Productivity in a dominant herbaceous species is largely unrelated to soil macronutrient stocks.

To predict ecosystem responses to anthropogenic change it is important to understand how and where plant productivity is limited by macronutrient availability. Nitrogen (N) is required in large quantities for plant growth, and is readily lost through leaching or gas fluxes, but reactive nitrogen can be obtained through dinitrogen fixation, and phosphorus (P) is often considered a more fundamental long-term constraint to growth and carbon sequestration in terrestrial ecosystems. Phosphorus limitation may be becoming more prevalent due to widespread pollution by atmospheric N. Assessments of the effects of macronutrient availability on productivity in natural ecosystems are however scarce. We measured standing biomass of bracken Pteridium aquilinum as a proxy for productivity across sites with similar climate but varied geology. Total above-ground biomass varied from 404 to 1947gm-2, yet despite 12-fold to 281-fold variation in soil macronutrient stocks these were remarkably poor at explaining variation in productivity. Soil total nitrogen, organic phosphorus, calcium, magnesium and zinc had no relationship with productivity, whether expressed as concentrations, stocks or element/C ratios, and nor did foliar N/P. Soil potassium (K) and molybdenum stocks both showed weak relationships with productivity. The stock of K in bracken biomass was considerably greater as a proportion of soil stock than for other nutrient elements, suggesting that this nutrient element can be important in determining productivity. Moisture availability, as indicated by environmental trait scores for plant species present, explained considerably more of the variation in productivity than did K stock, with less production in wetter sites. Soil N/C ratio and organic P stock were relatively unimportant in determining productivity across these bracken sites. It is possible that more-direct measures of N and P availability would explain variation in productivity, but the study shows the importance of considering other essential elements and other environmental factors when predicting productivity.

Atmospheric deposition of phosphorus to land and freshwater.

We compiled published and newly-obtained data on the directly-measured atmospheric deposition of total phosphorus (TP), filtered total phosphorus (FTP), and inorganic phosphorus (PO4-P) to open land, lakes, and marine coasts. The resulting global data base includes data for c. 250 sites, covering the period 1954 to 2012. Most (82%) of the measurement locations are in Europe and North America, with 44 in Africa, Asia, Oceania, and South-Central America. The deposition rates are log-normally distributed, and for the whole data set the geometric mean deposition rates are 0.027, 0.019 and 0.14 g m(-2) a(-1) for TP, FTP and PO4-P respectively. At smaller scales there is little systematic spatial variation, except for high deposition rates at some sites in Germany, likely due to local agricultural sources. In cases for which PO4-P was determined as well as one of the other forms of P, strong parallels between logarithmic values were found. Based on the directly-measured deposition rates to land, and published estimates of P deposition to the oceans, we estimate a total annual transfer of P to and from the atmosphere of 3.7 Tg. However, much of the phosphorus in larger particles (principally primary biological aerosol particles) is probably redeposited near to its origin, so that long-range transport, important for tropical forests, large areas of peatland and the oceans, mainly involves fine dust from deserts and soils, as described by the simulations of Mahowald et al. (Global Biogeochemical Cycles 22, GB4026, 2008). We suggest that local release to the atmosphere and subsequent deposition bring about a pseudo-diffusive redistribution of P in the landscape, with P-poor ecosystems, for example ombrotrophic peatlands and oligotrophic lakes, gaining at the expense of P-rich ones. Simple calculations suggest that atmospheric transport could bring about significant local redistribution of P among terrestrial ecosystems. Although most atmospherically transported P is natural in origin, local transfers from fertilised farmland to P-poor ecosystems may be significant, and this requires further research.

Life-history effects of arsenic toxicity in clades of the earthworm Lumbricus rubellus.

