Rapid amino acid cycling in arctic and antarctic soils

Amino acids constitute one of the largest inputs of organic nitrogen (N) to most polar soils and have been hypothesized to be important in regulating vegetational succession and productivity in Arctic ecosystems. Our understanding of amino acid cycling in these soils, however, is poor. The aim of this study was to investigate the size and rate of turnover of the amino acid pool in a range of Arctic and Antarctic soils. Our results indicate that in polar soils with either high or low ornithogenic inputs the amino acid pool is small in comparison to the inorganic N pool (NO^? ₃ and NH⁺ ₄). The free amino acid pool constituted only a small proportion of the total dissolved organic nitrogen (DON) pool in these soils. Here we show that these low concentrations may be due to rapid use by the soil microbial community in both Arctic and Antarctic soils. The turnover of the amino acid pool in soil was extremely rapid, with a half-life ranging from 2 to 24 h, indicating that this N pool can be turned over many hundred times each summer when polar soils are frequently unfrozen. The implications of amino acids in N cycling and plant and microbial nutrition are discussed.     

Impact of land use on soluble organic nitrogen in soil

Although it has been hypothesized that soluble organic nitrogen (SON) plays a central role in regulating productivity in some terrestrial ecosystems, the factors controlling the size of the SON pool in soil remain poorly understood. Therefore our principal aim in this work was to assess the impact of seven different land use systems (rough and managed grassland, deciduous and coniferous woodland, heathland, wetland and tilled land) on the size of the SON and inorganic N (NO^? ₃, NH⁺ ₄) pools in the surface soil layer (0–15 cm). After extraction with deionised water, we found that in most cases the size of the water extractable organic N (WEON) pool was similar in size to the inorganic N pool. In contrast, the KCl extractable organic N (KClEON) pool constituted the dominant form of soluble N in soils under all land uses, perhaps indicating that significant amounts were held on the soil exchange phase. In contrast to inorganic N, which varied significantly with land use, the size of the KClEON and WEON pool was similar for all land uses with the exception of KClEON in tilled land, where significantly lower amounts were observed. We conclude that SON constitutes an important soil N pool in a broad range of land uses, and that its role in microbial N assimilation, plant nutrition and ecosystem responses to atmospheric N deposition warrants further attention.     

Impact of Land Use on Soluble Organic Nitrogen in Soil

Although it has been hypothesized that soluble organic nitrogen (SON) plays a central role in regulating productivity in some terrestrial ecosystems, the factors controlling the size of the SON pool in soil remain poorly understood. Therefore our principal aim in this work was to assess the impact of seven different land use systems (rough and managed grassland, deciduous and coniferous woodland, heathland, wetland and tilled land) on the size of the SON and inorganic N (NO ₃ ^– , NH ₄ ⁺ ) pools in the surface soil layer (0–15 cm). After extraction with deionised water, we found that in most cases the size of the water extractable organic N (WEON) pool was similar in size to the inorganic N pool. In contrast, the KCl extractable organic N (KClEON) pool constituted the dominant form of soluble N in soils under all land uses, perhaps indicating that significant amounts were held on the soil exchange phase. In contrast to inorganic N, which varied significantly with land use, the size of the KClEON and WEON pool was similar for all land uses with the exception of KClEON in tilled land, where significantly lower amounts were observed. We conclude that SON constitutes an important soil N pool in a broad range of land uses, and that its role in microbial N assimilation, plant nutrition and ecosystem responses to atmospheric N deposition warrants further attention. SAFRD, University of Newcastle, Newcastle-upon-Tyne, NE1 7RU, U.K.     

