ACIDITY OF OPEN AND INTERCEPTED PRECIPITATION IN FORESTS AND EFFECTS ON FOREST SOILS s IN ALBERTA, CANADA

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1 41n11mnoPowniven ACIDITY OF OPEN AND INTERCEPTED PRECIPITATION IN FORESTS AND EFFECTS ON FOREST SOILS s IN ALBERTA, CANADA J. BAKER and DRAKE HOCKI Northern Forest Research Centre, Edmonton and MARVIN NYBORG University of Alberta, Edmonton Abstract. Emissions of SO, appear to have an acidifying effect on grossfall (open rainfall), throughfall, stemflow and soil solution at sites near major sources. Resulting effects on soil chemistry include elevated extractable acidity and aluminum and depressed exchangeable bases, especially Ca and Mg. These changes are mostly in the incipient phases in the study area. 1. Introduction Approximately 0.5 x 10 6 t SO 2 are emitted annually into the atmosphere throughout Alberta, mostly as a result of residual inefficiency of S recovery from sour (H2S containing) natural gas. An earlier paper in these proceedings describes the regional effects of these emissions on acidity of precipitation (Nyborg et al., 1976). Several large point sources of SO 2 are located in forested areas where they have strong local influences (Baker et al., 1973). Preliminary laboratory studies showed that solution and transformation of S compounds in the soil are likely to greatly influence the solubility, mobility and distribution of soil minerals and nutrients (Baker, 1976). This article reports early field data on chemistry of rainfall from two localities in Alberta affected by SO 2 emissions, and effects on forest soils from one of those localities. 2. Methods One locality surrounded two major S-extraction natural gas treatment plants near Rocky Mountain House, Alberta (lat ' N, long ' W), referred to as RMH, and is an area where soil studies are progressing. The other locality surrounded a processing plant on the Athabasca Oil Sands of northeastern Alberta near Fort McMurray (lat ' N, long ' W), and will be referred to as the GCOS area. Experimental sites were located at varying distances from the SO2 sources. Each site was equipped with plastic rainfall traps that included units to collect grossfall (uninterrupted rain), stemflow, and three throughfall (dripping off foliage) collection units. Collections were made throughout the growing season at weekly intervals or after each major rain event. Fallen snow was collected once from each site during the winter. All samples were analyzed for ph and S content after storage at 1 C for varying periods. Water, Air, and Soil Pollution 7 (1977) All Rights Reserved Copyright m 1977 by D. Reidel Publishing Company, Dordrecht-Holland

2 450 J. BAKER ET AL. At the RMH sites, soil was sampled from the litter surface (L-F-H), and at 0 to 5 cm, 5 to 15 cm and the 15 to 30 cm depths in the autumn when the soils were very wet. Samples were stored in plastic bags in cold rooms (1 C) until analyzed (for 4 mo). Field-moist samples were used in all analyses. Eight grams of moist L-F-H and 16 g of moist mineral soils (approximately equivalent to 4 and 12 g on an oven dry basis) were first extracted alternately with 25 ml each of warm (40 C) water and 1 N KCI, until 250 ml of leachate were collected. Each addition of water or salt was allowed to completely drain (1 h) before the addition of the next. Extraction was carried out using a V-shaped funnel equipped with a quantitative filter paper and a hardened No. 50 Whatman filter disc on the surface of the sample. These water-kc1 extracts were analyzed for ph, exchangeable acidity, Al, Ca, Mg, Mn, Fe, N, and P. Following the KC1 extraction, selected samples from sites near to and distant from the SO 2 source were sequentially further leached with 100 ml each of (40 C) 0.5 N NaF, 0.1 N NaOH and 0.5 N H 2S0, (after Jackson, 1958), for P and N determinations. Aliquots were digested with concentrated H 2S0, and a Cu catalyst. No attempt was made to differentiate between organic and inorganic P and N. Evaluations were made of the total amounts removed by the various extractants. Measurements for ph and KC1-extractable acidity were carried out on an automatic Metrohm E436 potentiograph. Titrations were done with 0.04 N Ca(OH) 2. The normal and a differentiated titration curve was carried out for each extract. Measurements of Al, Ca, Mg, Mn and Fe in KCI were made using a P.E. Atomic Absorption equipped with a digital concentration readout. Total P content in each of the four extracts was determined by the molybdate method using ascorbic acid as a reducing agent. Nitrogen evaluations were obtained by distillation of the digested extracts into boric acid and titrated to ph 4.7 with standard H 2SO4 using a dead-stop Radiometer titrator. Where S was determined, the Dean modification (Dean, 1966) of the method of Johnson and Nishita (1952) was used. 3. Results The ph values of grossfall, throughfall and stemflow for each major rainfall event of the 1974 growing season are shown in Figure 1 for 3 representative sites in the RMH area, one distant and two relatively near the SO 2 source. Included in Figure 1 are ph values of KC1 extracts from various soil layers of the given test sites. Average ph values of throughfall and stemflow for the entire season are shown in Figure 2 for sites near the SO 2 source in the RMH area, and are given in Table I for sites in the GCOS area. Table II gives the S content of the season's rains in the GCOS area, converted to S deposited in Kg ha-' mo- 1. The ph and the S content of snow from the two areas are shown in Table III. Notable in the rainfall data are the consistently lower ph values of intercepted rain than open rain, and the seasonal trend (Figure 1) towards slightly higher values from an initial minimum. Data for the constituents of the soil leachates are presented in Tables IV and V for

