Medium-term response of peat drainage water to changes in nitrogen deposition from the atmosphere

Wat. Res. Vol. 30, No. 9, pp. 2171-2177, 1996 Coovrkht K?I 1996 Elsevier ScienceLtd

Pergamon PIk soo43-1354(%)ooo83-8

MEDIUM-TERM TO CHANGES

LAILA YESMIN,

Printed’6 &earBritain. All rights reserved 0043-1354/96515.00+ 0.00

RESPONSE OF PEAT DRAINAGE WATER IN NITROGEN DEPOSITION FROM THE ATMOSPHERE SHIMNA

M. GAMMACK

and MALCOLM

S. CRESSER*Q

Department of Plant and Soil Science, University of Aberdeen, Aberdeen AB24 3UU, Scotland (First received June 1995; accepted in revised form February 1996)

Abstract-A 12-month glass-house simulation experiment has been used to investigate the response of drainage water solute chemistry of Cahna and mixed grass moorland peat microcosms to elevated deposition of ammonium- and nitrate-N. Uptake for both N deposition species was greater for grass than for Calluna vulguris turfs. Nitrification was negligible for the elevated ammonium-N treatments, which did not therefore enhance nitrate leaching, but did result in substantial increases in ammonium leaching for both vegetation types. However, the elevated nitrate treatments resulted in dramatically enhanced ammonium-N leaching and a more modest enhancement in the leaching of organic-N. The biological uptake minimises the acidification effect of the enhanced N depositions, with ca. 70% of the additional H+ inputs as HNO, being consumed in N transformation reactions. Copyright 0 1996 Elsevier Science Ltd Key words-nitrate deposition

leaching, ammonium leaching, peat moorland, drainage water chemistry, nitrogen

INTRODUCTION Most soils have a substantial capacity to absorb and retain N deposition from the atmosphere (INDITE,

1994). Deposited ammonium may be retained on cation exchange sites close to the surface. The extent of retention depends upon the past and present ammonium concentration of precipitation, the nature and amount of exchange sites, and the presence in deposition of competing cations such as Na+ (Duckworth and Cresser, 1991). Much of the retained ammonium may be subsequently converted to organic-N, the rate and extent of conversion being strongly temperature dependent (Duckworth and Cresser, 1991). Thus, organic-N may accumulate in surface soils in areas of high N deposition. Billett et al. (1990, 1993) have shown that this build-up rate may be substantial for forest soils. Such organic N accumulation did not result in a decrease in the C:N ratio, however, because of the relatively much greater rate of organic-C accumulation in the soils studied; indeed, the C:N ratio increased. More recently, for surface horizons of a Calluna moorland podzol, where the soils were all derived from granites or granitic tills, White, Gammack and Cresser (unpublished results) found that the C:N ratio increased with increasing total N deposition up to ca. 9 kg ha-’ yr-‘, but subsequently fell sharply with further increases in N deposition. *Author to whom all correspondence [Fax: (1224) 272 2681.

should be addressed

For peat soils in the U.K., the C:N ratio has been shown to fall sharply with increasing atmospheric N deposition along a pollution gradient, from about 40 to a plateau at 22 as the N deposition rate increased from 2 to 17 kg ha-’ yr-’ (Yesmin et al., 1995). The occurrence of this plateau suggests the limiting point for organic N accumulation as an N sink mechanism, although organic C and N could both still be accumulating at a relatively constant ratio close to 22. At this stage, the probability of enhanced ammonium leaching in drainage waters increases. An experiment with large Calluna moorland peat microcosms, collected from sites along a pollution gradient and subjected over 18 months to simulated polluted precipitation with a solute composition appropriate for each site, showed an approximate balance between ammonium inputs and outputs over a wide range of N deposition (Yesmin et al., 1995). Nitrification is generally negligible for most highly acidic, ombrotrophic peats, but along the N deposition gradient described above, annual nitrateN inputs and outputs were approximately balanced (Yesmin et a/., 1995). The nitrate leaching that occurs from hill peats in Scotland into rivers is probably, therefore, a substantial fraction of the incoming atmospheric nitrate deposition passing through the peat unchanged (Black et af., 1993). In unpolluted regions, nitrate is conserved via incorporation into soil microbial biomass (INDITE, 1994) or even via direct foliar uptake when precipitation is intercepted by vegetation (Edwards et al., 1985).

