Effects of simulated precipitation acidified with sulphuric and/or nitric acid on the throughfall chemistry of Sitka spruce Picea sitchensis and heather Calluna vulgaris

Environmental Pollution (Series B) 11 (1986) 255-270

Effects of Simulated Precipitation Acidified with Sulphuric and/or Nitric Acid on the Throughfall

Chemistry of Sitka Spruce Picea sitchensis and Heather Calluna vulgaris

U. Skiba, T. J. Peirson-Smith & M. S. Cresser

Department of Soil Science, University of Aberdeen, Aberdeen AB9 2UE, Great Britain

A B S T R A C T

Simulated acid rain was shown to increase the leaching of cations.[rom Sitka spruce and Calluna. The sum 0[" the cations ( Mg 2+, Ca 2 +, Mn 2 + K + and Na +) was linear O' related to the extent o.f H + uptake (neutralisation) by young Sitka spruce trees. The response o f Sitka spruce branches to a progressive decrease in [H + ] in the simulated rain over 1 h was a rapid decrease in Ca 2+, Mg z+ and K + leaching. For both simulated mist and simulated rain at pH 3,5 cation leaching was greater as a result q[ acidification with H2SO 4 rather than HNO 3 or an equinormal mixture of both acids; no significant d(fference was./ound between cation leaching when the rain was acidified with HNO 3 or the HNO3/H2SO 4 mixture. The leaching of total organic carbon, NO3, SOl- and Cl- was not affected by the acidity of the rain. However, some uptake of NO 3 was observed when Sitka spruce was exposed to simulated rain acidified with HNO 3.

INTRODUCTI ON

Rainwater passing through vegetation may be enriched in certain elements, including all the essential elements, and in organic substances (Tukey, 1970). Enhanced losses of especially Ca 2 +, M g 2÷ and K + occur from leaves in polluted environments (Ulrich, 1983), or when simulated acid mist or rain is applied (Fairfax & Lepp, 1975; Wood & Bormann,

255 Environ. Pollut. Ser. B 0143-148X/86/$03.50 © Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Great Britain

256 U. Skiba, T. J. Peirson-Smith & M. S. Cresser

1975; Scherbatskoy & Klein, 1983; Abrahamsen, 1984). Leaching losses were shown to be compensated by an accelerated rate of root uptake (Mecklenburg & Tukey, 1964). However, exceptionally high rates of leaching, especially for Mg 2+, may lead to deficiencies and eventually growth decline (Bosch et al., 1983; Tamm, 1984). In many studies no attempt has been made to include in the simulated precipitation the ions found in natural rain (e.g. Wood & Bormann, 1975; Craker & Bernstein, 1984). Some of these, however, have an appreciable effect on the quantity of nutrients leached. Tukey (1970) reported that K ÷ and Na + ions increased the quantities of nutrients leached, whereas Ca 2 + ions tended to inhibit leaching losses. Nitric acid may account for one-third to one-half of the acidity of rain (Fowler, 1980), a factor neglected in many studies reporting on the interactions of simulated acid precipitation with plants. Acidification with H2SO 4 alone is commonly employed.

The objective of this work was to compare the effects of simulated artificial rain and mist (containing the cations and anions at concentrations commonly observed for natural rain in the Aberdeen area) acidified with either nitric (HNO3) or sulphuric (H2SO4) acid, on the throughfall chemistry of Sitka spruce. Leaching losses of cations from large Sitka spruce branches subjected to acid rain and a simulated storm event are also reported. The Sitka results are compared with effects of acid and non-acid simulated rain on the cation leaching from Calluna.

EXPERIMENTAL PROCEDURE

Experiment 1

Twigs (2nd year growth; mean weight 30.8 ( + 8.6) g) were collected from a similar position on 2m high Sitka spruce trees in Kirkhill Forest, Aberdeen, north-east Scotland, in March 1984. The twigs were uniformly washed with distilled H20. After drying, each was then exposed to 10 ml of fine spray (simulated mist), from a purpose-built cross-flow pneumatic nebuliser (Edwards, 1984), of artificial rainwater (Edwards, 1984) adjusted to pH 3.0, 3.5~ 4-0, 4.5 or 5'0 with either HNO 3 (0.1 M) or H2SO 4 (0.05 M). Exposure to 10 ml of the fine spray was rsufficient to wet the twigs thoroughly without throughfall or stem flow occurring. Each treatment was applied to 3 twigs. The increase in weight due to wetting was recorded, to determine the amounts of mist retained by the twigs. After wetting, the

Acid rain throughfall in Sitka spruce 257

twigs were kept in a dust-free area at room temperature in test tubes part- filled with distilled water. The twigs were re-exposed to the mist treatment 5 times at three-and-a-half-day intervals. Finally they were subjected to a mist consisting of 110 ml of distilled water. The throughfall was collected in a tray, stored in glass vials, and then analysed for pH and cations (Ca 2 +, Mg 2 +, K + and Na +). The results were calculated as the additional cation concentrations in leachate (~ueq litre-1) over and above the concentrations attributable to deposition on the twigs during the five successive sprayings.

