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Field measurements of carbon leaching from leaves in beech and alder forests of northern Germany DIRK CZECH! and LUDGER KAPPEN!,2 1

2

Projektzentrum Okosystemforschung, Schauenburger Str. 112, D-24118 Kie1 Botanisches Institut der Universitat Kie1, Olshausenstr. 40, D-24098 Kiel

Accepted: April 4, 1996

Summary Carbon compounds were measured in solutes from crown drip and stem flow collected by rain gauges in a beech and an alder stand in northern Germany. The amount of leachate was calculated by using the concept of a canopy deposition model. Under this aspect leaching rate is the difference between canopy and bulk deposition. The calculations comprise direct leaching from leaves as well as carbon loss to honey-suckling insects and carbon loss used to compensate loss of inorganic substances. Leaching of organic carbon depends mainly on the amount of rainfall, the activity of phytophages and on the water repellance of the leaf cuticles. The annual loss of organic carbon by leaching was about 300 kg . ha- 1 per year in the alder forest, that was 1,4% of the annual gross carbon production. The loss in the beech forest was lower with about 60 kg C . ha- 1 year 1, corresponding to 0,8%. Key words: Fagus sylvatica, Alnus glutinosa, leaching, carbon, carbohydrates

Introduction Leaching is important in ecosystems by two reasons. First the loss of substances influences the carbon and nutrient balance of primary producers in the ecosystem. Secondly, leaching forms a nutrient source for faunal elements and microorganisms and thus feeds or perhaps diversifies the nutrient cycles. A great number of organic and inorganic substances were identified in leachates by laboratory investigations. Among the carbon compounds are sugars, pectins, sugar alcohols (CARLISLE et al. 1967, GUDERIAN et al. 1989, TuKEY 1970) also carbonic acids (HOFFMAN et al. 1980, SCHAEFER et al. 1989), amino acids, proteins (GROVER 1971, SCHERBATSKOY & KLEIN 1983), alcaloids and tannins. While these substances have mainly trophic character, vitamins and gibberellins that were leached from plant organs (KOZEL & TuKEY 1968) may have regulative effects on other organisms in the system. The amount of leaching of organic substances is correlated with environmental factors such as irradiance, temperature, or air pollution but to a low extent to pH level (GUDERIAN et al. 1989, MITCHELL 1968, SCHERBATSKOY & KLEIN 1983, TuKEY et al. 1958, TuKEY 1970). Most of the studies on leaching deal with the quality rather than the quantity of the leached substances. Thus,

we know very little about the amounts of carbon compounds that are leached out and the effect on the carbon balance of the ecosystem under consideration. Among the few studies respecting this is that by AMTHOR (1986) on forest ecosystems. In the present study we want to quantify the leaching rate of organic carbon components and to compare them with the total carbon production in a beech and an alder forest.

Material and methods Research site The research area "Bornhoved lakes" is located about 30 km south of Kiel in the south-western region of the east Holsatian lake and hill lands. An area of 50 ha comprises beech forest, mixed and alder forest, crop land and grass land. A beech forest (Asperulo-Fagetum) of 12 ha grows on acidic podzolic brown earth and para-brown-earth with pH values between 2.8 and 4.1 in the upper soil horizon (SCHLEUSS 1992). The trees are at the age of 97 years, 26 m tall and stand with 158 stems per ha. The alder forest - an Alnus glutinosa community - is at an age of 60 years. The stems are 17 m tall in average. they grow in alder bog peat that has locally fallen dry around 1930. FLORA (1997) 192

209

Methods The leaching rates of carbon compounds were determined as part of the total flow of substances in the forest ecosystem. The carbon compounds were quantified according to a compartment model developed by ULRICH (1981) and modified by SPRANGER (1992). The model takes into account that inorganic and organic matter deposition is dryas well as wet (Figure 1). Interactions between the tree crowns result from carbon leaching and uptake. Leachates leave the crown region via stem flow and crown drip. Crown drip was collected by six rain gauges (BLOCK & BARTELS 1985) in the beech forest and four in the alder forest. The stem flow was collected by means of funnels mounted as a spiral at the trunk of the beech trees. According to SPRANGER (1992) stem flow of alder is insignificant and therefore needed not to be measured. All substances from crown drip and stemflow together formed the canopy deposition as shown in equation (1). (1) CD CD

WD + DD + L canopy deposition (crow + stemflow)