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L. Yesmin er al.

2172

One difficulty with the interpretation of the results of regional surveys, or with experiments using soils collected from along a pollution gradient, is that it is impossible to state categorically that observed effects are attributable to an N deposition gradient effect, rather than, for example, effects of gradients in temperature or precipitation. Moreover, although using microcosms experiments collected from along a deposition gradient incorporate any effects attributable to long-term (over decades) soil change, they tell us nothing about the rates of change of drainage water chemistry, nor do they allow us to attribute any observed effects to one deposited N species rather than another. It was therefore decided to conduct an experiment to study the effects on the solute composition of peat drainage water of increasing N inputs as either ammonium sulphate or nitric acid. The treatments would be applied via realistically simulated precipitation over a 12 month period, using microcosms from a single site. Because of growing interest over recent years in the decline of Calluna vulgaris as a consequence of increasing N deposition, it was also decided to compare the effects of using peat microcosms with either Calluna or mixed grass species as the dominant vegetation. The main aim of the experiment would be to assess the rates at which ammonium-, nitrate- and organic-N increased in concentration in drainage water. To ensure that the N treatment application rates were appropriate for polluted U.K. sites, three treatments, corresponding to ambient and two and six times ambient total N deposition, would be applied.

MATERIALS AND METHODS It was felt imperative to add the N treatments in realistically simulated precipitation. The artificial rainfall was therefore added as a fine cone of spray from an array of 40 purpose-built pneumatic nebulisers fabricated from inert plastic and silicone rubber capillary tubing (Skiba et al., 1987). Thus, one nebuliser was placed above each of four replicate microcosms for each of the two vegetation types and five N treatments. Flow rates were regulated using capillary restrictions to gravity feed from bottles of simulated rainfall solution fixed above the system; individual showers lasted for 2-3 h. The nitrogen deposition was applied at 1x ,2 x and 6 x the existing total (wet + dry deposition, 1986-88) field dose for the site from which the microcosms had been collected, in the mean annual rainfall amount for the area. Wet deuosition and rainfall amount data were obtained from UKRGAR (1990), and dry deoosition data for the site from David Fowler. ITE. Edinburgh (personal communication). The total N’appli: cation rates were 12.1, 24.2 and 72.7 kg ha-’ yr-‘.

Turf collection and installation in greenhouse

Late in August, 1993, 20 large intact peat turfs were cut to a depth of ca. 250 mm from areas dominated by Calluna oulgaris, and a further 20 turfs were cut from areas dominated by mixed grass species. Both areas were well drained. After collection, the turfs were transported to Aberdeen, where they were trimmed, using a sharp electric carving knife, to 15Omm depth and to fit exactly into circular black plastic bowls of internal diameter 355 mm. Such trimming minimises damage to the roots and peat structure. The turfs were kept moist by regular light spraying with deionised water prior to installation in the greenhouse. Prior to use, each bowl had a hole drilled at the centre of its base, through which drainage water was collected by means of a plastic funnel. Shallow drainage grooves were also cut in a cross shape at the bottom surface of each bowl, centred on the hole, to facilitate drainage. The upper rim of the bowl was surrounded by a polyethylene collar ca. 200 mm high to minimise loss of the simulated rainfall and the risk of cross-contamination during spraying. The bowls containing the turfs were set up on supporting wooden blocks in an unheated greenhouse in a completely randomised designwith four replicates for each vegetation type and treatment. Simulated rainfall formulation and application

Five N treatments were applied as fine sprays of simulated rainfall, with compositions based on that of precipitation at the Glen Dye site, with or without additional N. The control artificial rain contained, with concentrations in pmoles 1-l in parentheses, NaCl (63), MgCb (7) MgSOd (I), CaS04 (4.5), K2S04 (2.3), HISO (27.5) (NH&S04 (18.7) and HNO, (43.8). The medium and high ammonium treatments were 59.2 and 221.4 uM with resoect to (NH4bS04, and the medium and high ’nitrate treatments‘ were 124.9 and 449.3 PM with respect to HN03, respectively, but otherwise had the same solute concentrations as the control. The resultant ionic composition of each treatment is listed in Table 1. The average annual rainfall for the Glen Dye area (1067 mm. UKRGAR. 1990) was aDDlied evenlv throuehout the year, in simulated showers -three times-per week. Each shower consisted of 677 ml of artificial rain (equivalent to approximately 6.8 mm rainfall). Collection of leachate samples

