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.     

The interactions between plant growth,vegetation structure and soil processes in semi-natural acidic and calcareous grasslands receiving long-term inputs of simulated pollutant nitrogen deposition

Regular applications of ammonium nitrate (35-140 kg N ha(-1) year(-1)) and ammonium sulphate (140 kg N ha(-1) year(-1)) to areas of acidic and calcareous grassland in the Derbyshire Peak District over a period of 6 years, have resulted in significant losses in both overall plant cover, and the abundance of individual species, associated with clear and dose-related increases in shoot nitrogen content. No overall growth response to nitrogen treatment was seen at any stage in the experiment. Phosphorus additions to the calcareous plots did however lead to significant increases in plant cover and total biomass, indicative of phosphorus limitation in this system. Clear and dose-related increases in soil nitrogen mineralization rates were also obtained, consistent with marked effects of the nitrogen additions on soil processes. High nitrification rates were seen on the calcareous plots, and this process was associated with significant acidification of the 140 kg N ha(-1) year(-1) treatments.     

Dissolved inorganic nitrogen budget for a forested catchment at Beddgelert, North Wales

An input-output budget for dissolved inorganic-N in a small forested catchment in North Wales is presented. From 1982 to 1990, bulk precipitation inputs averaged 10.3 kg ha(-1) year(-1), whereas throughfall inputs in 1983-1984 were 20.3 kg ha(-1) year(-1). Streamwater outputs were consistently larger than bulk precipitation inputs, averaging 14.6 kg ha(-1) year(-1). Inorganic-N in the forest stream was predominantly nitrate and concentrations were substantially higher than in a nearby moorland stream. Both streams showed seasonal trends in nitrate concentration, with highest concentrations occurring in summer in the forest stream but in winter in the moorland stream. Nitrate concentration in the forest stream increased with increasing soil temperature up to approximately 7 degrees C and decreased at higher temperatures. Nitrification is thought to be responsible for nitrate production at temperatures both below and above 7 degrees C, but root uptake becomes significant only at the higher temperatures. In the forest, dry deposition and cloudwater inputs of inorganic-N are responsible for increased nitrogen fluxes in throughfall compared with wet deposition. Mineralization and nitrification in excess of plant needs causes the organic soil horizons to act as a net source of dissolved inorganic-N. Nitrogen transformations in the soil lead to soil acidification at a rate of 1.0 keq ha(-1) year(-1).     

Catchment acidification-from the top down

Three main factors define the speed of catchment acidification: the total input of pollutants; the thickness and character of soils, including the nature of the bedrock; and the size of subcatchments. The aerial input of pollutants in the Harz is among the highest in Central Europe (e.g. SO4-S: 22-70 kg (ha year)(-1); NO3-N: 9-10 kg (ha year)(-1); NH4-N: 10-15 kg (ha year)(-1) and Cd: 2.6-8.7 g (ha year)(-1); Cu: 34-125 g (ha year)(-1); Pb: 150-380 g (ha year)(-1); Zn: 105-560 g (ha year)(-1)). Thick soil profiles (2-4 m) acidify from the top down. Whether the soils will neutralize incoming acids depends on their buffering capacity. The small headwater subcatchments acidify first and subsequently release acidic water with pH values down to < or = 40. Four brook zones can be divided by the composition of their biocoenoses. The latter depend on the degree of acidification. These zones are also characterized by different hydrochemical conditions.     

Fluxes of oxidised and reduced nitrogen above a mixed coniferous forest exposed to various nitrogen emission sources

Concentrations of nitrogen gases (NH(3), NO(2), NO, HONO and HNO(3)) and particles (pNH(4) and pNO(3)) were measured over a mixed coniferous forest impacted by high nitrogen loads. Nitrogen dioxide (NO(2)) represented the main nitrogen form, followed by nitric oxide (NO) and ammonia (NH(3)). A combination of gradient method (NH(3) and NO(x)) and resistance modelling techniques (HNO(3), HONO, pNH(4) and pNO(3)) was used to calculate dry deposition of nitrogen compounds. Net flux of NH(3) amounted to -64 ng N m(-2) s(-1) over the measuring period. Net fluxes of NO(x) were upward (8.5 ng N m(-2) s(-1)) with highest emission in the morning. Fluxes of other gases or aerosols substantially contributed to dry deposition. Total nitrogen deposition was estimated at -48 kg N ha(-1) yr(-1) and consisted for almost 80% of NH(x). Comparison of throughfall nitrogen with total deposition suggested substantial uptake of reduced N (+/-15 kg N ha(-1) yr(-1)) within the canopy.     