Exposures of Lumbricus rubellus to a series of arsenic concentrations in soil were used to assess life-stage (juvenile, adult) and genotype specific sensitivities, to calculate population growth rate (λ) and to assess patterns of As accumulation. Significant mortality was seen in juveniles at 125 mg/kg As, while growth and maturation was affected from 36 mg/kg and above. In adults, cocoon production at the highest concentration (125 mg/kg) was significantly reduced. Phylogenetic analysis was performed by comparison of mitochondrial sequences to establish genotypic variation among juveniles. Three clades with more than 7.5% divergent were described, with 70% of earthworms belonging to a single clade. Date of and mass at maturation was significantly different between clades, but clades were not differentially As sensitive. Parameter λ was reduced at 36 mg/kg As and was negative at 125 mg/kg As, suggesting impacts and population stability and potential extinction at environmentally relevant concentrations.

Role of riverine colloids in macronutrient and metal partitioning and transport, along an upland-lowland land-use continuum, under low-flow conditions.

An assessment is made of the role of riverine colloids in macronutrient (nitrogen, phosphorus and carbon), metal and trace element partitioning and transport, for five rivers in the Ribble and Wyre catchments in north-western England, under baseflow/near-baseflow conditions. Cross-flow ultrafiltration was used to separate colloidal (<0.45 µm >1 kDa) and truly dissolved (<1 kDa) fractions from river water. Clear patterns were observed, along the upland-lowland land use continuum, in the partitioning and transport of macronutrients and metals between the colloidal, truly dissolved and acid-available particulate (>0.45 µm, suspended) fractions. Of these operationally-defined fractions measured, colloids were generally more important for both macronutrient and metal transport in the upland than in the lowland rivers. The results suggest that organic moieties in truly dissolved form from sewage effluent may have a greater capacity to chelate metals. Organic-rich colloids in the upland moorlands and metal oxide colloidal precipitates in the industrial rivers had a higher capacity for binding metals than the colloidal fractions in the urban and agricultural lowland rivers. Aggregation of these colloids may provide an important mechanism for formation of larger suspended particulates, accounting for a higher degree of metal enrichment in the acid-available particulate fractions of the upland moorland and lowland industrial rivers, than in the lowland agricultural and urban rivers. This mechanism of transfer of contaminants to larger aggregates via colloidal intermediates, known as 'colloidal pumping' may also provide a mechanism for particulate P formation and the high proportion of P being transported in the particulate fraction in the uplands. The cross-flow ultrafiltration data also allowed refinement of partition coefficients, by accounting for colloids within the solids phase and replacing the filtered (<0.45 µm) fraction with the truly dissolved (<1 kDa) concentrations. These provided a clearer description of the controls on metal and P partitioning along the upland-lowland continuum.

Manganese in the upper Severn mid-Wales.

The concentrations of manganese (Mn) in the Upper River Severn (the Plynlimon catchments) are examined in relation to rainfall, cloud water, throughfall, stemflow and stream water concentrations where there is over 20 years of monitoring data available. Manganese concentrations are particularly low in rainfall and cloud water, with maximum concentrations occurring under low volumes of catch due to atmospheric "washout" of contaminants and dry deposition. There is strong Mn enrichment in throughfall and stemflow and this is probably linked to cycling through the vegetation. Manganese in the streams and groundwaters are primarily supplied from within-catchment sources. The highest concentrations occur within the tree canopy probably due to element cycling and in groundwaters due to mobilisation from the rock. Manganese concentrations in streams are at their lowest during spring and summer following long dry spells, with rapid increases following subsequent rain. There is no clear long-term trend in Mn concentration in the streams although there are increases in Mn concentrations for years when there is extensive felling of spruce plantation forest and in 1995 following a more extensive dry period. New high resolution monitoring picks up the effects of the rising limb of the hydrograph when concentrations rapidly increase, diurnal patterns during summer low-flow periods and contrasting dynamics between moorland and forested catchments.

Mercury in United Kingdom topsoils; concentrations, pools, and Critical Limit exceedances.