WD = wet deposition DD = dry deposition L

= leaching

The amount of dry deposition (DD) and possible absorption of lipophilic substances could not be measured directly. As a reference we measured the total deposition in the open field by means of 3 rain gauges in a nearby grass land. It comprises the wet (WD) and a small fraction of the dry deposition that went into the rain gauges during the rain-free periods. These two fractions together are taken as so-called bulk deposition (BuD) (SPRANGER 1992). The leaching rate was estimated by means of equation (2). (2) L"'CD-BuD Samples were collected every week within the period May to December 1991. Sodiumbisulfite was given into the rain gauges as a means for inhibiting the microbial destruction of organic substances in the collected leachates.

Aliquots of 50 ml were taken from the liquid in the rain gauges and filtrated cellulose-nitrate membranes. The solution was then colorimetrically analysed (DUBOIS et al. 1956) to measure the total amount of carbohydrates as glucose equivalents. For this purpose 5 ml H 2S04 and 50 f-tl of a phenolic reagent (80% phenol in distilled water) were added to 2 ml of the solute. After 10 minutes the extinction was measured in a photometer at 490 nm wavelength against that of a zero solution. As a standard glucose was taken. Organic carbon compounds (water soluble and particular carbon) were analysed by means of the continuous flow procedure (RFA method, Alkem Corp. GOULDEN & BROOKSBANK 1975). The solute samples were first acidified in order to release inorganic carbon as CO 2 which was washed out by nitrogen. Organic carbon was then oxidized with sodium tetraborate and potassium persulfate under UV light to CO 2 • This was indicated quantitatively by phenolphthalein and measured at 550 nm wavelength. The carbon particles in this analysis had a diameter of less than 0.2 .um, because the sample was membrane-filtrated. Each sample was measured with two replicates. The amount of leaves was estimated from the leaf area index (LAI), that was measured by an optical method (LAI 2000, Licor, Lincoln, Nebraska). The repellence of the cuticle, which determines the degree of leaching, was measured after a method of HAINES et al. (1985). This method bases on the measurement or estimation of the tangens delta of the sphere of an adherent water drop to the leaf surface.

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The collected solute in the forest contains carbon from two major sources. Besides leaching of photosynthetically fixed carbon from leaves the trees are also subjected to a carbon import from the atmosphere by dry and wet deposition on the crown surface (Fig. 1). Figure 2 depicts the monthly total carbon deposition in the open, next to the investigated forests, and compares with monthly precipitation. Bulk deposition of carbon and

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carbohydrate corresponds well with the amount of rainfall in most months except November and December. In these winter months the carbon deposition was comparatively too low. The highest bulk deposition rate was observed in June. Carbohydrates formed an average of28% of the organic bulk deposition. According to HOPMAN et al. (1980) and KAWAMURA & KAPLAN (1986) the other carbon fraction contains carbon compounds such as terpenoids as derivates from conifers, high molecular alkans and fatty acids which most likely were derivates from plant cuticles. Phenols and long-chained alkans are the compounds of motor exhausts. Dioctyl phthalate and dioctyl adipate are widly used plasticizers. Polycyclic aromatic carbohydrates and chlorinated carbohydrates are common products of the combustion of C based material (see also MATZNER 1984).

Leaching rates of beech and alder Comparing the leaching rates of the two forests (Fig. 3) we notice values from alder that are four times higher than those from beech. One reason for this difference is that the wettability of the cuticles of alder leaves was higher than that of beech leaves. The tangens delta values of the water drops were estimated to be by 10-20° in the first and 30-50° in the latter. Similar relations between wettability and leaching rates were already shown by TuKEY (1970) with other plant species (Pisum sativum, Beta vulgaris, Phaseolus vulgaris, Cucurbita pepo). The high leaching rates of alder in summer which correspond widely to high precipitation rates, reflected also a high activity of honey-dew suckling insects. This confirms AMSDORP'S (1991) observation of an increased population of phyllophagous beetle larvae in summer. Beech leaves, on the other hand, did not show any seasonal increase of phytophages or honey-dew sucklers. The relatively high leaching rates of both forests in November may be explained by the fact that the leaves of beech and alder were injured by nightly freezing, so that carbon compounds could leak from the damaged tissues and were washed out by rainfall. The leaching intensity was generally higher in the periphery of the beech forest than in the central part (Fig. 3), as SPRANGER (1992) showed also for the inorganic compounds. Since no rain gauges were placed under trees in the periphery of the alder forest, we can only assume this also for the alder. A correlation between monthly leaching rates and leaf area indices (LAI) could not be detected. After litter fall (December) the leaching rates from the trees decreases drastically.