Experiments 2, 3 and 4

In all the experiments described below, simulated precipitation was applied by a network of purpose-built cross-flow pneumatic nebulisers, each constructed by attaching a plastic Eppendorf pipette tip to a length of 0-7 mm diameter Technicon Autoanalyser peristaltic pump tubing, so that the ends of the pump tubing and the pipette tip were aligned. The length of the pump tubing used was selected to give the required rate of delivery to the nebuliser by gravity feed. The nebulisers were attached to a Dexion framework, 180cm above ground, and produced a rainfall intensity of 1 .12mmh -1 over an area of 3.80m 2.

Experiment 2 Sitka spruce trees (124 _+ 18 cm tall, 83 _+ 28 cm wide) from Craibstone Forest (Aberdeen, north-east Scotland) were carefully transplanted into buckets filled with a sandy loam (October 1984), and kept in a glasshouse. At weekly intervals (4 times) the trees were subjected for 6 h to a simulated precipitation adjusted to pH 3.5 with either HzSO 4 (0"05 M), H N O 3 (0' 1 M) or an equinormal mixture of both. A control treatment of non-acid rain (pH 5.3) was included. For each treatment 3 trees were each sprayed from 4 nebulisers. The soil in the plant pots was covered with polythene to avoid waterlogging when the trees were exposed to the 6 h raining period. For each tree the throughfall was collected in 4 5 funnels, depending on the size of the tree. Samples were taken at hourly intervals and were kept in plastic bottles until analysis for pH, Ca 2+, Mg 2+, Mn 2+, K +, Na +, NO3 and TOC (total organic carbon).

Experiment 3 Three 2 m long branches were sawn off 10 -12 m high Sitka spruce trees in June 1984 (Kirkhill Forest). Within 5 days of collection the branches were


washed by exposure to a simulated precipitation of distilled H20 for 1 h. One day later they were subjected to artificial rain at pH 3.1 (H2504) for 1 h: and finally after a further day to artificial rain o f p H 3.1 for 20 min, at pH 4 for 20 rain and at pH 5 for 20 min (all HzSOg-adjusted ) in order to simulate typical pH change during a storm event. The branches were fixed below the nebuliser frame at an angle of 20 ° to the horizontal. Throughfall was collected every 10 min from a sloping polythene sheet and stored in polythene bottles at 15 °C until analysed for H +, Ca 2+, Mg 2+ ,K + a n d N a +.

Exper imen t 4

Calluna vulgaris (flowering) was collected from Glenbuchat (Grampian region, north-east Scotland) in August 1984 and planted into 14 × 14cm square plant pots. One day after washing with distilled H20, the Calluna

was subjected either to artificial rain adjusted to pH 3.5 with HzSO 4 or a non-acidified control rain, for 4 h in both instances. For each treatment 4 plants were taken. Throughfall was collected in trays (10 × 20cm) at hourly intervals and analysed for pH, Ca z+, Mg z+, K + and Na +.

For all experiments the pH of the throughfall collected was determined immediately and converted to H ÷ concentrations in #eq litre- 1. Calcium, Mg 2 +, Na +, K + and TOC were determined within 1 day of collecting the throughfall and Mn 2+ and NO 3 within 1 month. Cations were determined by flame atomic absorption spectroscopy, NO3-N col- orimetrically using a Technicon Autoanalyser and TOC by a Tocsin aqueous carbon analyser. The cation, NO 3 and TOC concentrations were calculated as additional cation, NO 3 (#eq litre - x) or TOC leached (/~g ml-1) compared to unintercepted rain collected at tree-top height.

