Journal of Hydrology, 116 (1990) 167 183 167 Elsevier Science Publishers B.V., Amsterdam Printed in The Netherlands

[3]

PRECIPITATION, THROUGHFALL, SOIL SOLUTION AND STREAM- WATER CHEMISTRY IN A HOLM-OAK (Quercus ilex) FOREST

FERRAN RODA, ANNA AVILA and DAVID BONILLA

Centre de Recerca Ecologica i Aplicacions Forestals, Universitat Autonoma de Barcelona, 08193 Bellaterra (Spain)

ABSTRACT

Rod~, F., Avila, A. and Bonilla, D., 1990. Precipitation, throughfall, soil solution and streamwater chemistry in a holm-oak (Quercus ilex) forest. J. Hydrol., 116: 167-183.

Bulk precipitation, throughfall, soil solution at 20 and 40 cm depths, and stream water were monitored for 2 4 years in a holm-oak forest on schists in the Montseny Mountains (NE Spain). Bulk precipitation was mildly acidic, with Ca 2~ and SO~ as dominant ions. Canopy interactions produced a throughfall less acidic than bulk precipitation and enriched in all other ions. Large amounts of K ~ were leached from the canopy. Magnesium in net throughfall behaved similarly to K ~ , and it is concluded that leaching makes a major contribution to Mg ~ enrichment beneath the canopy. Judging from the moderate increase of Na* and Ca 2 in throughfall, dry deposition rates for both marine and continental aerosols were low in the studied stand, probably because of its sheltered topographic position within a well-vegetated massif, coupled with moderate tree height and low canopy roughness. Soil solution in the mineral soil was less acidic than throughfall. In common with most temperate forests, SO~ was the dominant mobile anion in the soil water, being largely accompanied by Ca 2~ . Potassium and NO:~ were depleted within the soil water with respect to throughfall, probably owing to biological uptake and cation exchange, and incorporation of K into clay lattices. Subsurface flow dominated the hydrology of the small forested catchment studied. Stream water was basic and rich in bicarbonate. Its chemistry revealed fast rates of weathering of sodium- and magnesium-bearing silicates (mainly albite and chlorite, respectively). Soil respiration and silicate hydrolysis resulted in HCO 3 being the dominant mobile anion in stream water. Calcium to chloride ratios were similar in bulk precipitation and in stream water, indicating that Ca 2 ~ release from weathering has been counteracted by plant uptake. Nutrient uptake by this aggrading forest strongly influences the solution dynamics of K ' , NO 3 and Ca ~ . It is concluded that: (1) this forest does not currently receive acidic atmospheric deposition; (2) the neutralization capacity of the soil-bedrock system is quite high; (3) biotic regulation and silicate weathering are the major processes shaping the solution biogeochemistry in this Mediterranean forest ecosystem.

INTRODUCTION

Changes in the ionic contents of water along its path through terrestrial ecosystems are highly informative of the biogeochemical processes taking place at different steps in the water fluxes. So-called water chemistry profiles have been frequently used to document such changes as water moves from

0022-1694/90/$03.50 © 1990 Elsevier Science Publishers B.V.

168 F. RODA ET AL.

precipitation to throughfall, to soil solution at increasing depths, and to ground water or stream water (e.g. Sollins et al., 1980; Binkley et al., 1982; Reynolds et al., 1986).

The objective of this paper is to bring together such information for an intensively studied holm-oak (Quercus ilex L.) forest in the Montseny Mountains (NE Spain); emphasis is placed on ion concentrations rather than fluxes because soil water fluxes are not available. Annual water fluxes are provided for precipitation, throughfall and stream water to allow conversion of weighted concentrations to fluxes; element fluxes in precipitation and through- fall are discussed by Ferres et al. (1984), and catchment element budgets by Avila and Rod~ (1988).

STUDY AREA

This study was carried out at La Castanya Biological Station in the Montseny Mountains (41°46'N, 2°21'E), 40 km NNE from Barcelona. Climate is montane Mediterranean. Mean annual air temperature is 9-10°C. Mean annual precipitation is 870 mm, but there is a large year-to-year variability in both the annual amount and the monthly distribution of precipitation. On average, rainfall is highest in spring and autumn and lowest in summer. Snowfall accounts, on average, for 3% of annual precipitation.

Slopes are very steep (30-35°). The bedrock is a low-grade metamorphic schist, with quartz, sericite, albite and chlorite as major minerals. The soil is a very stoney, sand-loam acidic dystric xerochrept, 0.4 1.5 m deep. At a depth of 4-23 cm, mean values of 10 soil samples gave: pH (H20) = 4.6, organic matter = 4.5%, base saturation = 44%; exchangeable cations (peqg 1): Ca = 17, Mg = 17, K = 1.7 and Na = 7.0.