Leaching and precipitation In order to describe the relationships between true leaching rates and precipitation the data from all months except July were taken into account. July was excluded in this case because this value was affected by strong insect excretion. Equation (3) describes the relationship between canopy precipitation (stem flow plus crown drip; see Fig. 1) and leached amounts of carbon (TOC) and carbohydrates (CH): (3) TOC (CH) =axCpb TOC = total organic carbon (g m-2 ground surface) CH = carbohydrates (g m-2 ground surface) CP = canopy precipitation (mm) a, b = parameters of the regression The regression curves in Figure 4 show a steep increase if canopy precipitation is low and a flattening (saturation curve) with increasing canopy precipitation. The high efficiency of low canopy precipitation rates can be explained by the fact that in periods with sporadic rainfall carbon compounds were accumulated on the leaf surfaces via dry deposition and transport from inside the leaf to the surface (SCHERBATSKOY & KLEIN 1983). This pool of soluble carbon compounds was washed off from the leaves. If rainfall was higher and more regular the leaching rates were more constant. During irregular and sporadic rain events the measured leaching rates could vary strongly because the time spans of dry and those of rainy weather were unevenly distributed over the weekly periods after which the measurements were regularly taken.

Annual leaching rates versus carbon production In order to estimate the whole amount of the leached carbon the weekly leaching rates were summed up for the year 1991 and compared with the annual total carbon production (TCP = net photosynthetic C uptake minus respiratory Closs), see Table 1. Four assumptions were made for this quantification: 1.) The leaching rates during the leafless months January to April were estimated equally and assumed to be as high as those measured in December. Leaching in the winter season is assumed to come from branches, stems, and tree epiphytes (algae, bryophytes, and lichens). 2.) Indirect effects: according to a study by AMTHOR (1986) carbon losses result partly from metabolic costs FLORA (1997) 192

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FLORA (1997) 192

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Table 1. Comparison of leaching rates for TOC, carbohydrates, and carbon-equivalents for cation leaching (measured in 1991) with the above-surface annual total carbon production of alder forest (data from ESCHENBACH 1995) and of beech forest (data from LANGE & SCHULZE 1986). Data for calculating C-equivalents for cation leaching were taken from BRANDING (1995). Leaching of total carbon (TOC)

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times higher than that from the beech forest (64 kg C ha- I a-I), although both forests had almost the same leaf area indices (about 4 m 2 m-2). The fraction of carbohydrates with 43% in the alder forest is also higher than in the beech forest (31 %). The loss of carbon, that was needed for replenishing of inorganic cations, was in the alder forest twice as high as that in the beech forest _ irrespective of the effect of nitrogen nutrition. The amount of leached carbon was estimated to be 1,4% of the annual gross primary production (GPP) for the alder trees and 0,8% for the beech trees, depending on the values necessary for the nitrogen metabolism (SPRANGER 1992). FLORA (1997) 192

213

Discussion In general, we could demonstrate with our data the order of magnitude of leaching in relation to the carbon production in forest ecosystems. Its magnitude varies with the tree species because of their anatomical and physiological properties. Tree or forest canopies from other regions revealed annual leaching rates within the same order of magnitude as shown in the present study. In a Quercus petraea canopy from England CARLISLE et al. (1967) measured a total of 174 kg Corgha-Ia-l with a fraction of carbohydrates of 36 kg C ha-1a-1 (20%), Particularly high were the leaching rates of apple foliage (Malus domestica) because apple has an enormous floral nectar production. DALBRO (1956) measured therefore a total of 360 kg Corgha-1a- l . By contrast, forest ecosystems (Shorea robusta stand; Pinus roxburghii stand) from the central Himalaya region revealed extremely low rates (c. 7 kg Corgha-la-l; MEHRA etal. 1985). However, in this case we assume that also the primary production is much lower than that of forests in the temperate lowland of central Europe. AMTHOR (1986) has calculated the amount of organic carbon equivalents that was necessary to compensate the leaching of nitrogen compounds. Basing on data from a Quercus petraea canopy in England it was 3.5 kg C ha- Ia-I (CARLISLE et al. 1967) and on those from Hubbard Brook Experimental Forest (EATON et al. 1973) it was 25 kg C ha-1a- 1. Consequently, we may have to add values of that order of magnitude to the figures we got for alder and beech forest. According to our data leaching rate was widely correlated with the amount of rainfall. The error of our data with respect to the effect of dry deposition was estimated low. The half-life period of carbohydrates outside of organisms is very low because of rapid microbial turnover. Other organic carbon compounds mainly of anthropogeneous origin are decomposed in various different ways after dry deposition (HOFFMAN et al. 1980, MATZNER, 1984). Thus, it is extremely difficult to get measurable figures of their quantities. Another source of error relevant for our calculations has been pointed out by ULRICH 1981 : Litter and fungal or algal biomass that occasionally dropped from the trees into the collecting containers is an additional source of carbon leaching. This effect could not totally be excluded although greatest care was taken by regularly controlling the rain gauges. It would be interesting to quantify the amount of carbon excreted by honey-dew sucklers and other animals, which may be quite significant as the investigations with apple trees and also ours with the alder forest showed. A separation of this fraction might be possible if leachate eluters would be applied that were 214