The median total mercury concentration in 898 UK rural topsoils, sampled between 1998 and 2008, was 0.095 µg g(-1). Approximate adjustment for unreactive metal produced an estimate of 0.052 µg g(-1) for reactive Hg. The highest concentrations were in the north and west, where organic-rich soils with low bulk densities dominate, but the spatial pattern was quite different if soil Hg pools (mg m(-2)) were considered, the highest values being near to the industrial north of England and London. Possible toxic effects of Hg were best evaluated by comparison with soil Critical Limits expressed as ratios of Hg to soil organic matter, or soil solution Hg(2+) concentrations, estimated by chemical speciation modelling. Only a few percent of the rural UK soils showed exceedance, and this also applied to rural soils from the whole of Europe. UK urban and industrial soils had higher Hg concentrations and more cases of exceedance.

The biogeochemistry of arsenic in a remote UK upland site: trends in rainfall and runoff, and comparisons with urban rivers.

Ten years of monitoring of rainfall and streams in the remote acidic and acid sensitive moorland and afforested moorland of upland mid-Wales reveals concentrations of arsenic (As) typically <1 µg L(-1). On average, the lowest concentrations occur within rainfall and they have declined over time probably in response to reductions in global emissions. There is a corresponding reduction within the streams except for forested systems where concentrations up to doubled following clear-fell. Within the streams there are both annual cycling and diurnal cycling of As. The annual cycling gives maxima during the summer months and this probably reflects the importance of groundwater inputs and mineralisation/desorption from the surface soil layers. Correspondingly, the diurnal cycling occurs during the summer months at low flow periods with As concentrations highest in the afternoon/evening. For the urban/industrial basins of northern England with historically a much higher As deposition, land contamination and effluent discharges, comparative data indicate As concentrations around three fold higher: strong seasonal patterns are observed for the same reasons as with the uplands. Across the sites, the As concentrations are over an order of magnitude lower than that of environmental concern. Nonetheless, the results clearly show the effects of declining emissions on rainfall deposition and some indication of areas of historic contamination. Arsenic is mainly present in the <0.45 fraction, but cross-flow filtration indicates that approx. 43% is in the colloidal phase at the clean water sites, and 16% in the colloidal phase at the contaminated sites. Part of this colloidal component may well be associated with organic carbon.

Background wet deposition of mercury in Great Britain.

Precipitation samples have been collected on a monthly basis from a network of 10 sites in Great Britain (GB) in order to estimate background mercury (Hg) deposition in the rural environment. Collection started in February 2005 and results presented here cover the period up to June 2009. The annual volume-weighted mean (AVWM) Hg concentrations range from 1.0 ng L(-1) at Cockley Beck in the Lake District in 2006 to 8.8 ng L(-1) at Heigham Holmes on the Norfolk Coast England in 2008. The largest validated solution concentrations were 33.7 ng L(-1) Hg measured at Cockley Beck in May 2008. The large difference in rainfall amount between sites means that the remote site at Cockley Beck has both the lowest long-term AVWM concentration (1.6 ng L(-1)) and the greatest annual flux is greatest at 43 mg ha(-1) yr(-1). Predicted deposition tends to be much greater in western Britain where the greater rainfall occurs. Because some observations are very close to the analytical detection limit (1.0 ng L(-1)), rigorous cleaning procedures, the use of replicate samplers to monitor contamination, and the inclusion of 'bottle blanks' are required to obtain valid measurements of Hg in the bulk deposition. Deposition in rural sites is equivalent to about 10% of the estimated magnitude of known emissions in GB.

Dynamic modelling of atmospherically-deposited Ni, Cu, Zn, Cd and Pb in Pennine catchments (northern England).

Simulation modelling with CHUM-AM was carried out to investigate the accumulation and release of atmospherically-deposited heavy metals (Ni, Cu, Zn, Cd and Pb) in six moorland catchments, five with organic-rich soils, one with calcareous brown earths, in the Pennine chain of northern England. The model considers two soil layers and a third layer of weathering mineral matter, and operates on a yearly timestep, driven by deposition scenarios covering the period 1400-2010. The principal processes controlling heavy metals are competitive solid-solution partitioning of solutes, chemical interactions in solution, and chemical weathering. Agreement between observed and simulated soil metal pools and surface water concentrations for recent years was generally satisfactory, the results confirming that most contemporary soil metal is from atmospheric pollution. Metals in catchments with organic-rich soils show some mobility, especially under more acid conditions, but the calcareous mineral soils have retained nearly all anthropogenic metal inputs. Complexation by dissolved organic matter and co-transport accounts for up to 80% of the Cu in surface waters.