Deposition of nitrogen-containing compounds to an extensively managed grassland in central Switzerland

During four intensive observation periods in 1992 and 1993, dry deposition of nitrogen dioxide (NO(2)) and ammonia (NH(3)), and wet deposition of nitrogen (N) were determined. The measurements were carried out in a small, extensively managed litter meadow surrounded by intensively managed agricultural land. Dry deposition of NH(3) was estimated by the gradient method, whereas eddy correlation was used for NO(2). Rates of dry deposition of total nitrate (= nitric acid (HNO(3)) + nitrate (NO(3)(-))), total nitrite (= nitrous acid (HONO) + nitrite (NO(2)(-))) and aerosol-bound ammonium (NH(4)(+)) were estimated using deposition velocities from the literature and measured concentrations. Both wet N deposition and the vertical NH(3) gradient were measured on a weekly basis during one year. Dry deposition was between 15 and 25 kg N ha(-1) y(-1), and net wet deposition was about 9.0 kg N ha(-1) y(-1). Daily average NO(2) deposition velocity varied from 0.11 to 0.24 cm s(-1). Deposition velocity of NH(3), was between 0.13 and 1.4 cm s(-1), and a compensation point between 3 and 6 ppbV NH(3) (ppb = 10(-9)) was found. Between 60 and 70% of dry deposition originated from NH(3) emitted by farms in the neighbourhood. It is concluded that total N deposition is exceeding the critical load for litter meadows, is highly correlated to local NH(3) emissions, and that NH(3) is of utmost importance with respect to possible strategies to reduce N deposition in rural regions.     

Effects of increased deposition of atmospheric nitrogen on an upland moor: nitrogen budgets and nutrient accumulation

This study was designed to investigate the effect of long-term (11 years) ammonium nitrate additions on standing mass, nutrient content (% and kg ha(-1)), and the proportion of the added N retained within the different compartments of the system. The results showed that more than 90% of all N in the system was found in the soil, particularly in the organic (Oh) horizon. Added N increased the standing mass of vegetation and litter and the N content (kg N ha(-1)) of almost all measured plant, litter and soil compartments. Green tissue P and K content (kg ha(-1)) were increased, and N:P ratios were increased to levels indicative of P limitation. At the lowest treatment, most of the additional N was found in plant/litter compartments, but at higher treatments, there were steep increases in the amount of additional N in the underlying organic and mineral (Eag) horizons. The budget revealed that the proportion of added N found in the system as a whole increased from 60%, 80% and up to 90% in response to the 40, 80 and 120 kg N ha(-1) year(-1) treatments, respectively.     

Size fractionated speciation of nitrate and sulfate aerosols in a sub-tropical industrial environment

Size fractionated chemical speciation of acidic aerosols were performed for ammonium sulfate, other sulfates, ammonium nitrate and other nitrates in a sub-tropical industrial area, Bina, India during December 2003 to November 2004. Analysis of variance (ANOVA) revealed highly significant temporal variations (p > .001) in the concentrations of nitrate and sulfate aerosols in all the three size fractions (fine, mid-size and coarse). Winter demonstrated utmost concentrations of ammonium sulfate, which ranged from 3.2 to 26.4 microg m(-3) in fine particles and 0.20-0.34 microg m(-3) in coarse particles. Ammonium sulfate was chiefly in fine mode (43.77% of total particulate sulfate) as compared to coarse particles (28.60% of total particulate sulfate). The major fraction Ammonium sulfate existed in different forms in atmospheric aerosols, for example NH4Fe(SO4)2, (NH4)2SO4, (NH4)3H(SO4)2 in fine particles, and (NH4)4(NO3)SO4+ in coarse particles. Other sulfate concentrations were also higher during winter ranging from 1.89 to 14.3 microg m(-3) in fine particles and 0.12-0.65microg m(-3) in coarse particles. Ammonium nitrate constituted the major fraction of total particulate nitrate all through the year and was principally in fine particles (the highest concentration in January i.e. 14.2 microg m(-3)). Other nitrates were mainly distributed in the fine particles (highest concentration in January i.e. 11.2 microg m(-3)) All the sulfate and nitrate species were mainly distributed in fine mode and have significant impact on human health.     