Controls on leaching of N species in upland moorland catchments

Spatial and temporal changes in mobility of N species have been studied for three UK upland river networks, the Etherow in the South Pennines, the Nether Beck in the Lake District and the Dee in NE Scotland. The catchments are subject to N deposition at 35.1, 22.0 and 10.8–15.6 kg N ha^?1 yr^?1, respectively. The NH⁺ ₄ leaching appears to be predominantly regulated by flow path in more polluted upland catchments. It is greatest where water draining acidified peaty soils contributes more to total discharge. Soluble organic matter may provide the dominant counter anion. In the Etherow and Dee catchments, which are dominated by acid mineral and organic soils, at high discharge NO^? ₃ also appears to be associated with greater input of water from acidified soils. In contrast, for the Nether Beck, higher NO^? ₃ concentrations are associated with tributaries draining soils contributing water with higher alkalinity, suggesting nitrification is important. For the Etherow and Dee, dissolved organic N (DON) appears to originate predominantly from acidified, peaty soils. Spiking experiments with peat soil from the Etherow catchment confirmed the limited capacity of these soils to utilize inorganic N inputs, favouring equilibration with NH⁺ ₄ inputs and leaching losses of inorganic N throughout the year.     

Controls on Leaching of N Species in Upland Moorland Catchments

Spatial and temporal changes in mobility of N species have been studied for three UK upland river networks, the Etherow in the South Pennines, the Nether Beck in the Lake District and the Dee in NE Scotland. The catchments are subject to N deposition at 35.1, 22.0 and 10.8–15.6 kg N ha^–1 yr^–1, respectively. TheNH ₄ ⁺ leaching appears to be predominantly regulated by flowpath in more polluted upland catchments. It is greatest where water draining acidified peaty soils contributes more to total discharge. Soluble organic matter may provide the dominant counter anion. In the Etherowand Dee catchments, which are dominated by acid mineral and organic soils, at high discharge NO ₃ ^– also appears to be associated with greater input of water from acidified soils. In contrast, for the Nether Beck, higher NO ₃ ^– concentrations are associated with tributaries draining soils contributingwater with higher alkalinity, suggesting nitrification is important. For the Etherow and Dee, dissolved organic N (DON) appears to originate predominantly from acidified, peaty soils. Spiking experiments with peat soil from the Etherow catchment confirmed the limited capacity of these soils to utilize inorganic N inputs, favouring equilibration with NH ₄ ⁺ inputs and leaching losses of inorganic N throughout the year.     

Fluxes of NO3-, NH4+, NO,NO2, and N2O in an Old Danish Beech Forest

The fluxes of the major nitrogen compounds havebeen investigated in many ecosystem studies over the world.However, only in few studies has attention been drawn to theimportance of the fluxes of minor gaseous nitrogen compoundsto complete the nitrogen cycle. In Denmark a detailed study onthe nitrogen cycle in an old beech forest has been implementedin 1997 at Gyrstinge near Sorø, Zealand. The study includesthe fluxes of the gases NO, N₂O and water mediatedtransport of NO₃ ^- and NH₄ ⁺. Measurementsof the fluxes of the gaseous compounds are performed withmicro-meteorological methods (eddy-correlation and gradient)and with chambers. Water mediated fluxes encompass rain,throughfall, stem-flow and leaching from the root zone. Thehydrological model is verified by TDR measurements. The findings show that the total water mediated N input tothe forest floor with throughfall and stemflow was 25.6 kg Nha^-1 yr ^-1, and open field wet deposition withprecipitation was 19.0 kg N ha^-1 yr ^-1. The internalcycling of N in the ecosystem measured as turnover oflitterfall and plant uptake was 100 kg N ha^-1 yr ^-1and 14 kg N ha^-1 yr ^-1, respectively. The fluxes ofthe gaseous N compounds NO and N₂O were of minorimportance for the total N turnover in the forest, NO_xemission being <1 kg N ha^-1 yr ^-1 and N₂Oemission from the soil being 0.5 kg N ha^-1 yr ^-1 withno significant difference between wet and dry soils.Concentrations of NO₃ ^- and NH₄ ⁺ in thesoil solution beneath the rooting zone are very small andconsequently the N leaching is almost negligible. It isconcluded that the nitrogen mass balance of this old beechforest ecosystem mainly is controlled by the input by dry andwet deposition and a large internal N cycle with a fast litterturnover. The nitrogen input tothe forest ecosystem which currently exceeds the critical loadby 5 kg N ha^-1 yr ^-1is mainly accumulated in the soil and no significant nitrateleaching is occurring.     