Throughfall and soil solutions were sampled in a 0.23 hectare permanent experimental plot that has been the site of studies on forest biomass, primary production and nutrient cycling (Terradas et al., 1980; Ferres et al., 1984). The plot lies at the base of a NW-facing slope. Slopes within the plot are 7-23 °, which is gentler than the surrounding hillslopes. It is covered by a pure, closed stand of holm-oak, 9-12 m high. In 1979, the tree layer (stem diameter at breast height/> 5 cm) had a density of 2010 stems hectare -1, and a basal area of 26.6 m 2hectare 1. The understorey is very sparse. Aboveground, biomass and net primary production of the tree layer were estimated to be 160 t hectare 1 and 9.3 t hectare y e a r 1, respectively (Ferrds et al., 1980, 1984; Ferr~s, 1985). Litter accumulates on the forest floor. Part of the plot was cultivated in the past; the age of the canopy trees is unknown but it is estimated at 60-90 years from current diameters and radial growth rates.

Stream water was sampled in a permanent, first-order stream draining a gauged catchment (TM9) whose outlet is 200m away from the experimental plot. TM9 is a N-facing, 4.3 hectare catchment ranging in elevation from 700 to 1035 m with a mean slope of 36 °. Bedrock and soils conform to the description given above. TM9 is completely covered by a dense holm-oak forest that was

PRECIPITATION ETC. IN A HOLM-OAK FOREST 169

coppiced for charcoal production until ~ 30 years ago and has remained undis- turbed since then.

METHODS

Field methods

Bulk precipitation data used in this paper were obtained in two distinct two-year periods. In 1978-1980, bulk precipitation was sampled biweekly with two collectors in a clearing near the experimental plot. In 1983-1985, bulk precipitation was sampled weekly with four collectors at a clearing adjacent to the outlet of TM9. Collectors were similar during both periods: a polyethylene funnel 19cm in diameter connected by a loop of PVC or Tygon tubing to a polyethylene bottle. Funnels were permanently open to the atmosphere. A porous plug in the funnel neck prevented insects and large debris from reaching the precipitation sample. After each weekly or biweekly interval, collectors were replaced with thoroughly cleaned ones. The cleaning process at the laboratory involved washing with detergent and tap water, soaking in diluted HC1, and repeated rinsing with distilled water. Conductivity control of the last rinse was used to ensure that no contamination by residual HC1 occurred. Precipitation amounts were measured in 1978-1980 with two totalizing funnel raingauges with oil inside the bottle to prevent evaporation, and in 1983 1985 with a standard raingauge, located in both cases adjacent to bulk precipitation collectors.

Throughfall was sampled at the experimental plot during 1978~1980. Throughfall amounts were measured with four (eight from January 17, 1980) throughfall gauges randomly placed within the plot. These gauges were identical to those used for precipitation amount. Throughfall, for chemical analysis, was sampled biweekly with eight other randomly located collectors (identical to bulk precipitation collectors). They were also changed for labora- tory-cleaned ones after each sampling.

Soil solutions were sampled from November 1983 to June 1986 in the ex- perimental plot with porous ceramic-cup Soil Moisture tension lysimeters. Four lysimeters were installed at a depth of 20 cm, and another 4 at 40 cm, though until January 1985 only two lysimeters were sampled at each depth. Lysimeters were emptied weekly, being evacuated after each sampling at - 6 5 kPa with a hand pump.

Stream water was sampled at the outlet of TM9. Weekly grab samples were supplemented by intensive sampling of storm events using a 24 bottle water sampler that automatically switched on when water at the weir reached a given stage. A stage recorder provided continuous data on streamflow at the thin- crested, 60 V-notch weir.

170

Laboratory methods

F. RODA ET AL.

Upon arrival in the laboratory, unfiltered samples were equilibrated at room temperature and analysed for conductivity, pH and alkalinity. The pH was measured potentiometrically with a combined electrode; the pH meter was calibrated with buffers of pH 4 and 7 or pH 7 and 9 according to sample pH. Alkalinity was measured by conductimetric titration. If particles were visible, the remainder of the sample was then filtered through 0.45pm pore-size membrane filters or acid-washed Whatman GF/F glass fibre filters. Frozen aliquots were stored until other analyses could be made.

Metallic cations were analysed on a single aliquot after addition of 400 ppm La (as LaC13) and 1% HC1 to samples, standards and blanks. Sodium and K were analysed by flame emission, and Ca '~ and Mg 2+ by atomic absorption spectrometry; samples were run at the Servei d'Espectroscopia de la Univer- sitat de Barcelona. In samples for 1978-1980, NO:~ was analysed by standard methods on a Technicon AutoAnalyzer II, and C1 by a manual colorimetric method using mercury thiocyanate (Rod/t, 1983). Ammonium, SO~ , and NO:~, and C1 in later samples were analysed by ion chromatography.