Responses to ammonium and nitrate additions by boreal plants and their natural enemies

Separate effects of ammonium (NH4+) and nitrate (NO3-) on boreal forest understorey vegetation were investigated in an experiment where 12.5 and 50.0 kg nitrogen (N) ha(-1) year(-1) was added to 2 m2 sized plots during 4 years. The dwarf-shrubs dominating the plant community, Vaccinium myrtillus and V. vitis-idaea, took up little of the added N independent of the chemical form, and their growth did not respond to the N treatments. The grass Deschampsia flexuosa increased from the N additions and most so in response to NO3-. Bryophytes took up predominately NH4+ and there was a negative correlation between moss N concentration and abundance. Plant pathogenic fungi increased from the N additions, but showed no differences in response to the two N forms. Because the relative contribution of NH4+ and NO3- to the total N deposition on a regional scale can vary substantially, the N load a habitat can sustain without substantial changes in the biota should be set considering specific vegetation responses to the predominant N form in deposition.     

Effects of atmospheric ammonia (NH3) on terrestrial vegetation: a review

At the global scale, among all N (nitrogen) species in the atmosphere and their deposition on to terrestrial vegetation and other receptors, NH3 (ammonia) is considered to be the foremost. The major sources for atmospheric NH3 are agricultural activities and animal feedlot operations, followed by biomass burning (including forest fires) and to a lesser extent fossil fuel combustion. Close to its sources, acute exposures to NH3 can result in visible foliar injury on vegetation. NH3 is deposited rapidly within the first 4-5 km from its source. However, NH3 is also converted in the atmosphere to fine particle NH4+ (ammonium) aerosols that are a regional scale problem. Much of our current knowledge of the effects of NH3 on higher plants is predominantly derived from studies conducted in Europe. Adverse effects on vegetation occur when the rate of foliar uptake of NH3 is greater than the rate and capacity for in vivo detoxification by the plants. Most to least sensitive plant species to NH3 are native vegetation > forests > agricultural crops. There are also a number of studies on N deposition and lichens, mosses and green algae. Direct cause and effect relationships in most of those cases (exceptions being those locations very close to point sources) are confounded by other environmental factors, particularly changes in the ambient SO2 (sulfur dioxide) concentrations. In addition to direct foliar injury, adverse effects of NH3 on higher plants include alterations in: growth and productivity, tissue content of nutrients and toxic elements, drought and frost tolerance, responses to insect pests and disease causing microorganisms (pathogens), development of beneficial root symbiotic or mycorrhizal associations and inter species competition or biodiversity. In all these cases, the joint effects of NH3 with other air pollutants such as all-pervasive O3 or increasing CO2 concentrations are poorly understood. While NH3 uptake in higher plants occurs through the shoots, NH4+ uptake occurs through the shoots, roots and through both pathways. However, NH4+ is immobile in the soil and is converted to NO3- (nitrate). In agricultural systems, additions of NO3- to the soil (initially as NH3 or NH4+) and the consequent increases in the emissions of N2O (nitrous oxide, a greenhouse gas) and leaching of NO3- into the ground and surface waters are of major environmental concern. At the ecosystem level NH3 deposition cannot be viewed alone, but in the context of total N deposition. There are a number of forest ecosystems in North America that have been subjected to N saturation and the consequent negative effects. There are also heathlands and other plant communities in Europe that have been subjected to N-induced alterations. Regulatory mitigative approaches to these problems include the use of N saturation data or the concept of critical loads. Current information suggests that a critical load of 5-10 kg ha(-1) year(-1) of total N deposition (both dry and wet deposition combined of all atmospheric N species) would protect the most vulnerable terrestrial ecosystems (heaths, bogs, cryptogams) and values of 10-20 kg ha(-1) year(-1) would protect forests, depending on soil conditions. However, to derive the best analysis, the critical load concept should be coupled to the results and consequences of N saturation.     