Biodegradation of Low Molecular Weight Organic Acids in a Limed Forest Soil

The effect of liming (3.45 and 8.75 t ha^-1 dolomite; 16 yr after application) on the biodegradation of three low molecular weight organic acids (citrate, oxalate and propionate) in forest soils was investigated. The concentration of organic acids in the soil solution followed the series propionate > citrate > oxalate with liming having no significant impact on soil solution concentrations (mean organic acid concentration = 8.7 ± 2.3 M). Organic acid mineralization by the soil microbial community was rapid in surface organic horizons (mean half-life for citrate = 2–6 h), with biodegradation rate gradually declining with soil depth. Concentration-dependent biodegradation studies (0 to 350 M) showed that the mineralization kinetics generally conformed well to a single Michaelis–Menten equation with V_max values following the series oxalate > citrate > propionate (mean = 9.8 ± 1.0 nmol g^-1 h^-1) and K_M values following the series oxalate = citrate > propionate (mean 168 ± 25 M). The V_max values declined with soil depth, which was consistent with a general reduction in microbial activity down the soil profile. Liming induced a significant increase in V_max for citrate with no change for propionate and reduction in V_max for oxalate. The latter was probably due to adsorption and precipitation of Ca-oxalate making it unavailable for microbial uptake. The higher adsorption/precipitation capacity for oxalate in the limed soils was confirmed by adsorption isotherms. Generally, liming increased soil microbial activity by approximately 10 to 35% with calculations based on soil solution concentrations indicating that organic acid mineralization constituted approximately 3 to 15% of the total soil respiration.     

Effects of Nitrogen Supply on the Sensitivity to O3 of Growth and Photosynthesis of Japanese Beech (Fagus crenata) Seedlings

To obtain basic information for evaluating critical levels of O₃ under different nitrogen loads for protecting Japanese beech forests, two-year-old seedlings of Fagus crenata Blume were grown in potted andisol supplied with N as NH₄NO₃ solution at 0, 20 or 50 kg ha⁻¹ year⁻¹ and exposed to charcoal-filtered air or O₃ at 1.0, 1.5 and 2.0 times the ambient concentration from 16 April to 22 September 2004. The O₃ induced significant reductions in the whole-plant dry mass, net photosynthetic rate at 380 μmol mol⁻¹ CO₂ (A ₃₈₀), carboxylation efficiency (CE) and concentrations of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and total soluble protein (TSP) in the leaves. The concentrations of Rubisco and TSP were negatively correlated with the concentration of leaf acidic amino acid, suggesting that O₃ enhanced the degradation of protein such as Rubisco. The N supply to the soil did not significantly change the whole-plant dry mass and A ₃₈₀, whereas it significantly increased the CE and concentrations of Rubisco and total amino acid. No significant interactive effects of O₃ and N supply to the soil were detected on the growth, photosynthetic parameters and concentrations of protein and amino acid in the leaves. In conclusion, N supply to the soil at ≤50 kg ha⁻¹ year⁻¹ does not significantly change the sensitivity to O₃ of growth and net photosynthesis of Fagus crenata seedlings.     

The Influence of Atmospheric N Deposition on Nitrous Oxide and Nitric Oxide Fluxes and Soil Ammonium and Nitrate Concentrations

The deposition of atmospheric N to soils provides sources of available N to the nitrifying and denitrifying microbial community and subsequently influences the rate of NO and N₂O emissions from soil. We have investigated the influence of three different sources of enhanced N deposition on NO and N₂O emissions 1) elevated NH₃ deposition to woodlands downwind of poultry and pig farms, 2) increased wet cloud and occult N deposition to upland forest and moorland and 3) enhanced N deposition to trees as NO ₃ ^– and NH ₄ ⁺ aerosol. Flux measurements of NO and N₂O were made using static chambers in the field or intact and repacked soil cores in the laboratory and determination of N₂O by gas chromatography and of NO by chemiluminescence analysis. Rates of N deposition to our study sites were derived from modelled estimates of N deposition, NH₃ concentrations measured by passive diffusion and inference from measurements of the ²¹⁰Pb inventory of soils under tree canopies compared with open grassland. NO and N₂O emissions and KCl-extractable soil NH ₄ ⁺ and NO ₃ ^– concentrations all increased with increasing N deposition rate. The extent of increase did not appear to be influenced by the chemical form of the N deposited. Systems dominated by dry-deposited NH₃ downwind of intensive livestock farms or wet-deposited NH ₄ ⁺ and NO ₃ ^– in the upland regions of Britain resulted in approximately the same linear response. Emissions of NO and N₂O from these soils increased with both N deposition and KCl extractable NH ₄ ⁺ , but the relationship between NH ₄ ⁺ and N deposition (ln NH ₄ ⁺ = 0.62 ln N_deposition + 0.21, r ² = 0.33, n = 43) was more robust than the relationship between N deposition and soil NO and N₂O fluxes.     