3 ACIDITY OF OPEN AND INTERCEPTED PRECIPITATION IN FORESTS , Idle.dde peek Aar rane-5 S-an Lake lee.alde Creek gross fall rh, fall Nt Ne V Ne \ a,t V V \ c NS v `e 1c v t - N6 \ c NC ne se NS V Ng nc,c 'C ne e se v ne sa se se 4,= i iii ,fa n ne-s Swan Lake oefe v ne n n n ' s. s n ; s.: s S' n n n,c \c \ \ N S i \ S \ c \ \ \ N N N.,c V Sz V. \.. N... N N %c V \. \ a V m V V \ c \ \ c V NS \ c,e,. n NE V V ke n4 4 v Fig. 1. Change in ph of gross fall throughfall and stemflow solutions from control and S-contaminated sites during the 1974 sampling period; soil ph values (KCI extracts) for corresponding sites are also given _ Scale 1 cm= 1.3 Km 57, , GAS PLANT s",, co 6.3 '4, , , Fig. 2. Average throughfall and stemflow (underlined) ph, June 15 to September 5, 1974, of selected sites in the RMH area.

4 452 J. BAKER ET AL. TABLE Average ph of grossfall and spruce stemflow during July to September, 1974; for sites in the GCOS area Site Distance (direction from SO 2 source (km) ph Grossfall Stemflow Controls (2) (SW) GCOS-I 3.2 (NW) GCOS-2 8 (NW) GCOS (E) GCOS (E) GCOS-6 9 (E) TABLE II Sulfur deposited by grossfall and by spruce throughfall a, GCOS area, during the 1974 growing season (July to September) Site Distance (direction) from SO 2 source (km) S Deposited (kg ha mo)-' Grossfall Throughfall Control- I 56 (SW) Control-2 24 (SW) GCOS-I 3.2 (NW) GCOS-2 8 (NW) GCOS-3 8 (S) GCOS 3A b 8 (S) a Data for stemflow not given owing to lack of a quantitative factor for conversion. h Aspen. individual sites in the RMH area. Averaged for sites near to and sites distant from the main SO 2 source, the most prominent features of the data are that for KCl extractable and exchangeable Al which are much higher in high exposure/impingement sites than in distant ones (Figures 3 and 4). Values for the ratio between exchangeable Al: extractable bases are plotted against soil ph, for samples of mineral soil, in Figure 4; showing a strong trend of reduced ph for greater Al: base ratio. 4. Discussion Notable in the rainfall data (Figure 1) is the trend towards increasing ph in successive events through the season. Also, although not as acidic as near some heavily industrial areas, the ph of intercepted precipitation, especially stemflow, is consistently much lower than open grossfall. The contained S also is significantly greater.