Microbiological characterization of the biological treatment of aircraft paint stripping wastewater

Leaching rates of the herbicide dichlorprop [(+/--2-(2,4-dichlorophenoxy)propanoic acid] and nitrate were measured together in field lysimeters containing undisturbed clay and peat soils. The purpose of the study was to investigate the leaching pattern of the two solutes in structured soils under different precipitation regimes. Spring barley (Hordeum distichum L.) was sown on each monolith and fertilized with 100 kg N ha(-1). Dichlorprop was applied at a rate of 1.6 kg active ingredient (a.i.) ha(-1). Each soil type received supplemental irrigation at two levels ('average' and 'worst-case'), giving total water inputs (irrigation and precipitation) of 664 and 749 mm year(-1), respectively. The larger water input approximately doubled the nitrate loads, from, on average, 11.6 to 21.8 kg N ha(-1) year(-1) in the clay soil and from 37.6 to 65.4 kg N ha(-1) year(-1) in the peat soil. In contrast, dichlorprop leaching was reduced by more than one order of magnitude when the water input was increased, from average amounts of 3.22 to 0.26 g a.i. ha(-1) during an S-month period in the clay and from 28.9 to 2.67 g a.i. ha(-1) in the peat. This leaching pattern of dichlorprop was explained in terms of preferential flow. The dried-out topsoil of 'average' watered monoliths may have allowed water flow in cracks, thus moving some of the herbicide rapidly through the topsoil to the subsoil. Once the compound reached the subsoil, degradation rates would be reduced and the herbicide residues would be stored for later leaching. Nitrate was presumably more evenly distributed in the soil matrix; therefore, water rapidly moving through macropores would not carry significant amounts of nitrate. In contrast, leaching would occur more evenly through the soil matrix, causing larger nitrate loads in the 'worst-case' watered monoliths. These results show that wet years may constitute a worst case scenario in terms of nitrate leaching, but not pesticide leaching, if macropore flow exerts a significant influence on leaching.     

Soil solution nitrogen and cations influenced by (NH4)2SO4 deposition in a coniferous forest

The effects of chronically enhanced (NH(4))(2)SO(4) deposition on ion concentrations in soil solution and ionic fluxes were investigated in a Picea abies plot at Grizedale forest, NW England. Soil cores closed at the base and containing a ceramic suction cup sampler were 'roofed' and watered every 2 weeks with bulk throughfall collected in the field. Treatments consisted of the inclusion of living roots from mature trees in the lysimeters and increasing (NH(4))(2)SO(4) deposition (NS treatment) to ambient + 75 kg N ha(-1) a(-1). Rainfall, throughfall and soil solutions were collected every 2 weeks during 18 months, and analysed for major cations and anions. NO(3)(-) fluxes significantly increased following NS treatment, and were balanced by increased Al(3+) losses. Increased SO(4)(2-) concentrations played a minor role in controlling soil solution cation concentrations. The soil exchange complex was dominated by Al and, during the experimental period, cores of all treatments 'switched' from Ca(2+) to Al(3+) leaching, leading to mean [Formula: see text] molar ratios in soil solution of NS treated cores of 0.24. The experiment confirmed that the most sensitive soils to acidification (through deposition or changing environmental conditions) are those with low base saturation, and with a pH in the lower Ca, or Al buffer ranges.     

Soil quality and vulnerability in a Mediterranean natural ecosystem of Central Italy

Wet and dry atmospheric depositions and soil chemical and microbiological properties were determined in a Mediterranean natural ecosystem of Central Italy near Rome (Castelporziano Estate). The monitoring of depositions permitted us to quantify the exceedances of S and N compounds (expressed as eqH(+)ha(-1)year(-1)) over the critical loads of acidity. Critical loads, i.e. the quantity of a substance which a part of the environment can tolerate without adverse effects occurring, were determined adopting the level 0 methodology following the UN/ECE Convention on Long-range Transboundary Air Pollution. Deposition data were available for the period 1992-1997, and acidity exceedances were referred to the main vegetation types present in the area. Results showed that most part of the Estate has a medium degree of vulnerability to acidification, and the corresponding risk of acidification deriving from the exceedances of atmospheric deposition was rather low. The study of soil chemical and microbiological properties included mainly total soil organic carbon (SOC), microbial biomass-C, biomass-C/SOC, soil respiration, and metabolic quotient (qCO2). Soil organic C metabolism has been discussed on the basis of the results from eight sampling sites.     