The influence of atmospheric n deposition on nitrous oxide and nitric oxide fluxes and soil ammonium and nitrate concentrations

The deposition of atmospheric N to soils provides sources of available N to the nitrifying and denitrifying microbial community and subsequently influences the rate of NO and N₂O emissions from soil. We have investigated the influence of three different sources of enhanced N deposition on NO and N₂O emissions 1) elevated NH₃ deposition to woodlands downwind of poultry and pig farms, 2) increased wet cloud and occult N deposition to upland forest and moorland and 3) enhanced N deposition to trees as NO^? ₃ and NH⁺ ₄ aerosol. Flux measurements of NO and N₂O were made using static chambers in the field or intact and repacked soil cores in the laboratory and determination of N₂O by gas chromatography and of NO by chemiluminescence analysis. Rates of N deposition to our study sites were derived from modelled estimates of N deposition, NH₃ concentrations measured by passive diffusion and inference from measurements of the ²¹⁰Pb inventory of soils under tree canopies compared with open grassland. NO and N₂O emissions and KCl-extractable soil NH⁺ ₄ and NO^? ₃ concentrations all increased with increasing N deposition rate. The extent of increase did not appear to be influenced by the chemical form of the N deposited. Systems dominated by dry-deposited NH₃ downwind of intensive livestock farms or wet-deposited NH⁺ ₄and NO^? ₃ in the upland regions of Britain resulted in approximately the same linear response. Emissions of NO and N₂O from these soils increased with both N deposition and KCl extractable NH⁺ ₄, but the relationship between NH⁺ ₄ and N deposition (ln NH⁺ ₄ = 0.62 ln N_deposition+0.21, r ² = 0.33, n = 43) was more robust than the relationship between N deposition and soil NO and N₂O fluxes.     

High Immobilization of NH4+ in Danish Heath Soil Related to Succession,Soil and Nutrients: Implications for Critical Loads of N

During recent decades heathlands havechanged into grasslands in regions with high atmosphericnitrogen deposition. In regions with intermediatedeposition level (e.g., Denmark) changes have been lesspronounced which may be due to delay or decrease inresponse of the ecosystem. The mor layer (O horizon) mayplay an important role for this delay due to high sinkstrength for N. In this study, the capacity for netNH₄ ⁺ immobilization and mineralization wasstudied during short- and long-term incubations (2–36 days)of mor samples from Danish dry inland heaths. High short-term capacity for net NH₄ ⁺ immobilization wasfound to be a general characteristic of Danish heath morlayers both under heather (Calluna vulgaris) andcrowberry (Empetrum nigrum ssp nigrum), the latterdominating late stages in heathland succession. The netNH₄ ⁺ immobilization was higher under youngcompared to old or dead vegetation, and higher on lessnutrient poor soils than on extremely nutrient poor soils.The addition of N, P and C stimulated CO₂ productionand net NH₄ ⁺ immobilization, but not net Nmineralization. The immobilization of ¹⁵NH₄ ⁺caused release of dissolved organic N, increased N anddecreased C/N ratio in the microbial biomass, and indicatedgrowth of microorganisms with other metabolic abilitiesthan the indigenous population. No evidence was obtained ofstabilization of immobilized ¹⁵NH₄ ⁺ intosoil organic matter during the experiment. On background ofthe results and current knowledge it was concluded that therecognition of the high capacity for net NH₄ ⁺immobilization in mor layers does not allow for a raiseof critical loads for N for northern dry inland heaths.     