Analytical, between-run precision of precipitation, throughfall and stream- water measurements was better than 8% coefficient of variation (CV) for all determinands and sample types (Rod~, 1983; Avila, 1988), except for NH4 (data not available) and for Na t and K * in 1978 1980 bulk precipitation that had, respectively, CV = 10% (Sx = 1.5peql :) and CV = 14% (s x - 0.6/~eql ~). Analytical accuracy was checked during 1983-1985 by analysing a synthetic sample at each run. The mean accuracy obtained was better than 7% for all determinands (Avila, 1988); however, data for NH4" are not available.

Volume-weighted mean (VWM) concentrations in bulk precipitation and throughfall were calculated by weighting each sample by the appropriate amount of precipitation or throughfall. VWM concentrations in stream water were calculated by weighting each sample by the amount of streamflow during the time interval represented by each sample. The limit between adjacent intervals was halfway between two consecutive samples. If a sudden stream flow change occurred between two consecutive samples, the limit between the respective time intervals was placed at the time when streamflow changed. The mean pH values were computed by transforming each sample pH to H' concentration, volume-weighting these, and converting back to pH.

RESULTS AND DISCUSSION

Bulk precipitation

Bulk precipitation chemistry was very similar in 1978-1980 and 1983-1985 (Table 1). Therefore, the following comments are based on the weighted average of the four available years. On an equivalent basis, bulk precipitation at Montseny is dominated by Ca 2~ and SO~ . For cations the order of

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172 F. RODA ET AL.

abundance is C a 2+ > Na + ~ N H ; > H + > Mg 2+ > K ' ; for anions, the order is SOl > C1 > NO~, although alkalinity was not systematically measured in precipitation samples (see below). Bulk precipitation at this site is rich in Ca 2~ (VWM = 65 peq l- 1) but has moderate concentrations of all the other analysed ions. In particular, VWM concentrations of Na ÷ and C1 (25 and 30peql 1, respectively) are rather moderate for a site that is only 27 km from the sea. The ratio between the equivalent VWM concentrations of NH; and NO3 is 1.23. Overall, bulk precipitation at Montseny is a Ca-enriched and H-depleted version of the precipitation chemistry prevailing over much of Europe, though the contents of SO24 and inorganic N are also much lower than in the high- deposition areas of central and northwest Europe.

On average, bulk precipitation at Montseny is only moderately acidic, with a mean pH of 4.75. However, precipitation is even less acidic than indicated by this mean pH: there is a positive alkalinity that has not been taken into account when computing the mean H" concentration. For the period 1983 1995, the VWM of the HCO3 concentration, estimated from the pH of each sample and assuming equilibrium with atmospheric CO2 is 33geql i. In a heathland catchment near TM9, Belillas (1989) measured a VWM alkalinity of 28peql ' in bulk precipitation during 1982-1984. However, annual means conceal an extremely large variation between events, with bulk precipitation pH ranging from 3.9 to 8.1 at our site. Strongly acidic precipitation (pH < 4.5) occurs in 27% of the sampling periods, which together deliver 18% of the mean annual precipitation.

So-called red rains have a marked effect on the mean precipitation chemistry at Montseny (Avila and Rodh, 1989). Red rains are produced by air masses from North Africa loaded with calcite-bearing silt (LSye-Pilot et al., 1986). During 1983 1985, red rains occurred at Montseny in eight weekly samples. These red rains had a VWM Ca 2+ concentration 15 times greater than other rains during this period, and also had high concentrations of all other ions, except H ÷ and NH; ; their mean pH was 7.68 and their estimated VWM HCO3 was 537 peq 1 1. Such highly concentrated and alkaline events therefore had a pronounced effect on annual VWM ion concentrations, even if they accounted for only 5.4% of the total precipitation.

Correlation and regression analyses were used to identify probable origins for ions in bulk precipitation at Montseny (Rod~, 1983; Avila, 1988). Sodium and C1 were highly correlated (r = 0.99), their regression line was close to that of seawater proportions, and the Na+/C1 ratio between VWM concentra- tions was only slightly below that for seawater. These observations indicate that almost all Na ~ and C1 in bulk precipitation at this site comes from sea-salt aerosols. Using Na + as the reference ion, and assuming no subsequent fractionation, sea-salt aerosols account for the following percentages of the VWM ion concentrations at Montseny: K + = 16, Ca 2+ = 2, Mg 2+ = 46, SOl = 6, and C1 = 94. Hence, continental sources deliver about half the Mg 2÷ and most of the K" and Ca 2+ in bulk precipitation at this site. Anth- ropogenic SO4 z- probably explains most of the 94% of 'excess' SO ]-, although biogenic SOl could also contribute to it.