Metal accumulation by stream bryophytes, related to chemical speciation.

Metal accumulation by aquatic bryophytes was investigated using data for headwater streams of differing chemistry. The Windermere Humic Aqueous Model (WHAM) was applied to calculate chemical speciation, including competitive proton and metal interactions with external binding sites on the plants. The speciation modelling approach gives smaller deviations between observed and predicted bryophyte contents of Cu, Zn, Cd and Pb than regressions based on total filtered metal concentrations. If all four metals, and Ni, are considered together, the WHAM predictions are superior at the 1% level. Optimised constants for bryophyte binding by the trace metals are similar to those for humic substances and simple carboxylate ligands. Bryophyte contents of Na, Mg and Ca are approximately explained by binding at external sites, while most of the K is intracellular. Oxide phases account for some of the Al, and most of the Mn, Fe and Co.

Simulating the long-term chemistry of an upland UK catchment: heavy metals.

CHUM-AM was used to investigate the behaviours of atmospherically-deposited heavy metals (Ni, Cu, Zn, Cd and Pb) in three moorland sub-catchments in Cumbria UK. The principal processes controlling cationic metals are competitive partitioning to soil organic matter, chemical interactions in solution, and chemical weathering. Metal deposition histories were generated by combining measured data for the last 30 years with local lake sediment records. For Ni, Cu, Zn and Cd, default parameters for the interactions with organic matter provided reasonable agreement between simulated and observed present-day soil metal pools and average streamwater concentrations. However, for Pb, the soil binding affinity in the model had to be increased to match the observations. Simulations suggest that weakly-sorbing metals (Ni, Zn, Cd) will respond on timescales of decades to centuries to changes in metal inputs or acidification status. More strongly-sorbing metals (Cu, Pb) will respond over centuries to millennia.

Simulating the long-term chemistry of an upland UK catchment: major solutes and acidification.

CHUM-AM was used to investigate changes in soil and water chemical variables in four moorland sub-catchments in Cumbria UK, to which non-marine S deposition has declined by 65% since the 1970s. The principal processes represented in the model comprise N and S uptake and release, water movements, the binding of cations by soil organic matter, chemical interactions in solution, and chemical weathering. CHUM-AM reproduced reasonably well the current soil pH and pools of N and S, and changes in streamwater chemistry over the period 1970-2000, notably decreases in the concentrations of alkaline earth cations and sulphate, and increases in pH. The model also predicts streamwater pH-Al relationships in agreement with observations. Predictive calculations suggest that constant atmospheric deposition of N at present rates will lead to N saturation and re-acidification, whereas a 50% reduction in N would stabilise soil and streamwater pH at about the present levels.

Potentially toxic metals in ombrotrophic peat along a 400 km English-Scottish transect.

Four samples of ombrotrophic peat were collected from each of 10 upland locations in a transect from the southern Pennines to the Highland Boundary Fault, a total distance of ca. 400 km. Bulk compositions and other properties were determined. Total contents of Al and heavy metals (Ni, Cu, Zn, Cd, Pb) were determined following digestion with hydrofluoric acid, and concentrations of metals extractable with dilute nitric acid were also measured. Supernatants obtained from aqueous extractions of the peat samples were analysed for pH, major cations and anions, dissolved organic carbon and dissolved metals, and concentrations of free metal ions (Al(3+), Ni(2+), etc.) were estimated by applying a chemical speciation model. Both total and HNO(3)-extractable metal concentrations varied along the transect, the highest values being found at locations close to industrial and former mining areas. The HNO(3)-extractable soil metal contents of Ni, Cu and Cd were appreciably lower than lowest-observed-effect-concentrations (LOEC) for toxicity towards microorganisms in acid, organic rich soils. However, the contents of Zn at two locations, and of Pb at five locations exceeded LOECs, suggesting that they may be exerting toxic effects in the peats. Soil solution concentrations of free heavy metal ions (Cu(2+), Zn(2+), Cd(2+), Pb(2+)) were substantially lower than LOECs for toxicity towards vascular plants, whereas concentrations of Al(3+) were near to toxic levels at two locations.