A conceptual model of spatially heterogeneous nitrogen leaching from a welsh moorland catchment

Soil- and stream-water data from the Plynlimon research area, mid-Wales, have been used to develop a conceptual model of spatial variations in nitrogen (N) leaching within moorland catchments. Extensive peats, in both hilltop and valley locations, are considered near-complete sinks for inorganic N, but leach the most dissolved organic nitrogen (DON). Peaty mineral soils on hillslopes also retain inorganic N within upper organic horizons, but a proportion percolates into mineral horizons as nitrate (NO^? ₃), either through incomplete immobilisation in the organic layer, or in water bypassing the organic soil matrix via macropores. This NO^? ₃ reaches the stream where mineral soilwaters discharge (via matrix throughflow or pipeflow) directly to the drainage network, or via small N-enriched flush wetlands. NO^? ₃ in hillslope waters discharging into larger valley wetlands will be removed before reaching the stream. A concept of catchment ‘nitrate leaching zones’ is proposed, whereby most stream NO^? ₃ derives from localised areas of mineral soil hillslope draining directly to the stream; the extent of these zones within a catchment may thus determine its overall susceptibility to elevated surface water NO^? ₃ concentrations.     

Effects of Long-Term Application of Ammonium Sulphate on Nitrogen Fluxes in a Beech Ecosystem at Solling, Germany

To study the effects of elevated inputs of acidity and nitrogen (N), 1000 mmol m^-2 a^-1 of ammonium sulphate (NH₄NO₃) equivalent to an input of potential acidity of 2000 mmol m^-2 a^-1 was applied annually for 11 yr between 1983 and 1993 in a beech forest at Solling, Germany. Most of the applied NH₄ ⁺ was nitrified in the litter layer and in the upper mineral soil. N in soil leachate quickly responded to the elevated input, but most of the applied N was stored in the soil or left the ecosystem via pathways other than soil output. Leaching of N from the soil increased until the last year of N addition. After the last N application, N fluxes decreased rapidly to low values. The buffering of acidity produced by the nitrification of the applied NH₄ ⁺ was caused mainly by three different processes: (i) sulphur (S) retention, (ii) release of aluminium, (iii) release of base cations. Retention of S took place mostly in the subsoil. 72% of the S input was recovered in output after 14 years of the experiment. Due to the increased fluxes of mobile anions with soil solution, outputs of cations increased drastically.     

The effect of nitrogen enrichment on the carbon sink in coniferous forests: Uncertainty and sensitivity analyses of three ecosystem models

Estimates of the global carbon sink induced by nitrogen enrichment range vary widely, from nearly zero to 2.3 Gt C year^?1. It is necessary to reduce this uncertainty if we are to make accurate predictions of the future magnitude of the terrestrial carbon sink. Here, we present a Monte Carlo approach to uncertainty and sensitivity analysis of three ecosystem models, Century, BGC and Hybrid. These models were applied to a coniferous forest ecosystem in Sweden. The best estimate of the change in total carbon content of the ecosystem with the cumulative change in nitrogen deposition over 100 years, ΔC_total/ΔN_deposition was 20.1 kg C (kg N)^?1 using the pooled mean, with a pooled standard deviation of 13.8 kg C (kg N)^?1. Variability in parameters accounted for 92% of the total uncertainty in ΔC_total/ΔN_deposition, and only 8% was attributable to differences between models. The most sensitive parameters were those which controlled the allocation of assimilate between leaves, roots and stem. In particular, an increase in allocation to fine roots led to a large reduction in ΔC_total/ΔN_deposition in all models, because the fine roots have a very high turnover rate, and extra carbon allocated there is soon lost through mortality and decomposition.     