RESULTS

Experiment 1

This experiment showed the effect of simulated mist, acidified to different pH values (pH 3.0-5.0) with H2SO 4 or HNO 3, on cation leaching from Sitka spruce twigs. On average 2.65 ( + 0.26) ml of mist droplets remained on the twigs after each spraying, depositing a considerable amount of acids, cations and anions on to the needles. When the twigs were sprayed with l l 0 m l of H20, 37.5 (+ i.7)ml were collected as throughfall,

Acid rain throughfiTll in Sitka spruce

TABLE 1 Neutralisation of Acid Mist Deposited on Sitka Spruce Twigs

259

Simulated mist Total input o f H +

p H (during 5 sprayings) (l~moles H + )

H + in ~throughfall ~

Incoming mist Incoming mist H2SO 4 H N O 3

(#moles H +) (l~moles H + )

3.0 13.3 0.012 0.041 3-5 4.20 0.072 0.022 4.0 1.33 0.076 0-023 4.5 0.42 0-053 0-027 5.0 0.133 0.028 0.014

a Means of triplicates.

containing the redissolved acids, ions originating from the mist and any materials leached from the twigs due to the acid treatments. As Table 1 shows, the deposited H2SO 4 and H N O 3 were neutralised to a large degree. The deviation from the mean pH of 3 twigs for each treatment was too large to allow meaningful comparison between various treatments.

The total amounts of cations deposited on to the twigs from 5 treatments were: 7.9/~g Ca 2+ , 1.98/~g Mg 2+ , 5.3 ~g K + and 21.3 #g Na + . Cation leaching was observed for all twigs (Fig. 1). A comparison between

100

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4.o 4.5 5 ~ o p , a.o a'.5 4'.0 4'.5 5 .0 pH

Fig. 1. The effect of mist acidified with H2SO 4 (striped bars) or HNO 3 (open bars) on the leaching of cations from Sitka spruce twigs. Significant difference between treatments:

*p < 0.1; **p < 0.05.

260 U. Skiba, T. J. Peirson-Smith & M. S. Cresset

H2SO 4- and HNO3-adjusted mists showed a significant increase for HzSO 4 mist in the amounts of Mg 2÷, K ÷ and Na t leached from twigs subjected to pH 3.5 and pH 4.0 mists, and also, for Mg 2+, to pH 4.5 mist. With less acidic mists (pH 4.5 and 5.0), the differences between the amounts of cations (except Mg 2 ÷) leached by mists containing H 2 S O 4 or

HNO 3 were not significant. Calcium leaching was only significantly higher for H2SO4-adjusted mists than for HNO3-adjusted mists at pH 4.0. The lack of any significant difference between cation leaching from twigs treated with mists acidified with H2SO 4 or HNO 3 to pH 3.0, and also to pH 3-5 for Ca 2 ÷ leaching, was possibly due to precipitation ofCaSO 4 on the needle surface at high concentrations of H 2 S O 4 as the water evaporates.

Experiment 2

In this experiment the throughfall chemistries of young Sitka spruce trees subjected to simulated rain acidified to pH 3.5 with H2SO 4, HNO 3, or a mixture of H2SO 4 and HNO 3, were compared. For a 6h period, the highest concentrations of cations, TOC and NO 3 were present in the throughfall collected over the first hour of exposing Sitka spruce to acid or non-acid precipitation. In subsequent hours the rate of decrease in cation leaching with time was lower. A typical 'leaching graph' is shown in Fig. 2. The initial higher concentration ofleachates in the throughfall was found to be due to dry deposition (details to be published). The effect was more pronounced for spruce kept out of doors after a long dry period than after a rainy period. To minimise dry deposition, trees were kept in a glasshouse between treatments.

Fig. 2.

100-

7 o"

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0

\

; 2 3 4 5 6 h

The effect of time upon leaching of Ca 2 + from Sitka spruce exposed to simulated acid rain (H2SO 4, pH 3.5) for 6 h.


The mean H + concentration of the incoming simulated rain, 0.30 mM, was reduced to 0.23 mr~ (+ 0.039); both figures are mean concentrations of 4 weekly 6h raining sessions of all 3 treatments. No significant difference in neutralisation capacity was observed when the 3 acidification treatments or the 4 weekly repeats of the treatments were compared. For all treatments, cation and TOC leaching were always higher in the first week than in subsequent weeks, due to dry deposition of the cations. In the following weeks, changes in cation and TOC concentrations were not significant, except for the leaching of K +, which decreased with time for all treatments in the same way (Fig. 3).

T ,ot 25

o , 1 2 3 4 w e e k s

Fig. 3. The effect of successive weekly simulated rain events upon leaching of K + from Sitka spruce exposed to simulated acid and non-acid rain for 6 h (means of all treatments)

for each event.