5 ACIDITY OF OPEN AND INTERCEPTED PRECIPITATION IN FORESTS 453 TABLE III Soluble S content and ph of snow Direction and distance (km) Sampling S in snow ph of of site from source date (1974) kg ha-' snow GCOS area S, 4.8 February S, 9.6 February NW, 3.2 (on river ice) February SW, 56 February E, 2.4 April E, 4 April SW, 24 April RMH area N, 1.0 N, 12.8 April 4 April NW, 6.4 April S, 1.0 April S, 4.0 April SE, 11.7 April SE, 20 April E, 1 April NE, 9.6 April NE, 24 April Northern February Saskatchewan (`Control') Included in Figure 1 are ph values of KC1 extracts from various soil layers of the given test site. Comparing soil extract ph values with those of stemflow, grossfall and throughfall, suggest that the acidic nature of these solutions, particularly near the SO2 source, is having an acidifying effect on the soil, especially in the two surface layers of soil. The minimum ph value of rain in the first-sampled event of the season, perhaps is due to a greater portion of surface-absorbed SO 2 being collected in the first sampling period. Within any sampling date, there seems to be a positive relationship between grossfall and throughfall, i.e. an increase or decrease in the ph of grossfall is usually accompanied by an increase or decrease in throughfall. This relationship is not seen in the ph of stemflow and suggests that the acidic nature of bark may play as great or greater role in the ph of stemflow as surface-absorbed SO 2. With both throughfall and stemflow, the leaching of organic acids from vegetation likely contributes to the acidity of these solutions. Greater acidity of grossfall should create conditions for greater leaching of organic acids. There is a general trend for grossfall and throughfall ph values to increase over the sampling season which suggests that with frequent summer rains there is little chance for SO 2 accumulation in the atmosphere or on the surfaces of foliar material. This

6 TABLE IV P and N, in mg kg- 1, removed sequentially by various extractants from soils sampled at sites near to and distant from the SO 2 source, RMH area Idlewilde Ck. (distant from SO 2 source, 32 km W) L-F-H 0-5 cm 5-15 cm cm P KCI NaF NaOH H 2SO 4 Total N KCl NaF NaOH H 2SO 4 Total Total Radiant Ck. L-F-H (distant from 0-5 cm SO 2 source 5-15 cm km W) cm Total Average for L-F-H distant sites 0-5 cm cm cm Total Aquitaine-S L-F-H (near SO 2 source 0-5 cm km S) 5-15 cm cm Total

7 Aquitaine-N L F H (near SO 2 source 0-5 cm km N) 5-15 cm cm Total Swan Cl. L F H (near SO, source 0-5 cm I km S) 5-15 cm I cm Total Swan Lk. L F H (near SO 2 source 0-5 cm km SE) 5-15 cm cm nil nil Total Average for L F near sites 0-5 cm cm cm Total

8 TABLE V Acidity, exchangeable cations and ph of 1 N KCI extracts for soils sampled at sites near to and distant from the SO 2 source, RMH area Sites KCI ext. ph Acidity - meg Cations - meg 100 g-' 100 g-' KC1 ext. to to ph 5.5 ph 8.2 Al Ca Mg Fe Mn Idlewilde Ck. L-F-H (distant from 0-5 cm nil nil SO, source, 5-15 cm nil nil km W) cm nil nil Radiant Ck. L-F-H (distant from 0-5 cm nil SO, source, 5-15 cm nil nil km W) cm nil nil Ricinus L-F-H 6.1 nil nil nil (distant from 0-5 cm nil SO 2 source, 5-15 cm nil nil km SE) cm nil nil Jack Fish Lk. L-F-H 6.1 nil nil (distant from 0-5 cm 5.5 nil nil nil SO 2 source, 5-15 cm nil km NXW) cm nil Average for four L-F-H distant sites 0-5 cm nil nil cm nil nil cm nil nil Aquitaine-S L-F-H (near SO, source, 0-5 cm km S) 5-15 cm nil cm nil nil Total a bases Al bases Ca/Mg

9 Gulf L-F-H (near SO, source, 0-5 cm nil km SW) 5-15 cm nil cm nil nil Pr. Ck. Camp L-F-H 5.8 nil nil (near SO, source, 0-5 cm nil nil km NW) 5-15 cm nil cm nil Pr. Ck. Ran. Sta. L-F-H (near SO, source, 0-5 cm nil nil km N) 5-15 cm nil nil cm nil nil Aquitaine-N L-F-H (near SO 2 source, 0-5 cm km N) 5-15 cm nil cm nil Swan Ck. 1.-F-H 4.3 l (near SO 2 source, 0-5 cm nil km S) 5-15 cm nil nil cm nil nil Swan Lk. L-F-H (near SO, source, 0-5 cm km SE) 5-15 cm nil nil cm nil nil Average for seven L-F-H near sites 0-5 cm cm nil cm nil nil a Excluding Al.