Growth and Nitrogen Availability of Red Pine Seedlings under High Nitrogen Load and Elevated Ozone

To evaluate the effect of increasing nitrogen (N) deposition and tropospheric ozone (O₃) concentrations on N-saturated forest ecosystems, we investigated the response of Japanese red pine (Pinus densiflora), an N-saturation sensitive tree species, to increasing N load under elevated O₃ concentrations. One-year-old seedlings of red pine were treated with three levels of N supply (0, 50 and 100 mg N L^-1 fresh soil volume) under two levels of atmospheric O₃ concentration (< 5 and 60 ppb) for two growing seasons. Nitrogen treatment did not stimulate dry matter production of the seedlings. Growth inhibition was observed in the highest N treatment under low O₃ and in the two higher N treatments under elevated O₃. Irrespective of the O₃ concentration, increasing N supply negatively affected root growth and mycorrhizal development in fine roots, resulting in a reduction in P and Mg uptake from the soil. Net photosynthetic rate was significantly reduced by both the highest N treatment under low O₃ and the two higher N treatments under elevated O₃, together with decreased N-availability to Rubisco. Nitrogen assimilated from NO₃ ^- to amino acid in the needles was not affected by the treatments. However, needle protein concentration was reduced by the highest N-treatment under low O₃ and by the two higher N-treatments under elevated O₃. These results suggest that elevated O₃ potentially disturbs the N-availability in the form of protein including Rubisco, and may advance the negative effects of excessive N-deposition on N-sensitive plant species in N-saturated forests.     

Effect of fresh green waste and green waste compost on mineral nitrogen, nitrous oxide and carbon dioxide from a Vertisol

Incorporation of organic waste amendments to a horticultural soil, prior to expected risk periods, could immobilise mineral N, ultimately reducing nitrogen (N) losses as nitrous oxide (N₂O) and leaching. Two organic waste amendments were selected, a fresh green waste (FGW) and green waste compost (GWC) as they had suitable biochemical attributes to initiate N immobilisation into the microbial biomass and organic N forms. These characteristics include a high C:N ratio (FGW 44:1, GWC 35:1), low total N (<1%), and high lignin content (>14%). Both products were applied at 3 t C/ha to a high N (plus N fertiliser) or low N (no fertiliser addition) Vertisol soil in PVC columns. Cumulative N₂O production over the 28 day incubation from the control soil was 1.5 mg/N₂O/m², and 11 mg/N₂O/m² from the control + N. The N₂O emission decreased with GWC addition (P < 0.05) for the high N soil, reducing cumulative N₂O emissions by 38% by the conclusion of the incubation. Analysis of mineral N concentrations at 7, 14 and 28 days identified that both FGW and GWC induced microbial immobilisation of N in the first 7 days of incubation regardless of whether the soil environment was initially high or low in N; with the FGW immobilising up to 30% of available N. It is likely that the reduced mineral N due to N immobilisation led to a reduced substrate for N₂O production during the first week of the trial, when soil N₂O emissions peaked. An additional finding was that FGW + N did not decrease cumulative N₂O emissions compared to the control + N, potentially due to the fact that it stimulated microbial respiration resulting in anaerobic micro sites in the soil and ultimately N₂O production via denitrification. Therefore, both materials could be used as post harvest amendments in horticulture to minimise N loss through nitrate-N leaching in the risk periods between crop rotations. The mature GWC has potential to reduce N₂O, an important greenhouse gas.     

A Conceptual Model of Spatially Heterogeneous Nitrogen Leaching from a Welsh Moorland Catchment

Soil- and stream-water data from the Plynlimon research area, mid-Wales, have been used to develop a conceptual model of spatial variations in nitrogen (N) leaching within moorland catchments. Extensive peats, in both hilltop and valley locations, are considered near-complete sinks for inorganic N, but leach the most dissolved organic nitrogen (DON). Peaty mineral soils on hillslopes also retain inorganic N within upper organic horizons, but a proportion percolates into mineral horizons as nitrate (NO ₃ ^– ), either through incomplete immobilisation in the organic layer, or in water bypassing the organic soil matrix via macropores. This NO ₃ ^– reaches the stream where mineral soilwaters discharge (via matrix throughflow or pipeflow) directly to the drainage network, or via small N-enriched flush wetlands. NO ₃ ^– in hillslope waters discharging into larger valley wetlands will be removed before reaching the stream. A concept of catchment nitrate leaching zones is proposed, whereby most stream NO ₃ ^– derives from localised areas of mineral soil hillslope draining directly to the stream; the extent of these zones within a catchment may thus determine its overall susceptibility to elevated surface water NO ₃ ^– concentrations.     