PRECIPITATION ETC. IN A HOLM-OAK FOREST 173

TABLE 2

Multiple regressions of SOl and NO~ against weekly concentrations of Ca z' , NH~ and H + in bulk precipitation at La Castanya (Montseny) for the period 1983 -1985. Weekly samples with red rains were not included in the regressions

R e g r e s s i o n a r e n

SO] - 0.54 Ca ''~ + 0.58 NH~ + 0.47 H + 2.4 0.88 75

NO:~ = 0.17 Ca ~' + 0.44 NH~ + 0.20 H - 1.9 0.82 75

~The independent terms were not significantly different from zero in either regression. All regres- sion coefficients were significant (P < 0.05) in both regressions. Data from Avila (1988); concentrations in peq 1 '.

Multiple regressions of SO~- and (separately) NO~ on Ca 2., NH~ and H ÷ account for most of the variat ion of weekly concentrat ions of both anions in non-red rains during 1983 1985 (Table 2). (SO42 and NO~ in non-red rains represent, respectively, 86 and 89% of total inputs of each anion in bulk precipitation during this period.) Assuming Ca 2+, NH; and H ÷ to be uncor- related among themselves, regression coefficients in Table 2 can be interpreted as the proportion of the equivalents of each cation that accompanies SO42 and NO~-. Although the cations are correlated, the regression coefficients have been found to be ra ther robust in stepwise multiple regressions. Thus, the data in Table 2 suggest that 54% of the Ca 2+ equivalents in bulk precipitation would go with SO24 , and 17% with NO~. For NH~, 58% would go with SO~ and 44% with NO3 (these percentages add to slightly over 100%, probably owing to the combined effects of their standard errors, non-zero independent terms, and cross-correlations among the independent variables). For H ÷, 47% would go with SO42 and 20% with NO3. As C1 is accompanied by Na ~, most of the missing Ca 2* probably goes with HCO~, while the missing H ÷ should go partly with HCO3 and partly with organic anions. Applying the above percentages to the VWM concentrat ions in non-red rains during 1983-1985, SO] equivalents in bulk precipitation are 42% neutralized by Ca z÷, 31% by N H : , and 20% by H ÷ (totalling 93%). Nitrate equivalents are 35% neutralized by Ca 2+, 62% by NH~ and 22% by H ÷ (totalling 119%). Although, as commented above, such percentages have important uncertainties, the overall picture that emerges from this exercise is that the inorganic S and N chemistry of bulk precipitation at Montseny is dominated by sulphates and nitrates of calcium and ammonium, whereas sulphuric and nitric acids play only a minor role. To this picture, calcium bicarbonate has to be added as a major component.

Throughfall

Throughfall under holm-oak at Montseny is on average more concentrated and less acidic than bulk precipitation (Table 3). On an equivalent basis, Ca 2÷ is still the dominant cation, but K ÷ now stands second: the sequence of cation abundance is Ca z÷ > K + > Mg 2÷ > Na * > H * (NH4 was not analysed). Unfor-

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PRECIPITATION ETC. IN A HOLM-OAK FOREST 175

tunately, SO~ was not analysed in throughfall at Montseny. However, in the holm-oak forest at Prades (Tarragona, NE Spain), where throughfall chemistry is rather similar to that at Montseny, annual VWM concentration of SO42-- was 136 peq l- ' (Bellot, 1989). This means that the order of abundances for anions in throughfall at Montseny is probably SO~ > C1 > NO3, the same as in bulk precipitation.

Enrichment factors for each ion have been calculated in Table 3 as the ratio of annual VWM concentrations in throughfall to those in bulk precipitation. Assuming the annual stemflow amount in this stand to be 9% of annual precipitation, as at Prades (Bellot, 1989), mean throughfall concentrations would be 1.33 times greater than in bulk precipitation simply because of concentration owing to interception losses. Except for H~, all enrichment factors in throughfall are greater than 1.33 (Table 3): there is a net increase of the amounts of ions in throughfall with respect to bulk precipitation. The highest enrichment is for K ~ (24 times), followed by Mg 2÷ (3.6 times) and then by Na ~, Ca 2÷, NO3 and C1 (1.6-2.2 times).

On an annual basis, holm-oak throughfall at Montseny is less acidic than bulk precipitation, with a mean pH of 5.10. In Prades, holm-oak throughfall pH is very similar (mean 5.08) and has a VWM alkalinity of 48 peq 1 1 (Betlot, 1989). Data for individual sampling periods at Montseny reveal that when precipita- tion pH is less than ~ 5.0, throughfall is less acidic than precipitation, but when precipitation pH is greater than ~ 5.8, throughfall is more acidic than precipitation (Rodh, 1983). Hence, on an event basis, the holm-oak canopy partly neutralizes the incoming free acidity if precipitation is clearly acidic, but it acidifies precipitation when the latter is mildly acidic. This is reflected in throughfall having a smaller pH range (4.7-6.2 for most samples) than bulk precipitation (4.4 6.6 for most samples).