FLORA (1997) 192

described by IBROM (1993). With this respect our leaching rates are not strictly identical to those defined by TuKEY (1970).

Acknowledgement This study was supported by the German Ministry for Science and Technology (BMBF), Project Ecosystem Research BornhOved Lakes.

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e h

IBROM, A (1993): Die Deposition und die Pflanzenauswaschung (Leaching) von Pflanzennahrstoffen in einem Fichtenbestand im Solling. Ber. Forschungszentrum Wald Okosyst. Gottingen, Reihe A, Bd. 105. KAWAMURA, K. & KAPLAN, I. R. (1986): Compositional change of organic matter in rainwater during precipitation events. Atmosph. Environ. 20: 527-535. KOZEL, P. C. & TuKEY, H. B. (1968): Loss of gibberellins by leaching from stems and foliage of Chrysanthemum multiflorum "Princess Anne". Amer. J. Bot. 55: 1184-1189. LANGE, O.-L. & SCHULZE, E.-D. (1986): CO 2-Assimilation. In: ELLENBERG, H., MAYER, R. & SCHAUERMANN, J.: Okosystemforschung, Ergebnisse des Sollingprojektes. Ulmer, Stuttgart, 136-149. MATZNER, E. (1984): Annual rates of deposition of polycyclic aromatic hydrocarbons in different forest ecosystems. Water, Air and Soil Pollution 21: 425-434. MEHRA, M. S., PATHAK, P. C. & SINGH, J. S. (1985) : Nutrient movement in litter fall and precipitation components for central Himalayan forests. Ann. Bot. 55: 153-170.

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t :

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999-1002.

MITCHELL, C. A (1968): Detection of carbohydrates leached from above-ground plant parts. Thesis presented at Cornell University for the degree of M. S. SCHAEFER D. A, LINDBERG S. E. & HOFFMAN W. A (1989): Fluxes of undissociated acids to terrestrial ecosystems by atmospheric deposition. Tellus B 41: 207-218. SCHERBATSKOY T., & KLEIN R. M. (1983): Response of spruce and birch foliage to leaching by acidic mist. J. Environ. Quality 12: 189-195. SCHLEUSS, U. (1992): Boden und Bodenschaften einer norddeutschen Modinenlandschaft - Okologische Eigenschaften, Vergesellschaftung und Funktionen der Boden im Bereich der Bornhoveder Seenkette. EcoSys, Beitr. Okosystemforsch. SuppI. Bd. 2, 1992, Kiel. SPRANGER, T. (1992): Erfassung und okosystemare Bewertung der atmosphlirischen Deposition und weiterer oberirdischer Stoffflusse im Bereich der Bomoveder Seenkette. Dissertation thesis presented to Geografisches Institut, University of Kiel. TuKEY, H. B., Jr. (1970): The leaching of substances from plants. Ann. Rev. Plant. Physiol. 21: 305-324. - & WITTWER, S. H. (1958): Loss of nutrients by foliar leaching as determined by radioisotopes. Proc. Am. Soc. hort. Sci. 71: 496-506. ULRICH, B. (1981): Theoretische Betrachtung des Ionenkreislaufs in Waldokosystemen. Z. Pflanzenemahr. Bodenk.

144: 647-659.

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