess Nitrogen Deposition.

The effects of ozone and acid mist on Scots pine saplings.

It has been suggested that the forest decline ("Neuartige Waldschäden") seen recently in parts of West Germany is due to the direct effects of ozone combined with acid mists, rather than soil-mediated effects of acid deposition. It has been proposed that ozone (a) makes the needles of affected conifers more susceptible to leaching by acid mist and (b) damages the photosynthetic apparatus, giving rise to diminished carbohydrate reserves which reduce the ability of affected trees to replace the leached nutrients. This nutrient deficiency (especially of Ca and Mg) is a characteristic symptom of the Waldschäden, which progresses through growth decline, needle loss, and eventually death. Parts of this hypothesis were tested in a preliminary experiment in which 3-year old Pinus sylvestris (Scots pine) saplings were exposed to 4 different O3 levels, with and without acid mist (pH 3) treatment, for 56 days between July and September, 1983 in outdoor 'solardome' fumigation chambers. The visual symptoms observed at >100 µg m-3 were more characteristic of the chlorotic mottle seen on O3-affected trees in the USA than the general chlorosis of affected stands in Germany. O3 at mean concentrations of >200 µg m-3 for 56 days reduced the fine root biomass and accelerated the senescence of older needles, in keeping with field effects observed in Germany. However, these O3 levels increased, rather than decreased, the concentrations of most elements in the needles. Acid mist had no effect on needle concentrations, and there was no O3-acid mist interaction. O3 up to 300 µg m-3 also had no effect on the amount of ions leached from the needles, whereas acid mist increased the leaching of some ions, and again there was no interaction. The only nutritional effect of O3 was to reduce the foliar uptake of NO -3 from the acid mist solution. An aphid infection part way through the experiment caused a large increase in leaching, particularly of K, and affected the intermediate O3 and watersprayed plants most. Caution is needed in extrapolating these results to the field, as the experiments were of short duration on young trees with fully-formed needles, growing in a soil better supplied with nutrients than field soils. Nevertheless, these preliminary results do not support the hypothesis of an O3-mediated increase in foliar leaching as the major cause of forest decline nor were the symptoms of O3-injury on Scots pine comparable with those reported in the field.

Chromium uptake and transport in barley seedlings (Hordeum vulgare L.).

Potassium chromate is more toxic to the growth of barley in solution culture than chromic chloride, though apparent uptake of the latter is much faster. Inhibitor studies indicate that CrO4 (2-) uptake is "active" whereas Cr(3+) uptake is passive, demonstrating that the two forms do not share a common uptake mechanism. Studies on the form of Cr inside root cells show that in plants fed CrO4 (2-) the Cr remains largely unchanged whereas in plants fed Cr(3+) a little CrO4 (2-) (0.5 per cent) is produced. This conversion is dependent on the presence of living material and is probably enzymatic. Chromate uptake follows Michaelis-Menten kinetics at low concentration and is competitively inhibited by sulphate. Transport of chromium up the root is very slow, accounting for the low levels of Cr in the shoots. Chromate is transported better than Cr(3+) though still to a very limited extent. These experiments provide a physiological basis for previous observations.

Evidence from inhibitor studies that the endophyte synthesises nitrogenase in the root nodules of Alnus glutinosa L. Gaertn.

Nitrogenase activity (acetylene reduction assay) in the nodulated non-leguminous angiosperm Alnus glutinosa is inhibited within minutes when plants are exposed to a gas phase containing 90% oxygen On returning the plants to air, nitrogenase activity recovers within a few hours, both in the presence of cycloheximide, which inhibits protein synthesis on 80 S (eukaryotic) ribosomes, and in the absence of inhibitor. When chloramphenicol, which inhibits protein synthesis on 70 S (prokaryotic) ribosomes, is added instead of cycloheximide, recovery from oxygen inhibition does not occur, or occurs only slowly. The effects of chloramphenicol are specific to the D-threo-isomer which indicates a direct inhibition of protein synthesis. Erythromycin has a similar effect to chloramphenicol. Protein biosynthesis in non-nodulated roots is inhibited by cycloheximide but not by chloramphenicol. The results are interpreted as evidence that the nitrogenase within Alnus glutinosa root nodules is synthesised by the microbial symbiont.