10 458 J. BAKER ET AL EXTRACTABLE ACIDITY (KCL) EXTRACTABLE AL (KCL) EXTRACTABLE CA (KCL) 9 ::::: EXTRACTABLE MG (KCL) ; CONTROL S-ALTERED CONTROL S-ALTERED CONTROL S-ALTERED CONTROL S-ALTERED 0-5 co cm CM Fig. 3. Effect of S-products on certain soil characteristics ' mineral samples only 03 0,# # o 01 o r 0 90, 0 0 ' ph Fig. 4. A plot indicating the general relationship between ph and the Al/bases (mainly Ca + Mg) ratio..

11 ACIDITY OF OPEN AND INTERCEPTED PRECIPITATION IN FORESTS 459 feature is not seen in the ph values of stemflow throughout the sampling season and may confirm the suggestion that acid groups leached from bark surfaces may be more responsible for the acidity found in stemflow. The basis for sequential extraction of P and N lay in the hypothesis that if SO, fallout increased the acidity of the soil solution, there would be greater solubility and mobility of soil constituents, particularly phosphorus, which is normally considered fairly immobile (Fried and Broeshart, 1967). In addition, if solubilization of P-bearing constituents took place, the released P should be found in association initially with aluminum, and, as the soil ph decreased further, in association with iron. Work presently in progress in this laboratory indicates that, for forest soils, the extractants used remove P fractions which differ somewhat from those removed from aerated cultivated soils, as originally suggested by Jackson (1958). This sequential leaching approach should be useful in detecting shifts in distribution of soil phosphorus between different cationic species, especially Al and Fe. Current data (Table IV) show that both P and N are concentrated in the litter surface (L F H horizon) and thus the source of much of these constituents probably is organic. This fact should make any effect of SO 2 on the distribution of soil-p more easily detectable. Generally, NaOH removed more P and N than the other extractants, suggesting alkaline hydrolysis and liberation of organically bound P, particularly from the L F H horizon. In mineral soils, P may be removed by anion exchange, OH ions liberating phosphates bound to Al and Fe (Fried and Broeshart, 1967). Mineral soil samples from sites near the SO 2 source show KCI extracts with much higher exchangeable Al values than soils from distant sites. This suggests that the acidifying effect of SO, in the soil solution brought about a modification of the cation composition of the soil exchange complex and that AP + is much more prominent in SO 2 contaminated soils. There is in fact, a consistent relationship between the aluminum content of the KC1 extract and acidity, both to ph 5.5 and ph 8.2 (unpublished preliminary results) than between acidity and ph, suggesting that ph alone may not be a reliable parameter to describe changes in soil acidity. 5. Conclusions After two years of studies, data show that SO 2 appears to be having an acidifying effect on grossfall, throughfall, stemflow and soil solution at sites near major sources. Potassium chloride extractable acidity and aluminium are greater in soils suspected of SO, contamination, while exchangeable bases (especially Ca and Mg) are lower. The acidifying effect of SO, at present is not as obvious on soil anionic constituents as that on soil cationic constituents. Nitrogen and P cycling are vitally linked with forest growth. Increased soil acidification could ultimately bring about the removal of these from the root zone or create conditions whereby these elements would be chemically or physically fixed so as to be unavailable for stand use.

12 460 J. RAKER ET AL. References Baker, J.: 1976, Bench Studies of the Effects of SO 2 on Soils, Can. Dept. Environ, Can. For. Serv. Bimonthly Res. Notes, in press. Baker, J., Hocking, D., and Nyborg, M.: 1973, in D. Hocking and D. Reiter (eds.), Proceedings of a Workshop on Sulfur Gas Research in Alberta, Inf. Rept. NOR-X-72, Northern Forest Research Centre, Edmonton. Dean, A. G.: 1966, Analyst 91(1085), 530. Fried, M. and Broeshart, H.: 1967, The Soil-Plant System, Academic Press, N.Y. Jackson, M. L.: 1958, Soil Chemical Analysis, Prentice-Hall, Engelwood Cliffs, N.J. Johnson, C. M. and Nishita, H.: 1952, Anal. Chem. 21(4), 736. Nyborg, M., Crepin, J. M., Hocking, D., and Baker, J.: 1976, Effect of Sulphur Dioxide on Precipitation and on the Sulphur Content and Acidity of Soils in Alherta, Canada, Proc. 1st International Symp. on Acid Precipitation and the Forest Ecosystem, Columbus, Ohio.