Ammonium, nitrate and nitrite nitrogen and nitrate reductase enzyme activity in groundnut (Arachis hypogaea L.) fields after diazinon, imidacloprid and lindane treatments

Impacts of diazinon (O,O-diethyl O-2-isopropyl-6-methylpyrimidin-4-yl phosphorothioate), imidacloprid [1-(6-chloro-3-pyridylmethyl)-N-nitroimidazolidin-2-ylideneamine] and lindane (1,2,3,4,5.6-hexachlorocyclohexane) treatments on ammonium, nitrate, and nitrite nitrogen and nitrate reductase enzyme activities were determined in groundnut (Arachis hypogaea L.) field for three consecutive years (1997 to 1999). Diazinon was applied for both seed- and soil-treatments but imidacloprid and lindane were used for seed treatments only at recommended rates. Diazinon residues persisted for 60 days in both the cases. Average half-lives (t1/2) of diazinon were found 29.3 and 34.8 days respectively in seed and soil treatments. In diazinon seed treatment, NH4(+), NO3(-), and NO2(-) nitrogen and nitrate reductase activity were not affected. Whereas, diazinon soil treatment indicated significant increase in NH4(+)-N in a 1-day sample, which continued until 90 days. Some declines in NO3(-)N were found from 15 to 60 days. Along with this decline, significant increases in NO2(-)N and nitrate reductase activity were found between 1 and 30 days. Imidacloprid and lindane persisted for 90 and 120 days with average half-lives (t1/2) of 40.9 and 53.3 days, respectively. Within 90 days, imidacloprid residues lost by 73.17% to 82.49% while such losses for lindane residues were found 78.19% to 79.86 % within 120 days. In imidacloprid seed-treated field, stimulation of NO3(-)N and the decline in NH4+NO2(-)-N and nitrate reductase enzyme activity were observed between 15 to 90 days. However, lindane seed treatment indicated significant increases in NH4(+)-N, NO2(-)-N and nitrate reductase activity and some adverse effects on NO3(-)N between 15 and 90 days.     

Comparison of throughfall and soil solution chemistry between a high-density Corsican pine stand and a naturally regenerated silver birch stand

In Flanders, critical loads for acidification and eutrophication are exceeded in the majority of the forest stands, and many previously nitrogen limited forest ecosystems have become nitrogen saturated. The present study investigates whether a naturally regenerated stand of silver birch (Betula pendula Roth) contributes less to the acidification and eutrophication of the forest soil than a high-density plantation of Corsican pine (Pinus nigra ssp. laricio Maire). Throughfall deposition of inorganic nitrogen was about 3.5 times higher in the Corsican pine stand than in the birch stand. Potassium throughfall deposition was significantly higher under birch due to higher canopy leaching. Magnesium throughfall deposition was significantly higher under the pine canopy due to higher dry deposition. The lower nitrogen throughfall deposition in the birch stand was reflected in a 60% lower nitrate percolation at 1m depth compared with pine. Nitrate soil percolation is linked to losses of aluminium and base cations.     

L-asparaginase and L-glutaminase activities in submerged rice soil amended with municipal solid waste compost and decomposed cow manure

The field study was conducted to evaluate the effect of municipal solid waste compost (MSWC) as a soil amendment on L-asparaginase (LA) and L-glutaminase (LG) activities. Experiments were conducted during the wet seasons of 1997, 1998 and 1999 on rice grown under a submerged condition, at the Agriculture Experimental Farm, Calcutta University at Baruipur, West Bengal, India. The treatments consisted of control, no input; MSWC, at 60 Kg N ha(- 1); well-decomposed cow manure (DCM), at 60 Kg N ha(- 1); MSWC (30 Kg N ha(- 1)) + Urea (U) (30 Kg N ha(- 1)); DCM (30 Kg N ha(- 1)) + U (30 Kg N ha(- 1)) and Fertilizer, (at 60:30:30 NPK kg ha(- 1)) through urea, single superphosphate and muriate of potash respectively). LA and LG activities alone and their ratio with organic-C (ratio index value, RIV), straw and grain yield were higher in DCM than MSWC-treated soils, due to higher amount of biogenic organic materials like water-soluble organic carbon, carbohydrate and mineralizable nitrogen in the former. The studied parameters were higher when urea was integrated with DCM or MSWC, compared to their single applications. The heavy metals in MSWC did not detrimentally influence the above-measured activities of soil. In the event of long term MSWC application, changes in soil quality parameters should be monitored regularly, since heavy metals once entering into soil persist over a long period.     