To compare the effects of acid rain containing H 2 S O 4 , H N O 3 and H2SO4/HNO 3 and non-acid rain on the throughfall chemistry of Sitka spruce, the mean concentrations of the 4 weekly treatments were taken (Fig. 4). A test of significance of the difference between all possible pairs was performed (Table 2). The different treatments did not have any significant effect on the amount of TOC leached from Sitka spruce. Cation leaching from Sitka spruce exposed to H2SO4-acidified rain was significantly higher than from trees exposed to non-acid rain. For Ca 2 +, K + and Na + leaching, the difference between H2SO 4- and HNO 3- adjusted rain was significant, as was the difference betweenHzSO 4- and H2SO4/HNO3-acidified rain for Ca 2+ leaching. With HNO 3- and HNO3/H2SO4-acidified rain, only the leaching of Mg 2+, Ca 2+ and Mn 2 + was significantly different from the leaching with non-acid rain. The differences between leaching with HNO 3- or HNO3/H2SO4-acidified rain were, however, not significant for any cation. A factorial analysis of the variance verified the overriding increase in the cation leaching from Sitka spruce exposed to H2SO4-acidified rain.

The increase in base cation leaching (E[Mg 2÷ + C a 2+ + Mn 2+ + K + + Na+]#eq litre-1) was compared with the simultaneous decrease


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Fig. 4. A comparison of the cations and total organic carbon (TOC) in throughfall of Sitka spruce subjected to 6 h simulated rain events, acidified to pH 3.5, with H2SO4(S ), HNO3(N), an equinormal concentration of both (S/N) or a non-acid control (pH 5-3) (C). Mean concentrations of throughfall from 3 trees, 6 h and 4 weekly samples are shown.

in [H +] of the incoming acid rain (Table 3). The results obtained in the first week were excluded, because of the high levels of dry deposition leading to higher cation, anion and TOC concentrations in the throughfall. The correlation coefficient for the remaining pairs of all acid treatments combined (0.807) was significant at the 0.5 ~o level. Owing to the low numbers of replicates and the limited accuracy of pH measurements, the correlation for pairs of individual treatments was not significant at the 5 ~o level. The linear regression for all pairs was calculated as E cations = 0.022 + 0.98 [H + ], indicating a linear relationship between cation leaching and decrease in [H ÷] and approximate 1:1 cation exchange. The positive intercept, and the observation that E base cations generally exceeded the amount of H ÷ apparently neutralised (Table 3), suggests that weak acids may also play a minor role. It can be assumed therefore that cation exchange for H ÷ ions on the leaf surface was the major process by which cation leaching and neutralisation of acid rain was brought about. This agrees with Tukey's (1970) observations on the leaching of nutrients from foliage.

In a previous experiment by the authors the leaching of Cl - , NO3 and


TABLE 3 A Comparison of Base Cations and the H + Concentration of

Throughfall

Rain acid(fled Increase in base cations ~ Decrease in H + b with (peq litre 1) (#eq litre l)

H2SO 4 Week 2 140 76 Week 3 176 124 Week 4 152 143

H N O 3 Week 2 76 5 I Week 3 70 66 Week 4 80 78

H2SO4/HN03 Week 2 76 46 Week 3 55 83 Week 4 44 18

Non-acidified rain Week 2 56 2 Week 3 34 2 Week 4 29 3

Mean cation leaching of 3 trees over 6 h (throughfall-input). b Mean [H + ] of 3 trees over 6 h (input-throughfall). Correlation coefficient for all acid treatments = 0.81.

S O ] - from Sitka spruce was found not to increase with increasing acidity of HzSO4-adjusted rain. In this experiment the throughfall was only analysed for NO 3. Leaching of NO 3 was observed from Sitka spruce trees subjected to non-acid and acid rain adjusted with H2SO 4 or a mixture of H2SO4/HNO 3 to pH 3.5. With Sitka spruce exposed to HNO3-acidified rain, however, after an initial leaching of NO3 for the first 3 h, uptake of NO 3 was observed for all 4 weeks of treatment. The means of the hourly results over all 4 weeks plotted against time (h) are shown in Fig. 5. The pattern of the NO3 uptake/leaching graph was very similar to the trend of base cation leaching over the 6 h period. Uptake of NO 3 and also NH~- was reported by Abrahamsen (1984) for Norwegian throughfall studies, and thought to be caused by the limited supply of N in Scandinavian forests.