The leachate volume from each peat turf was measured gravimetrically every week, 24 h after application of the third shower, by which time drainage had effectively ceased. Drainage water was collected weekly in darkened plastic bottles and stored at 4°C until analysis. Peat drainage water analysis

The pH and ammonium, nitrate and total N concentrations were determined every 2 weeks on volume-weighted bulked leachate samples representative of each 2-week

Table I. Concentrations (flmok 1-l) of ions in simulated precipitation for the control (TI), elevated nitrate (rl and fi) and elevated ammonium (TXand Ts) treatments Ion

TI

T2

T3

T4

T3

Ca’+

9.0

9.0

9.0

9.0

9.0

M&+ Site selection

The site chosen for this experiment was Glen Dye in north-east Scotland (grid reference NO 642864). Glen Dye receive a moderate amount of N deposition (INDITE, 1994), being in neither a pristine nor a highly polluted area of the U.K., and was chosen because it might give more realistic results for short- to mid-term simulation experiments than microcosms from a pristine site.

Na+

16 63

K+ H+ NH:

4.6 98.8 31.3

4.6 180 37.3

Cl-

77

NO,-

43.8

71 125 108

SO?

108

16 63

16 63

4.6 98.8 118 11 43.8 189

16 63

4.6 504 37.3 17 449 108

16 63

4.6 98.8 443 17

43.8 513

The above values may be converted to mol, ha-’ yr-’ by multiplying by 10.67.

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Peat drainage water response to N deposition

PH~!I.

Week pH 4.4 4.21

pH 4.4 4.2t______!%!!!?_Tfl_a-___----___

3.2 31, 2

6

Medium nitrate

31, 2

10 14 18 22 26 30 34 38 42 46 50 Week

6

r-7__ 10 14 18 22 26 30 34 38 42 46 50 Week

High nitrate

PH

Week

Week

Fig. 1. Simulated rain pH (broken line) and drainage water pH (dotted line represents drainage water from peat turfs dominated by grass species, solid line by Calluna vulgaris) over 12 months. Bars represent standard errors of the mean.

period. The pH of the leachate solution was determined using a combined glass/calomel electrode suitable for solutions of low ionic strength. Nitrate-N and total N were determined using a Technicon AutoAnalyser II, method No.-280-82E. The total N method used depends upon

photochemical oxidation of ammonium and organic-N to nitrate-N by persulphate and UV radiation. Ammonium-N was determined by flow injection analysis, using a standard Tecator manifoid. Organic-N was determined by subtraction of the sum of NOT-N and NH:-N from total N.

pH. The pH differed significantly between vegetation types, however, with Calluna vulgaris microcosms consistently giving significantly lower leachate pH values than the mixed grass microcosms (p < 0.001). When individual vegetation types were considered separately, increased N treatment level still significantly reduced the drainage water pH. Nitrogen species in drainage water

RESULTS Peat drainage water pH

Simulated rain and drainage water pH, the latter measured every 2 weeks, are plotted as a function of time in Fig. 1. Table 2 shows how the drainage water volume-weighted mean H+ concentration values and associated standard errors for drainage water from the four replicate turfs varied with treatment and vegetation type. Considering the results for the whole 12 months of the experiment, increased N deposition resulted in a significant decrease in drainage water

Nitrate-N, ammonium-N and organic-N concentrations in drainage water over the 12 months of the experiment are shown in Figs 2-4, together with the input concentrations in the simulated rain solutions. There were significant differences in leachate concentrations for all N species between Calluna and mixed grass microcosms. Nitrate. There was appreciable nitrate uptake overall for all treatments and both vegetation types (Fig. 2), although this tended to be lower through winter to mid-late spring (weeks 12-33). However, the grass microcosms apparently made much more

Table 2. The effect of treatment upon volume-weighted mean H+ concentrakms over I2 months, and the corresponding DH values, of drainage water from ueats supporting Culluna- and grass-dominant vegetation Calluna-dominant

Grass-dominant

(l(g I-‘)

SE

Mean pH

Mean H+ (BBI-9

246 240 285 348 375

18.8 10.3 19.5 15.5 18.2

3.61 3.62 3.55 3.46 3.43

159 168 186 186 232

Mean H+

Treatment Control Medium NO, Medium NH: High NO; Hiah NH:

SE

Mean pH

8.15 5.44 3.23 11.5 23.7

3.80 3.77 3.13 3.73 3.63

L. Yesmin et al.

2174

1 0.8 0.6 .. . .. . . ~.~

Week

Week .2

z

0.3

High ammonium

Week

Week

Fig. 2. Nitrate-N concentrations in drainage water from peat turfs dominated by grass species (dotted line) and C. vulgaris (solid line) over 12 months, shown with the input nitrate-N concentration in simulated rain (broken line). Bars represent standard errors of the mean. The results for elevated ammonium and elevated nitrate treatments have been plotted on different scales.

use of nitrate inputs from all treatments than the did. Generally, nitrate Calluna uulgaris microcosms uptake by plant and/or microbial biomass was

10 8 6i

greatest over the first few weeks of the treatment application, but there was a very marked decrease in nitrate concentration in drainage water from the

Control

Week 5 $ 10 c &8 =‘d 6 t

Medium ammonium

9

P) SM

Medium nitrate

F4 6

Week

.z10 9

EJ 10 3

81 ~~~~

2

Week

High ammonium

6 10 14 18 22 26 30 34 38 42 46 50 Week

Fig. 3. Ammonium-N concentrations in drainage water from peat turfs dominated by grass species (dotted line) and C. vulgaris (solid line) over 12 months, shown with the input ammonium-N concentration in simulated rain (broken line). Bars represent standard errors of the mean.

Peat drainage water response to N deposition

2175

Week 2 4,

z4 O3

High ammonium

I

O 3;

2

2

1

1

0,

2

‘, ” ” ” 6 10 14 18 22 26 30 34 38 42 46 50 Week

o!2

, High nitrate

6 10 14 18 22 26 30 34 38 42 46 50 Week

Fig. 4. Organic-N concentrations in drainage water from peat turfs dominated by grass species (dotted line) and C. vulgaris (solid line) over 12 months. Bars represent standard errors of the mean.

Culluna microcosms for the high nitrate treatment from early May to late August (weeks 34-50). There was a consistent seasonal effect in the leachate nitrate concentration for the grass and Cullunn microcosms for the control and increased ammonium treatments, with a trough around weeks 3242 (late April to the end of June). This was also observed for the medium nitrate treatment, and for the high nitrate treatment for the grass microcosms only. Ammonium. The drainage water ammonium concentration increased steadily over time for the control and for all enhanced N deposition treatments, and was conspicuously and consistently greater under C&mu than under grass (Fig. 3). Under Cullunu, concentrations of ammonium in leachate generally exceeded those in the simulated rainfall, except for the high ammonium treatment. Even for this treatment, output ammonium concentration exceeded input ammonium concentration by the end of the experiment. It is readily apparent that, compared with the control and medium nitrate treatments, the high nitrate treatment significantly increased the leachate ammonium concentrations, under both Calluna and mixed grass vegetation. Seasonal trends were much less regular for leachate ammonium concentration than for nitrate concentration. Organic N. The organic-N concentration versus time plots show rather variable trends (Fig. 4), although the concentrations tended to be greater under Cullunu than under the mixed grasses,

especially at the higher N treatments. The high ammonium treatment appears to be causing an increase in leachate organic-N concentration under both vegetation types. Annual N jluxes The annual inputs and output fluxes of the different N species are shown in Fig. 5. This clearly shows that there was nitrogen retention for all treatments. There was greater retention of N under grass than under Callunu, especially for the increased nitrate treatments. The amount of N retained increased markedly with total N input. In the treatments where the extra N input was applied as ammonium, nitrate uptake by the ecosystems was substantial, with very little nitrate in the leachate, regardless of total N load or vegetation type. For the control, medium and high ammonium treatments, 78-94% of the nitrate input was retained. Nitrate leaching increased with nitrate inputs, however. Ammonium output tended to increase with total N input. Under Cullunu, ammonium outputs were greater than inputs at low levels of ammonium input (control, medium and high nitrate treatments), but ammonium retention occurred at higher levels of ammonium input under Cullunu and under grass for all treatments. For the same total input of nitrogen, N retention was greater with elevated ammonium than with elevated nitrate. Ammonium-N leaching and organic-N leaching both tended to increase with increasing nitrate application rate.