A number of mechanisms may be responsible for the observed changes in acidity when rainwater wets and washes the forest canopy. Neutralization of rainwater by the canopy may be achieved through: (1) washing out of dry-depo- sited neutralizing substances (e.g. CaCO3); (2) exchanging H + for cations on exchange sites in plant surfaces or in intercellular free spaces (Mecklenburg et al., 1966; and (3) leaching of salts of weak acids, organic or not, whose anions would protonate under acidic conditions (Hoffman et al., 1980). Conversely, acidification of rainwater by the canopy can arise from washing out of dry- deposited acidic substances (e.g. H2SO4, HNO3) or leaching of organic acids. In this holm-oak forest, exchange with free H ÷ in bulk precipitation can account for only 10% of the sum of equivalents ofNa ÷, K ÷, Ca 2+ and Mg 2÷ removed from the canopy in net throughfall.

The origin of ions removed from the holm-oak canopy by precipitation is difficult to determine. Both sea-salt aerosol impaction and dry deposition of continental particles seem rather low in this forest, as indicated by the moderate enrichment factors of Na ÷ and Ca 2÷ (Table 3). Even if all the Na + removed from the canopy was of marine origin, sea-salt aerosol impaction could only account for 2.6% of the Mg 2÷ and 17% of the C1 in net throughfall

176 F. RODA ET AL.

fluxes. If all the Ca 2 in net throughfall came from dry deposition, and assuming that the soluble Ca/Mg ratio for continental particles impacted onto the canopy was the same as that for continental ly derived Ca 2+ and Mg 2÷ in bulk precipitation (mean Ca/Mg = 8 on an equivalent basis (Rod~, 1983)), dry deposition would account for 20% of the net throughfall flux of Mg 2~. If Ca 2+ was substantially leached in this stand, the estimated contribution of dry deposition for Mg 2~ in net throughfall would decrease.

On the other hand, there is little doubt that the large amounts of K + in the net throughfall of this forest are derived from leaching of plant tissues. Net throughfall fluxes of K + in this stand are high throughout the year but show a distinct maximum in late spring and early summer when flowering and leaf senescence occur in holm-oak. In addition, net throughfall fluxes of K- correlate more strongly with throughfall amounts and with the intensity of organic coloration of throughfall than any other analysed ion (Rod~, 1983), strongly suggesting a leaching origin of K ÷. It can be hypothesized that leaching would also be an important source for those ions behaving in through- fall as K ÷ does. Of all the analysed ions, only the net throughfall fluxes of Mg 2~ (and also of PO~ , not dealt with in this paper) show a strong positive correla- tion with those of K * (r = 0.76, P < 0.01). Net throughfall fluxes of Na- and Ca 2~ are weakly or not significantly correlated with either throughfall amounts or net throughfall fluxes of K~.

To summarize the stochiometrical and correlational evidence discussed above, leaching seems to be a major source of Mg ~+ in net throughfall. The moderate amounts o fNa ~ and Ca 2 removed from the canopy could be easily the result of dry deposition of, respectively, marine and continental particles, though it cannot be excluded that some leaching of both cations also occurs.

There is a small net annual enrichment of NO3 in the throughfall of this forest (Table 3). However, this apparently small canopy exchange may or may not hide a higher flux of dry depositions of NO3 to the canopy (as H N Q vapour, NO2, or NO3 aerosols) coupled with a higher canopy uptake of this ion.

Soil solution

Soil solution chemistry was not very different at 20 and 40 cm depths, and most of the observed differences were due to one of the lysimeters at 40 cm consistently having higher concentrat ions than all the others. Therefore, data from both soil depths have been pooled (Table 4). Concentrations given in this table for the soil solution are arithmetic means because the water volume within tension lysimeters is a poor indicator of water fluxes in the soil. This has to be borne in mind because concentrat ions in bulk precipitation, throughfall and stream water have been volume-weighted. Lysimeters remained dry from June or July until the autumn rains. As the first soil solution samples that could be taken after the summer drought typically had both high concentra- tions for most ions and lower water columes, VWM concentrat ions in the soil solution would be for most ions somewhat lower than those given in Table 4.

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178 F. RODA ET AL.

The soil solution in this holm-oak forest is midly acidic, with a mean pH of 5.65. However, this pH value is largely due to the high CO2 pressures in the soil atmosphere: soil solution samples typically rose 1 pH unit when they were equilibrated with the air, implying a soil pCO2 about 10 times atmospheric (this may well be an underestimate as some degassing could already have taken place in the lysimeters).

The soil solution chemistry is dominated by Ca ~ and SO~ , followed by Mg 2~, C1 and Na ÷ (Table 4). Compared with throughfall values, soil water Mg 2+ concentrations increase 5 times, Na + 4 times, and Ca 2+ and C1 3 times. On the other hand, the mean K- concentration is reduced by 60%, and NO:~ by 90% with respect to throughfall. Many biogeochemical processes may influence the chemistry of water as it infiltrates and resides in the soil. Evapo- transpiration tends to increase the concentrations of all ions. For atmophilic elements such as S and C1 this could be the dominant process, though S is known to undergo rapid microbial transformations in forest soils (Fitzgerald et al., 1983). Silicate weathering releases Na +, K-, Ca 2+ and Mg 2÷ while consuming protons and increasing the alkalinity. Released cations may become involved in secondary mineral formation; a fate specially important for K + in these soils where illite is abundant. Cations in the soil solution are in dynamic equilibrium with cation exchange sites in the solid phase. At the same time, microbial and plant uptake of nutrients tend to lower their concentration in solution, an effect most clearly seen at our site for NO3 and perhaps K ÷ , both ions being strongly depleted in soil solution with respect to throughfall.