100"

7 7

• •

e Z c ! o

t z o

- 2 0

1 2 3

Fig. 5. The effect of time upon the leaching and uptake of N 0 3 ( O O) by Sitka spruce exposed to simulated rain acidified to pH 3.5 with HNO 3 compared to the simultaneous leaching of base cations (Z Ca 2÷, Mg 2+, Mn z+, K +, Na +) (V- -V) . Mean concentrations of weekly throughfall samples collected every hour from 3 Sitka spruce

trees.

Experiment 3

in experiments 1 and 2, the effects of acid precipitation on the cation leaching and neutralisation capacity were studied for young Sitka trees (about 6 years old) and 2nd year growth twigs from similarly aged trees. Here we report on the effect of simulated precipitation acidified to pH 3.1 with H2SO, , on the cation leaching and H + uptake of mature Sitka spruce branches (from ca. 25-year-old trees). Acid precipitation was partially neutralised, with a decrease in [H + ] from 0"8 mM H + in the incoming rain to 0.56mMH + in the throughfall mean of 6 samples collected every 10min. The highest cation concentration was collected in the first throughfall sample (Fig. 6).

Analysis of rainwater samples through storms often shows a drop in pH immediately after the beginning of the storm (Edwards et al . , 1984). With time the pH of the rain tends to increase to its initial value at the start of the storm or a higher value. In order to simulate such a storm event an initial low pH value (pH 3.1) was chosen, increasing to pH 4 and then pH 5. Figure 6 shows that for the simulated storm event the amount of Mg 2 +, Ca 2+ and K + leaching decreased with increasing pH, and was significantly lower than when subjecting the same branch to pH 3"1 simulated rain for 1 h. The amount of Na + leaching was unaffected by the pH of the rain. In spite of exposing the branches to a simulated storm only 1 day after subjecting them to pH 3. I rain for 1 h, the initial leaching of

266 U. Skiba, T. J. Peirson-Smith & M. S. Cresset

• 51 ] v

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10 20 30 40 50 60 min

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150"

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Fig. 6. Cation leaching from Sitka spruce branches from mature trees exposed to simulated rain acidified to pH 3.1 with HESO 4 for 60min ( V - - V ) , and 24h later to simulated rain acidified to pH 3-1 for 20 min, to pH 4 for 20 min and to pH 5 for 20 min

(O-- -Q) . *p < 0.1: ***p < 0.01.

Ca 2+, Mg z+ and K + was similar. This shows that even a branch not connected to a root system is capable of replenishing cations.

Experiment 4

In this experiment the effects of simulated acid (pH 3.5, H2SO4-adjusted) and non-acid precipitation (pH 5.3) on the leaching chemistry of Calluna was studied.

The results are summarised in Table 4. The throughfall chemistry from Calluna was altered in a similar way to that of Sitka spruce. Acid rain was partially neutralised and more Ca 2 + and Mg z + leached with pH 3-5 rain than with non-acid rain. The K + leaching, however, was not altered by acid rain; and it was found that the K + concentration was much higher under Calluna than under Sitka spruce. For the acid and non-acid

AcM rain throughIall in Sitka spruce 267

TABLE 4 Analysis of the Throughfall Chemistry of Calluna Subjected to Simulated Non-acid or

Acid Precipitation

Cations leached (peq litre- l ) Throughjall analysis Arti[icial rain

p H 3.5 ~ p H 5 ~

Ca 2÷ 79.5+ 11 30.5+ 15 Mg 2+ 63.3 + 23.3 30.8 + 10 K + 100 _+56.4 100 _+90 Na + 36.5 + 17.4 34.3 _+ 21.3

Sum of base cations leached 279.3 Change in H + (incoming rain-throughfall) 170

195.6 - 1 2

" Mean of 4 plants and 4 hourly samples.

treatment, 4 plants of different ages were used, which is probably partly responsible for the high deviations from the mean. The trends in cation leaching, however, were the same for all ages. Anions: C1-, NO 3 and SO 2- were not affected by acid precipitation.

As observed for Sitka spruce, the sum of base cations leached from Calluna (peq iitre-1) was of a similar order to the decrease in H + ions when subjecting Calluna to pH 3.5 rain. With pH 5 artificial rain the sum of base cations leached was lower than when using acid rain and the H + concentration increased slightly. The slightly poorer agreement between E base cations and [H + ] for the Calluna may reflect the shorter period of simulated rainfall used.