L. Yesmin et al.

control

High nitrate

Medium nitrate

i

Grass

Medium ammonium

High ammonium

control

Medium ammonium

High ammonium

I

-401

-40 -60

control

control

Medium nitrate

High nitrate

-601

Fig. 5. Annual inputs and outputs of N species for peat turfs dominated by grass species and C. vulgaris subjected to simulated rain with field (control) and elevated levels of nitrate- and ammonium-N. Positive values (above the line) represent inputs in simulated rain, negative values (below the line) represent outputs in drainage water.

DISCUSSION

The acidification of drainage water (Fig. 1) as a result of increased depositions of ammonium sulphate and nitric acid is to be expected as a consequence of the mobile anion effect (Wright et al., 1988). In other words, the increase in leachate nitrate concentration for the elevated nitrate treatments (Fig. 2) and in sulphate concentration for the elevated ammonium sulphate treatments results in a higher H+ concentration in soil solution, and hence in drainage water. Sulphate leaching was not monitored in this experiment, but would almost certainly have increased in response to the ammonium sulphate treatments. The only pH trends with time possibly attributable to temporal changes in nitrate leaching were for the high nitrate treatment. Under the mixed grasses, as nitrate concentration increased steadily between weeks 38 and 52 (Fig. 2), there was a corresponding decrease in pH (Fig. l), while under Calluna, the fall in nitrate concentration between weeks 32 and 48 was associated with a corresponding pH increase. At least three mechanisms could contribute to the effect of vegetation type upon drainage water pH. Firstly, the effect could be partly attributable to greater nitrate and, possibly, sulphate uptakes by grass compared with C&ma. The reduced mobile anion concentration resulting, for example, from increased nitrate uptake by grass, as is clearly visible in Fig. 2, would increase the pH. Secondly, the effect of vegetation type on pH could be partly a

consequence of change in the nature of the plant litter and litter decomposition rate. Thirdly, greater uptake of nitrate over several years would be expected to result in an increase in the effectiveness of this important proton sink process (INDITE, 1994) for grass compared with Caliuna. In the latter context, when all N inputs and outputs are known, as in this experiment, the N fluxes may be used to calculate theoretical proton flux effects associated with N transformations, using the organic-N pool as a baseline reference (INDITE, 1994; Reuss and Johnson, 1986; Van Breemen er al., 1983). On this basis, organic-N leaching has no effect; a mole of ammonium input releases a mole of protons, a mole of ammonium leached consumes a mole of protons, a mole of nitrate input consumes a mole of protons, and a mole of nitrate leached releases a mole of protons. The N-derived proton fluxes thus calculated suggest that the N transformations associated with the ambient N input treatment partly ameliorate the effect of the H+ input in the rain, by 0.27 and 0.48 kg H+ ha-’ yr-’ for grass- and Culluna-dominant turfs, respectively. Increasing the N deposition load two- and six-fold by increasing ammonium input exacerbated the direct acidification from H+ inputs, by 0.53 and 3.62 kg H+ ha-’ yr-’ for grass-dominant turfs and by 0.12 and 3.38 kg H+ ha-’ yr-’ for Calluna-dominant turfs, respectively. If, on the other hand, the two- and six-fold increases in N deposition were as nitric acid, the effects of N transformations were increasingly ameliorative as the nitric acid input increased. The N transformation-

Peat drainage water response to N deposition derived neutralisation amounted to 0.95 and 4.06 kg H+ ha-’ yr-’ for grass-dominant turfs, and to 0.96 and 3.42 kg H+ ha-’ yr-’ for Culluna-dominant turfs, respectively. The substantial proton sink effect associated with nitrate deposition/transformation agrees with observations in a previous experiment using moorland microcosms collected from along a pollution gradient (Yesmin et al., 1995). It should be realised, of course, that most of the neutralisation effect will be spent neutralising protons associated with nitrate in the nitric acid inputs. In the present experiment, for the medium and high N treatments, 72% and 71%, respectively, of the H+ associated with nitric acid inputs was effectively neutralised as a consequence of N transformations. Overall, therefore, the peat may still be acidified by high levels of acid deposition, but not to anything like the extent expected from the deposited proton flux alone. It should be emphasised, however, that the response to increased N deposition was being studied during a transitionary phase. The response to increasing nitrate deposition of peats under predominantly Calluna moorland is likely to be a rapid increase in nitrate leaching through to rivers. This breakthrough has already been reported in a study of riverbank soil effects on Scottish upland rivers (Black et af., 1993). Sustained high levels of nitrate deposition also eventually result in elevated ammonium concentrations in associated drainage waters. Sustained high levels of ammonium deposition rapidly cause an increase in ammonium leaching. While this must result in increased ammonium concentrations in river water draining from peats, ammonium passing through peats unchanged will not, on its own, acidify either the peat or the associated drainage water. Any acidification effect upon drainage water is thus likely to be associated with mobile accompanying anions, usually predominantly sulphate in the U.K. The enhancement of nitrate leaching as a consequence of elevated ammonium deposition is very small, because the nitrification rates in highly acidic upland peats are very small (INDITE, 1994). Trends in the concentrations of organic-N over the duration of this experiment were rather variable. In experiments using peats from along a pollution gradient, high N deposition resulted in a marked decrease in the peat C:N ratio and an increase in organic-N leaching (Yesmin et al., 1995). This reflects the high capacity of peats to function as N sinks over several years, before significant increases in mobile organic-N occur. CONCLUSIONS