Stream water

Stream water at TM9 has high pH (mean 7.44) and alkalinity (VWM = 332 #eq 1 '). Bicarbonate is the dominant anion followed by SO~- and C1 , and Na + and Ca 2~ are the dominant cations (Table 4). Nitrate concentra- tions are very low. Compared with throughfall, concentrations in stream water are much higher for all ions except H ÷, K +, and NO3 (Table 4). Compared with soil solution, stream water is more diluted for all ions except Na ~ and HCO~.

There are pronounced temporal variations of the streamwater chemistry at TM9. Baseflow has much higher concentrations of Na ÷, Ca 2+, Mg + and HCO3 than the annual VWM figures given in Table 4. Concentrations of these ions in baseflow steadily increase with time after major storm events, alkalinity lying in the range 500-800peql ' a few weeks after peak flows. Alkalinity values around 900peq 1-1 w e r e repeatedly measured at baseflow during the summer of 1983, after a dry winter and spring. These high alkalinities were accompanied by high levels of Na ÷, Ca 2+ and Mg 2÷.

Changes of streamwater chemistry during 11 intensively sampled storm events were analysed in detail by Avila (1988). Considerable differences of solute behaviour were found from one event to another. However, much of the variation of solute response was associated with the antecedent hydrological conditions in the catchment. Except for H ÷ and SO~ , VWM concentrations in

PRECIPITATION ETC. IN A HOLM-OAK FOREST 179

stormflow were lower in events with wet antecedent conditions (WAC) than in events with dry antecedent conditions (DAC). However, the difference was large but not significant for K ÷ and NO3, owing to huge variations in these ions between events with DAC. Events with WAC were characterized by little or no increase of K ÷ and NO3 and by having either a large dilution of Na ÷, Ca 2+ and Mg 2÷ early in a wet period or almost no dilution after repeated copious rainfall. Events with DAC were characterized by showing, in most cases, a large increase of K ÷ and NO~ and by having either a dilution or increase of Na ÷, Ca 2* and Mg 2.. Irrespective of antecedent conditions, HCO3 was diluted and SO42 and C1- increased in stream water in all the sampled events.

During events showing no substantial increase of K + and NO3 in stream- water, runoff was probably generated by subsurface flow, since this indicates that water stayed in the soil long enough for the biological and mineralogical retention mechanisms of these ions to be effective. Based on such assumptions, subsurface flow generated 97% of all streamflow during storm events, and 99% of the annual water yield (Avila, 1988). The increased K * and NO3 concentra- tions in stream water observed in events with DAC were probably not produced by Hortonian overland flow on the slopes but by precipitation throughfall directly entering the stream channel and by Hortonian overland flow on the dry reaches of the channel. Supporting this interpretation, events with DAC drained on average only 1.3% of the event precipitation.

WATER CHEMISTRY PROFILES

When ion concentrations in bulk precipitation, throughfall, soil solution and stream water are compared in the holm-oak forest at Montseny, the highest concentration is found for: (1) H ÷ in bulk precipitation, (2) K ÷ and NO~ in throughfall, (3) Ca 2÷, Mg 2÷, SO~- and C1 in soil water, and (4) Na * and HCOj in stream water (Table 4).

For K +, the maximum concentration in the water profile is in throughfall because of the high amounts of this ion leached from plant surfaces in this forest. Mean K ÷ concentrations decrease sharply between throughfall and the mineral soil solution, in spite of increased K ÷ leaching from the litter layer. The reduction is probably accomplished through biological uptake, retention on exchange sites, and incorporation into clay lattice. These mechanisms were so effective in retaining K + that its concentration in stream water was never > 20#eq1-1 during 2 years of monitoring (except during two short summer storms), even though the K ~ VWM concentration in throughfall is 86 peq l-1.

The same is true for NO3, although throughfall enrichment is less dramatic than for K ÷ and NO3 is depleted in soil water to a much larger extent than K ÷. The main NO3 retaining mechanism in the soil is probably biological uptake; denitrification must be of minor importance in these well-drained and well- aerated soils. Indeed, trenched plot experiments in a nearby holm-oak stand have revealed that NO~ greatly increases in the soil solution when nutrient uptake by tree roots is prevented from otherwise undisturbed soil monoliths

TA

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PRECIPITATION ETC. IN A HOLM-OAK FOREST 181

(Bonilla and Rodh, 1990). No seasonal variation of NO3concentrations have been found in the stream water of TM9, in contrast to many temperate forest streams where NO3 is relatively high in winter and early spring, and low in summer (e.g. Likens et al., 1977; von Rehfuess, 1981). Winter soil temperatures on N-facing slopes at La Castanya are 3-4°C most of the time. The lack of seasonality of NO3 in stream water at TM9 suggests that microbial and/or plant uptake of N remains active through at least most of the winter in this forest.