Effects of Acid Rain on Competitive Releases of Cd, Cu, and Zn from Two Natural Soils and Two Contaminated Soils in Hunan, China

Leaching experiments of rebuilt soil columns with two simulated acid rain solutions (pH 4.6–3.8) were conducted for two natural soils and two artificial contaminated soils from Hunan, south-central China, to study effects of acid rain on competitive releases of soil Cd, Cu, and Zn. Distilled water was used in comparison. The results showed that the total releases were Zn>Cu>Cd for the natural soils and Cd>Zn≫Cu for the contaminated soils, which reflected sensitivity of these metals to acid rain. Leached with different acid rain, about 26–76% of external Cd and 11–68% external Zn were released, but more than 99% of external Cu was adsorbed by the soils, and therefore Cu had a different sorption and desorption pattern from Cd and Zn. Metal releases were obviously correlated with releases of TOC in the leachates, which could be described as an exponential equation. Compared with the natural soils, acid rain not only led to changes in total metal contents, but also in metal fraction distributions in the contaminated soils. More acidified soils had a lower sorption capacity to metals, mostly related to soil properties such as pH, organic matter, soil particles, adsorbed SO₄ ²⁻, exchangeable Al³⁺ and H⁺, and contents of Fe₂O₃ and Al₂O₃.     

Topographic Controls of Nitrogen,Sulfur, and Carbon Transport from a Tolerant Hardwood Hillslope

The lateral down-slope movement of water, NO₃ ^-, NH₄ ⁺, SO₄ ^2-, H⁺ and DOC through an ablation till was examined from 1987 to 1990 for a one hectaresoil catena on a steep hillslope with uniform forest cover at the Turkey Lakes Watershed (TLW), Ontario, Canada. Natural variation in the export of nutrients from the soil profile via soil water to Little Turkey Lake was assessed in relation to nutrient distribution in soil at different topographic positions.Subsurface throughflow exhibited dramatic differences in nutrientconcentrations and fluxes with slope position, largely reflectingthat of the soil horizons through which the water passed. GreaterNO₃ ^-, SO₄ ^2-, and DOC concentrations in subsurface water in the upper, well-drained hillslope were a reflection of enrichment by contact with more acidic, more developed podzols, and more favorable soil physical and biological conditions for NO₃ ^- retention in solution.Nutrient inputs to the lake were strongly influenced by increaseddown-slope transport of water, and increased SO₄ ^2-, N, and C retention in wetter, less-developed podzolic soils that characterize lower slope positions. An understanding of water movement and soil development variation withtopographic position was required to accurately estimate nutrient budgets for steep slopes at TLW.     

Implications of Heterogeneous Chemistry of Nitric Acid for Nitrogen Deposition to Marine Ecosystems: Observations and Modeling

Size resolved particle composition and nitric acid (HNO₃)measurements from the ASEPS'98 experiment conducted in the BalticSea are used to provide observational evidence of substantialgas-particle transfer of oxidized nitrogen (N) compounds in themarine boundary layer. We then focus on the importance ofHNO₃ reactions on sea salt particles in determining spatio-temporal patterns of N dry deposition to marine ecosystems.Modelling results obtained assuming no kinetic or chemical limiton HNO₃ uptake and horizontally homogeneous conditions withnear-neutral stability, indicate that for wind speeds 3.5 – 10 ms^-1 transfer of HNO₃ to the particle phase to formparticle nitrate (NO₃ ^-) may decrease the N depositionvelocity by 50%. We extend this research using the CHEM-COASTmodel to demonstrate that, in a sulphur poor environment undermoderate wind speeds with HNO₃ concentrations representativeof those found in the marine boundary layer, inclusion ofheterogeneous reactions on sea spray significantly reducesmodelled NO₃ ^- deposition in the near coastal zone.