The experiment confirmed the limited capacity (Black et al., 1993) of peats to function as sustained

2177

sinks for high levels of nitrate deposition. It showed that ammonium leaching increases relatively rapidly in response to elevated ammonium deposition, and confirmed that little of the ammonium deposited to peats is nitrified. It highlighted dramatic differences between the response of peats supporting Callunadominated vegetation compared with those supporting predominately grasses. This has serious implications when attempting to make impact assessments of elevated N deposition on soils and associated streams. Acknowledgements-The

authors are indebted to the U.K. Department of the Environment Air Quality Division and to NERC for financial support for this study, and to the International Rotary Foundation who funded Ms. Yesmin’s studentship. The authors also wish to thank the landowner for permission to work at Glen Dye. REFERENCES

Billett M. F., Fitzpatrick E. A. and Cresser M. S. (1990) Changes in the carbon and nitrogen status of forest soil organic horizons between 1949/50 and 1987. Environ. Pollut. 66, 67-79.

Billett M. F., Fitzpatrick E. A. and Cresser M. S. (1993) Long-term changes in the nutrient pools of forest soil organic horizons between 1949/50 and 1987, Alltcailleach Forest, Scotland. Appl. Geochem., Suppl. Issue, No. 2, 179-183. Black K. E., Lowe J. A. H., Billett M. F. and Cresser M. S. (1993) Observations on the changes in nitrate concentrations along streams in seven upland moorland catchments in northeast Scotland. War. Res. 27, 1195-I 199. Duckworth C. M. S. and Cresser M. S. (1991) Factors influencing nitrogen retention in forest soils. Environ. Pollur. 72, l-21. Edwards A. C., Creasey J. and Cresser M. S. (1985) Factors influencing nitrogen inputs and outputs in two Scottish upland catchments. Soil Use Manage. 1, 83-87. INDITE (1994) Impacts of Nitrogen Deposition in Terrestrial Ecosystems. Report of the United Kingdom Review Group on Impacts of Atmospheric Nitrogen Deposition on Terrestrial Ecosystems, Department of the Environment, London. Reuss J. 0. and Johnson D. W. (1986) Acid deposition and the acidification of soils and waters. Ecological Studies, Vol. 59. Springer-Verlag, New York. Skiba U.. Edwards A.. Peirson-Smith T. and Cresser M. S. (1987) ‘Rain simulation in acid rain research-techniques, advantages and pitfalls. In Chemical Analysis in Environmental Research (Edited by Rowland A. P.), DD. 1623. ITE Svmnosia 18. ITE. Abbots Rioton. UKRGAR (1990) &ii Deposiiion ii the United’Kingdom 19861988. Third Report of the United Kingdom Review Group on Acid Rain. Department of the Environment, London. Van Breemen N., Mulder J. and Driscoll C. T. (1983) Acidification and alkalinization of soils. Plant Soil 75, 28S308.

Wright R. F., Norton S. A., Brakke D. F. and Frogner T. (1988) Experimental verification of episodic acidification of freshwaters by seasalts. Nature 334, 422424. Yesmin L., Gammack S. M., Sanger L. J. and Cresser M. S. (1995) Impact of atmospheric N deposition on inorganicand organic-N outputs in water draining from peat. Sci. Total Environ. 166, 201-209.