Ca 2+, Mg 2. and SO~ have their maximum concentrations in the mineral soil solution (though soil water in or just below the thin organic horizon was not sampled). This is not entirely the result of evapotranspiration of infiltrated throughfall because the concentration of these three ions has also increased relative to that of C1 (Table 5). Decay of organic matter, cation exchange with soil colloids, and silicate weathering in the upper mineral soil could, alone or in combination, account for the moderate increase of Ca 2+ and Mg '~÷ in the soil solution relative to C1 . As there are no throughfall SO42 data at Montseny, it is not possible to assess the role of dry deposition of S to the canopy in the increased SO~ /C1 ratio in soil solution with respect to that in bulk precipita- tion (Table 5). However, the mean equivalent SO~ /C1 ratio in holm-oak throughfall at Prades is 2.13 (Bellot, 1989). If this throughfall ratio were also valid for Montseny, the increased ratio in soil solution could derive from mineralization of organic S.

At first sight, it is surprising that mean concentrations of Ca 2+, Mg 2" and SO~ decrease from soil solution to stream water. Since accumulated evapo- transpiration increases along the water path through the ecosystem, con- centrations of those ions that are not strongly retained within the system would steadily increase from precipitation to stream water. This should apply to SO~ and, probably, also to Ca 2~ and Mg 2÷. An important clue comes from the C1 concentration which decreases from soil solution to stream water. Taking C1 to be an atmophilic, conservative tracer this indicates that different kinds of water are being sampled in tension lysimeters and at the catchment outlet. The stream is being fed by water that is poorer in SO~ and C1 than the soil solution at 20 and 40cm depths, probably because it has remained within the catchment in places subjected to less accumulated evapo- transpiration than the upper soil horizons.

Relative to C1 , Mg 2- concentration increases from soil solution to stream water whereas Ca 2+ remains the same (Table 5). This means that weathering of Mg-bearing silicates (mainly chlorite) also occurs at soil depths > 40 cm or in rock fissures. The constancy of Ca 2" relative to C1 indicates either that there is no such deep weathering for Ca, or that it is counteracted by Ca uptake by deep roots. The major role of plant uptake in the Ca dynamics of this forest has been discussed by Avila and Rodh (1988).

In contrast, Na" and HCO3 have their maximum concentrations in stream water (Table 4). Na + greatly increases, both in absolute terms and relative to C1 , from soil solution to stream water. On the other hand, Na+/C1 ratios are

182 F. RODA ET AL,

near ly the same in soil so lu t ion as in bu lk p rec ip i t a t ion (Table 5), sugges t ing t ha t mos t w e a t h e r i n g of Na-bea r ing s i l ica tes (main ly albi te) t akes p lace deep in the soil profi le or in rock fissures, wi th a lmos t no ne t re lease of Na ÷ to the soil so lu t ion in the uppe r soil. Hence , Na ~ behaves l ike Mg ~÷ at depth but not in the uppe r 40 cm.

The m e a n a lka l in i ty in s t r e am wa te r is 5 t imes h igher t h a n in the soil solut ion. B i c a r b o n a t e is the dom i nan t mobi le an ion in the s t ream, whereas in soil wa t e r a t 20 and 40 cm depths it is SO42 . This chemica l change is a by- p roduc t of the hydro lys i s of s i l icates wi th H2 CO3 from soil r e sp i ra t ion as p ro ton donor.

H + is the only ana lysed c o m p o n e n t to have its m a x i m u m c o n c e n t r a t i o n in bu lk prec ip i ta t ion . P a r t i a l neu t r a l i za t ion of the incoming free ac id i ty t akes p lace t h r o u g h the canopy in te rac t ions . F u r t h e r neu t r a l i za t i on occurs in the mine ra l soil t h r o u g h slow disso lu t ion of any dry-deposi ted ca l ca reous dust, ca t ion exchange , ca t ion re lease f rom decay ing o rgan ic mat te r , and s i l ica te wea ther ing . These neu t ra l i z ing processes are sufficiently s t rong to overcome the effects of n i t r i f ica t ion (quite low in this soil; Boni l la and Rod~ 1990), and of net ca t ion u p t a k e by the agg rad ing fores t (a ma jo r process).