Predicting the release of metals from ombrotrophic peat due to drought-induced acidification.

Ombrotrophic peats in northern England and Scotland, close to industrial areas, have substantial contents of potentially toxic metals (Al, Ni, Cu, Zn, Cd and Pb) and of pollutant sulphur, all derived from atmospheric deposition. The peat sulphur, ordinarily in reduced form, may be converted to sulphuric acid under drought conditions, due to the entry of oxygen into the peats. The consequent lowering of soil solution pH is predicted to cause the release of metals held on ligand sites of the peat organic matter. The purpose of the present study was to explore, by simulation modelling, the extent of the metal response. Chemical variables (elemental composition, pH, metal contents) were measured for samples of ombrotrophic peats from three locations. Water extracts of the peats, and samples of local surface water, were also analysed, for pH, dissolved organic carbon (DOC) and metals. Metal release from peats due to acidification was demonstrated experimentally, and could be accounted for reasonably well using a speciation code (WHAM/Model VI). These data, together with information on metal and S deposition, and meteorology, were used to construct a simple description of peat hydrochemistry, based on WHAM/Model VI, that takes into account ion-binding by humic substances (assumed to be the "active" constituents of the peat with respect to ion-binding). The model was used to simulate steady state situations that approximated the observed soil pH, metal pools and dissolved metal concentrations. Then, drought conditions were imposed, to generate increased concentrations of H2SO4, in line with those observed during the drought of 1995. The model calculations suggest that the pH will decrease from the initial steady state value of 4.3 to 3.3-3.6 during rewetting periods following droughts, depending upon assumptions about the amount of potentially mobile soil S. The pH decreases will be accompanied by increases in concentrations of dissolved metals (Mg, Al, Ca, Ni, Cu, Zn, Cd, Pb) of an order of magnitude or more, depending upon assumptions about the replenishment of soil metal pools by deposition. In the most realistic scenario for present conditions, the severity of pH depressions will gradually decline due to the relatively slow depletion of the soil S pool by droughts. However, the magnitudes of heavy metal pulses will decline quite rapidly (over two or three droughts) because current and future metal deposition is unable to compensate for leaching losses from the soil pools.

Metals in bulk deposition and surface waters at two upland locations in northern England.