DISCUSSION

Simulated acid wet deposition was neutralised, acid mist to a greater extent than acid rain. The extent of neutralisation of simulated rain acidified with H2SO 4 was of a similar magnitude for Sitka spruce and Calluna. Field studies have also shown decreases in H + ions in throughfall collected underneath birch, larch, Sitka spruce and pine (Miller, 1984). The neutralisation of acid precipitation was associated with enhanced Ca 2 +, Mg 2 *, Mn 2 +, K * and Na * leaching from Calluna. Comparing acid (pH 3.5) and non-acid (pH 5-3) rain, Mg 2+ and Ca 2*


leaching was increased by the same factor for both Sitka spruce and Calluna, ca. 2.5 x for Ca z+, 2 x for Mg 2+. Increased foliar leaching of Ca 2 +, Mg 2 + and K + with increasing acidity of a simulated mist has also been rePorted for sugar maple and silver birch (Wood & Bormann, 1975). Scherbatskoy & Klein (1983) showed that yellow birch and white spruce seedlings lost larger amounts of Ca 2 + and K + as the pH of the simulated mist was decreased. The linear relationship between the sum of base cations leached and H + neutralised indicates cation exchange as the mechanism for increased foliar leaching with increased acidity of rain. This has also been postulated as a possible mechanism to account for extra cation leaching with increased acidity of rain by Wood & Bormann (1975) and by Parker (1983).

Simulated acid rain for 8 h on Sitka spruce trees (experiment 2) resulted in the leaching of Ca 2+, Mg 2+ and K + equivalent to only ca. 0.1 ~ - - f o r Ca 2+, Mg 2 + - of the total foliar Ca 2+ and Mg 2+, and 0"03 3o of total foliar K ~. This calculation is based on an estimation of about 150000 needles per tree having a cation composition similar to that reported by the Forestry Commission (1970). Radioisotope studies by Tukey et al.

(1958) indicate that more than 25'/'{; of total foliar Na + and Mn 2+ and between 1 04, and 10°0 of foliar Ca 2+, Mg 2+ and K + could be leached within 24h. The observed increase in cation leaching due to acid precipitation is therefore relatively small. Plants with adequate nutrient supply are capable of withstanding losses of metabolites by brief infrequent rains (Tukey, 1980); and Lausberg (1935) suggested that losses of cations are replenished within 3 4 days after a long storm. Therefore it is not surprising to find virtually no significant differences in the cation concentrations of Sitka spruce throughfall from 4 successive weeks. Increase in cation leaching with acid precipitation is therefore probably of little importance for plants growing in nutrient-rich soil. However, it could lead to deficiencies on nutrient-poor soils, especially where leached cations may be lost in water draining rapidly downslope primarily through the upper soil horizons. Acid precipitation itself brings about a depletion of part of the nutrient reserves of the soil (Hutchinson, 1980). This, combined with enhanced foliar leaching, could lead to nutrient deficiencies, especially Mg 2+ deficiency, observed in German forests (Bosch et al., 1983).

The differences in cation leaching with acid rain and mist containing H2SO 4, HNO 3 or both cannot yet be easily explained. Certainly NO~ uptake may play a significant role. Weak complex formation between

Acid rain throughJall in Sitka spruce 269

Ca 2 + and SO 2- to form CaSO ° in solution becomes significant at SO I - concentrations above 10-4M (Lindsay, 1979), so that for HzSO 4 with a SO 2- concentration ofd.6 x 10-4M in the synthetic rain at pH 3.5, this species may facilitate exchange of H + for Ca 2+. Magnesium could behave in a similar way. For Na + and K +, NaSO~- and KSO£ formation may be relevant. These two ions behaved similarly in experiments 1 and 3 with respect to the difference between H2SO 4- and HNO3-adjusted rain. Whatever the mechanism, it is clear that sulphuric acid may cause greater cation leaching than nitric acid at the same pH. Thus where leaching is causing deficiency, S pollution may be more injurious than N pollution.

A C K N O W L E D G E M E N T S

The authors are indebted to the UK Department of the Environment and the NERC for financial support, to Michael N. Court for advice on statistical analysis, and to the Aberdeen University Forestry Department for providing young Sitka spruce trees.

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Documents

Effects of simulated precipitation acidified with sulphuric and/or nitric acid on the throughfall chemistry of Sitka spruce Picea sitchensis and heather Calluna vulgaris