When wa te r leaves the c a t c h m e n t it is a basic so lu t ion r ich in b ica rbona te . As no ca lc i te is k n o w n to occur in these m e t a m o r p h i c schists, and as Na * (not a c c o m p a n i e d by C1- ) is the dominan t ca t ion in the net ou tpu t of this c a t c h m e n t (Avila and Rod~, 1988), the ma jo r neu t r a l i za t ion process seems to be s i l ica te wea ther ing .

ACKNOWLEDGEMENTS

This work was pa r t l y funded by CAICYT projec t 2129/83. The con t inued coope ra t ion of the D e p a r t m e n t d 'Agr icu l tu ra , R a m a d e r i a i Pesca de la G e n e r a l i t a t de C a t a l u n y a is g ra te fu l ly acknowledged .

REFERENCES

Avila, A., 1988. Balan~ d'aigua i nutrients en una conca d'alzinar al Montseny. Doctoral Disserta- tion, Universitat de Barcelona.

Avila, A. and Rod~, F., 1988. Export of dissolved elements in an evergreen-oak forested watershed in the Montseny mountains (NE Spain). Catena (supplement), 12:1 11.

Avila, A. and Rod/t, F., 1989. Les pluges de fang al Montseny. In: III Trobada d'Estudiosos del Montseny. Diputaci6 de Barcelona, Barcelona, pp. 67 71.

Belillas, C., 1989. Balance de nutrientes y efecto del fuego en cuencas de landa (La Calma, Montseny). Doctoral Dissertation, Universitat Autbnoma de Barcelona.

Bellot, J., 1989. Angdisis de los flujos de deposicibn global, trascolacidn, escorrentla cortical y deposicidn seca en el encinar mediterr~neo de l'Avic (Sierra de Prades, Tarragona). Doctoral Dissertation, Universidad de Alicante.

Binkley, D., Kimmins, J.P. and Feller, M.C., 1982. Water chemistry profiles in an early- and a mid-successional forest in coastal British Columbia. Can. J. Forest Res., 12: 240-248.

Bonilla, D. and Rod~, F., 1990. Nitrogen cycling responses to disturbance: trenching experiments in an evergreen-oak forest. In: A.F. Harrison, P. Ineson and O.W. Heal (Editors), Nutrient cycling in Terrestrial Ecosystems. Elsevier, Barking, pp. 179~189.

PRECIPITATION ETC. IN A HOLM-OAK FOREST 183

Ferr~s, L., 1985. Creixement radial i producci5 prim~ria neta aeria a l 'alzinar de La Castanya (Montseny, Barcelona). Orsis, 1:71 79.

Ferr~s, L., Rod~, F., Verdfi, A.M.C. and Terradas, J., 1980. Estructura y funcionalismo de un encinar montano en el Montseny. II. Biomasa arbdrea. Mediterr~nea, 4:23 36.

Fer~es, L., Rod~, F., Verdti, A.M.C. and Terradas, J., 1984. Circulacidn de nutr ientes en algunos ecosistemas forestales del Montseny (Barcelona). Mediterr~inea Ser. Biol., 7: 13~166.

Fitzgerald, J.W., Ash, J.T., Strickland, T.C. and Swank, W.T., 1983. Formation of organic sulfur in forest soils: a biologically mediated process. Can. J. Forest Res. 13:1077 1082.

Hoffman Jr., W.A, Lindberg, S.E. and Turner, R.R., 1980. Precipitation acidity: the role of the forest canopy in acid exchange. J. Environ. Qual., 9: 9~100.

Likens, G.E., Bormann, F.H., Pierce, R.S., Eaton, J.S., and Johnson, N.M, 1977. Biogeochemistry of a Forested Ecosystem. Springer, New York.

LSye-Pilot, M.D., Martin, J.M. and Morelli, J., 1986. Influence of Saharan dust on the rain acidity and atmospheric input to the Mediterranean. Nature, 321:427 428.

Meckleburg, R.A., Tukey Jr., H.B., and Morgan, J.V.. 1966. A mechanism for the leaching of calcium from foliage. Plant Physiol., 41: 610~613.

Reynolds, B., Neal, C., Hornung, M. and Stevens, P.A., 1986. Baseflow buffering of s treamwater acidity in five mid-Wales catchments. J. Hydrol., 87:167 185.

Rod~, F., 1983. Biogeoqulmica de les aigfies de pluja i de drenatge en alguns ecosistemes forestals del Montseny. Doctoral Dissertation, Univers i ta t Autonoma de Barcelona.

Sollins, P., Grier, C.C., Cromack, K., Fogel, R. and Fredriksen, R.L., 1980. The internal element cycles of an old-growth Douglas-fir ecosystem in western Oregon. Ecol. Monogr., 50: 261-285.

Terradas, J., Ferr~s, L., Ldpez Soria, L., Rod~, F. and Verdfi, A.M.C., 1980. Estructura y funcionalis- mo de un encinar montano en el Montseny. I. Planteamiento del estudio y descripcidn del ~irea experimental. Mediterr~nea, 4: 11-22.

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