Concentrations of aluminium and minor metals (Mn, Ni, Cu, Zn, Sr, Cd, Ba, Pb) were measured in precipitation and surface water at two upland locations (Upper Duddon Valley, UDV; Great Dun Fell, GDF) in northern England for 1 year commencing April 1998. At both locations, the loads in bulk precipitation were at the lower ends of ranges reported for other rural and remote sites, for the period 1985-1995. The deposited metals were mostly in the dissolved form, and their concentrations tended to be greatest when rainfall volumes were low. The concentrations of Cu, Zn and Pb in deposition were correlated (r2 > or = 0.40) with concentrations of non-marine sulphate. Three streams, ranging in mean pH from 5.07 to 7.07, and with mean concentrations of dissolved organic carbon (DOC) < 1 mg l(-1). were monitored at UDV, and two pools (mean pH 4.89 and 6.83, mean DOC 22 and 15 mg l(-1)) at GDF. Aluminium and the minor metals were mainly in the dissolved form, and in the following ranges (means of 49-51 samples. microg l(-1)): Al 36-530. Mn 4.4-36, Ni 0.26-2.8, Cu 0.25-1.7, Zn 2.1-30, Cd 0.03-0.16, Ba 1.9-140, Pb 0.10-4.5. Concentrations were generally higher at GDF. Differences in metal concentrations between the two locations and between waters at each location, and temporal variations in individual waters, can be explained qualitatively in terms of sorption to solid-phase soil organic matter and mineral surfaces, complexation and transport by DOC, and chemical weathering. The UDV catchments are sinks for Pb and sources of Al, Mn, Sr, Cd and Ba. The GDF catchments are sources of Al, Mn, Ni, Zn, Sr, Cd and Ba. Other metals measured at the two locations are approximately in balance. Comparison of metal:silicon ratios in the surface waters with values for silicate rocks indicates enrichment of Ni and Cu, and substantial enrichment of Zn, Cd and Pb. These enrichments, together with high metal deposition in the past, make it likely that concentrations of the metals in the surface waters are governed by release from catchment pools of atmospherically-deposited metal. The catchments appear to be responding on a time scale of decades, possibly centuries, to changes in metal deposition. For the more acid waters at UDV, the calculated free-ion concentrations of Al are similar to published LC50 values for acute toxicity towards fish. The free-ion concentrations of Ni, Cu, Zn and Cd in all the surface waters are one-to-four orders of magnitude lower than reported LC50 values for fish.

Accumulation of Al, Mn, Fe, Cu, Zn, Cd and Pb by the bryophyte Scapania undulata in three upland waters of different pH.

Measurements were made of the contents of Al, Mn, Fe, Cu, Zn, Cd and Pb in Scapania undulata in three streams (D2, D5, D11) in the English Lake District. The stream waters had average pH values of 5.35 (D2), 5.81 (D5) and 7.26 (D11), the main differences in other major chemical components being in Mg, Al, Ca and alkalinity. There was generally more metal accumulation in the older parts of the plants, but this was not significant in all cases. Extents of accumulation varied with stream pH and dissolved metal concentration. For Al, accumulation was greatest in streams D2 and D5. Mn accumulated most in D5 and Fe was without preference. Cu, Zn and Cd accumulated mostly in the plants in stream D11 and Pb accumulated more in D5 and D11. In terms of enrichment factors (amount of metal in the plants divided by stream water concentration) the sequence was Zn < Cd < Cu < Mn < Pb < Al < Fe. Laboratory experiments supported the findings of the field data, providing evidence that uptake increases with pH at constant total metal concentration. The results are interpreted qualitatively in terms of the chemical speciation of the metals in the stream water and competition between metal ions and protons at the plant-water interface. It is suggested that Al, Cu, Zn, Cd and Pb behave according to chemical complexation, whereas redox processes and/or colloidal interactions may be significant for Mn and Fe.

Reversal of acidification in tributaries of the River Duddon (English Lake District) between 1970 and 1998.

Long-term changes in stream water chemistry in the upper Duddon catchment (southwest Lake District, UK) were investigated. Ten streams were sampled and analysed weekly during 1998, and the results compared with data for the early 1970s and 1986. The waters exhibited a range of pH, average values for 1998 being 5.04-7.04. For all the streams, the average pH in 1998 was greater than that during 1971-73. Statistical analysis was carried out, using the 1970s data to estimate the magnitude of inter-annual variation, and taking discharge into account on the basis of antecedent rainfall. The results showed that for two of the streams the pH increase was significant at the 2.5% level, while for a further three it was significant at the 10% level. Comparison of the 1998 concentrations of nitrate and non-marine sulphate with data obtained for five streams in 1973-74 showed that average nitrate concentration had increased from 11 to 20 microeq dm(-3) while that of non-marine sulphate had decreased from 94 to 50 microeq dm(-3). For four of the streams, comparisons were also made between the 1998 data and those for 1986. In three cases, pH in 1998 was generally higher, and Al generally lower, than the values for 1986, but in the fourth case little difference was evident. The present results support observations for five nearby standing waters, strengthening the evidence for a general reversal of acidification in the southwest part of Lake District, due to a decline in the deposition of pollutant sulphur.