Cumulative impact of 40 years of industrial sulfur emissions on a forest soil in west-central Alberta (Canada)

The impact of 40 years of sulfur (S) emissions from a sour gas processing plant in Alberta (Canada) on soil development, soil S pools, soil acidification, and stand nutrition at a pine (Pinus contorta x Pinus banksiana) ecosystem was assessed by comparing ecologically analogous areas subjected to different S deposition levels. Sulfur isotope ratios showed that most deposited S was derived from the sour gas processing plant. The soil subjected to the highest S deposition contained 25.9 kmol S ha(-1) (uppermost 60 cm) compared to 12.5 kmol S ha(-1) or less at the analogues receiving low S deposition. The increase in soil S pools was caused by accumulation of organic S in the forest floor and accumulation of inorganic sulfate in the mineral soil. High S inputs resulted in topsoil acidification, depletion of exchangeable soil Ca2+ and Mg2+ pools by 50%, podzolization, and deterioration of N nutrition of the pine trees.     

Ammonium-nitrogen transformation and nitrogen retention in broiler manure supplemented with a soil amendment containing nitrifying bacteria

The effect of a soil amendment on ammonium nitrogen transformation and nitrogen retention in broiler manure was evaluated. Prior to incubation, broiler manure was mixed with autoclaved soil or non-autoclaved soil in different ratios to make 1 kg mixtures; broiler manure:non-autoclaved soil=9:1, 5:5, and 1:9 or broiler manure:autoclaved soil=9:1, 5:5, and 1:9. The non-autoclaved soil treatment reduced either numerically or significantly NH(4)(+)-N concentration compared to the autoclaved soil treatment during the 8-wk incubation. Total-N concentration of the non-autoclaved soil treatments was lower than the autoclaved soil treatments from 4 to 8 wk. The lowest manure to non-autoclaved soil treatment (M:S=1:9) had considerably more nitrite and nitrate; however, the higher ratio manure to non-autoclaved soil treatments (M:S=9:1 and 5:5) had slightly higher total nitrite and nitrate levels compared to the same ratio of autoclaved soil treatments. The moisture level of the 9:1, 5:5, and 1:9 M:S treatments were approximately 70, 45, and 30%, respectively. The results indicated that nitrifying bacteria in the non-autoclaved soil reduced the ammonium nitrogen concentrations of poultry manure by converting NH(3) or NH(4)(+) to NO(2)(-) or NO(3)(-). However, the higher moisture levels in treatments with greater manure to soil ratios (M:S=9:1 and 5:5) created anaerobic conditions that allowed for denitrification and greater N losses.     

Ecosystem effects of atmospheric deposition of nitrogen in The Netherlands

Atmospheric deposition of inorganic N, mainly ammonium volatilized from manure produced in intensive stockbreeding, on sensitive terrestrial and aquatic ecosystems in The Netherlands is in the order of 40 to 80 kg ha(-1) year(-1). Proven effects of this deposition are (i) eutrophication with N, leading to floristic changes (ii) acidification of base-poor sandy soils and of moorland pools, leading to higher concentrations of dissolved, potentially toxic metals such as Al3+, and (iii) increased levels of nitrate in groundwater below woodlands. In acid forest soils, but not in soils under heathland, nitrification and leaching of nitrate is common. However, in very poor sandy forest soils and at very high ammonium inputs, nitrification may be too slow to prevent the development of high concentrations of ammonium. Both excessive acidification and excessive levels of ammonium probably play an important role in the general forest decline, which is most severe in the southern and central parts of the country, where ammonium inputs are highest.     

Cadmium uptake by tobacco as affected by liming, N form, and year of cultivation

Tobacco is able to accumulate cadmium and reduction of cadmium content can reduce health hazards to smokers. Soil pH and form of N fertilizers are among the factors affecting Cd uptake by tobacco. This hypothesis was tested in an acid soil in northern Greece by a four year field experiment. The variability of Cd uptake by tobacco was attributed to the variation of soil Cd availability as affected by soil pH. Liming with 3000 kg Ca(OH)(2) ha(-1) increased soil pH by 0.8 units and decreased extractable with DTPA soil and leaf Cd by 40% and 35%, respectively. The ammonium fertilizer caused the opposite, but weaker, effects. Liming reduced soil Cd more in the ammonium treatment than in nitrate or combined N treatments. The year of cultivation strongly affected soil and leaf Cd. Four years after tobacco cultivation, soil pH was reduced by 0.5 units, whereas soil and leaf Cd reduction was more than 60% in the limed treatments. Liming affected Cd uptake only in the first three years of cultivation.