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Abstract "Heft 23"

Freiburger Bodenkundliche Abhandlungen

Schriftenreihe des

Institut für Bodenkunde und Waldernährungslehre
der Albert-Ludwigs-Universität Freiburg i.Br.
Schriftleitung: F. Hädrich

Heft 23

Bodo Heyn

Elementflüsse und Elementbilanzen in Waldökosystemen der Bärhalde - Südschwarzwald

Freiburg im Breisgau 1989

ISSN 0344-2691


Within the framework of the research program "Geochemistry of. environmentally relevant trace elements" (sponsored by the German Research Foundation, DFG) from 1976 till 1979 the Department of Soil Science and Plant Nutrition of the Albert-Ludwigs-University Freiburg i. Br. measured different element flows within a small catchment area. Specifically, the precipitation, the canopy drip, the litter fall, seepage from the topsoil and subsoil as well as run-off in the streamlet were measured. For the program, the flows were analyzed for 16 main and trace elements (Al, Ca, Mg, K, Na, P, Si, Fe, Mn, Be, Cd, Co, Cu, Ni, Pb, Zn). 6 typical ecosystems were selected in the Bärhalde research area. These sites differ in soil type, vegetation, elevation and relief position.

At the upper slope higher amounts of main elements are deposited with the precipitation. A correlation between large particle size, shorter transport distance and heavier rain-out at this site was stated (chapter 4.2). Those elements which are transported over a longer distance (mainly trace elements, bound to aerosole particles less than 2 /jm) have a lower deposition rate at the upper slope than in the lower one. There are considerable differences between the soil inputs (canopy drip and litter fall) in the ecosystems of the Bärhalde. This is mainly due to different site properties, including age of trees, elevation and relief position. Specific behaviours of the elements during the atmospheric transport (chapter 4.3), during deposition (chapter 4.5) and during their incorporation into the biocycle (chapters 4.6 and 4.7) influence the soil input by wet and dry deposition as well as the biogenic litter input (chapter 7.).

The statistical evaluation of the random samples showed data matrices which were distributed normally or slightly inclined. The chronological and spatial variability in and between the ecosystems differs among the elements. The deposited amounts of the elements differ between the ecosystems, in almost all cases significantly so (chapter 4.4). The input with precipitation varies between upper and lower slope or different elements from 2 to 220 %. For trace elements Zn, Cu, Co and Pb almost no differences could be found, whereas for K, Be and particularly P the deviation was maximal (chapter 4.21). The remaining 9 elements showed deviations between 30 and 40 %. Because the variability over the research period at the different registration sites was 10 to 25 % for all elements, no general trend with the altitude for the wet deposition could be found. In total, the trace element input with the wet deposition vn the Bärhalde area is comparable with other low-charged areas (NORNBERG et al., 1984, MIES et al., 1987). Some trace elements (Cd, Zn, Pb) have higher emission loads than low-charged areas. The balance calculations for the whole catchment were based on a mean precipitation and element input (chapter'4.1).

From the results of the turn-over measurements, groups of elements with different flows in the compartments can be separated. By interception deposition, at the upper slope (chapter 7.) with exclusively old spruce stands, there was an additional element input for all elements. The gain for the trace elements is again less than for the main elements. The deposition by interception was not measured at the Bärhalde, it was only calculated from the difference between element flow in the canopy drip compared with the rainfall. Because this difference was always positive for the older exposed stands at the upper slope (within exception of Ni and Be) this was interpreted as an additional input by dry deposition or interception. The differences between precipitation, canopy drip and litter fall showed that interception is the dominant process. For Ni no interception could be calculated, for Pb it is questionable. For the elements Be, Cu, Co and Al the amount of interception deposition is relatively low. For K the deposition by interception at the ridge is as high as the input. The calculated interception deposition for other elements is above the precipitation input.For example, for Na, Si and Cs the interception deposits were twice the amount of deposits with rainfall. For Mg the rate is four times higher than the precipitiation input and for Mn it is as much as eight times higher.

By the example of Mn (chapter 7.9.) it is shown that the amount of leaching is correlated with the total biomass in the tree crown. The level of the Mn contents in the crown compartments and the age of the trees have no influence on the leaching. The canopy drip is additionally concentrated by washing the interception deposition. The high turnover in the ecosystems runs contrary to the low store in the plants. Also for K a higher turnover than store was found. The process of leaching clearly concentrates the elements K, Mn, Cd and Ca in the canopy drip. This has to be assumed also for Si and Mg. The high acidity of the rainfall increases the leaching. This will affect the supply of the spruce trees, especially with Mg, which is already limited in the Bärhalde area. A tendency towards adsorption in the tree crown was calculated for Na. Higher Na contents in older plant organs correspond with this observation. While the Na input with precipitation is low, there is a much higher Na transport in the canopy drip at the upper slope. This additional amount was attributed to interception. Na may be imported by fine particles which reach the area with the far transport (chapter 4.3). The soil input for this podsol stand is thereby increased by 70 % Na. Very low Na contents in the vegetation and low Na inputs with the litter fall show that Na is minimally involved in the biocycle. In all ecosystems Na, with its high solubility, is exported quickly out of the catchment area with the seepage or the lateral slope waterflow. Adsorption in the tree crown was found for the elements Al and P as well as the trace elements Fe, Zn, Cu, Pb, Ni and Co. It was not always possible to decide whether and to what extent a plant uptake through the needle surfaces took place. For the element Cu with relatively high content in the crown of the old stand (P130) in spite of low general plant supply, plant uptake can be assumed. The assumption of RAISCH (1983) that because of the high acidity in this site the Cu turnover is higher and the plant uptake by the roots is increased may also explain the higher plant contents. The raised mobility of Cu in this profile is shown by the higher losses with the seepage. The uptake of other elements by the vegetation surface is very low. The elements Al, Fe, Pb and Cu have a similar distribution in the tree compartements. Therefore, they m-ay have similar modes of distribution. The contents of Fe and Pb in the vegetation is low. In the spruce litter their content is 4- to 10-fold compared with the living biomass of the crown. Therefore, we calculate with the adsorption of aerosoles on the litter surfaces (similar to the Soiling, MAYER, 1981). The higher deposits of Al and P from the litter appear to originate not only from absorption of aerosole particles but also from higher plant uptake from the soil. For the element P the internal turnover with the biocycle is the dominant feature. Amounts of P transported with the solutions are very small.

In general, the soil inputs from litter and atmosphere are involved in the biocycle to a large extent. Root uptake of such elements is relatively easy from generally acid soils with a high quota of mobile ions. When comparing the element input from the atmosphere and from the dying biomass (without leaching), taking into account the degree of supply of the vegetation, the turnover in the biocycle becomes evident. More than half of the soil input of nutrients P, K, Ca, Mg, Mn and Fe originates from plant litter. Also for Al, the high plant content results from a higher bio-input than atmospheric input. Looking only at the actually measured inputs without correction for leaching and adsorption, it is found that at the upper slope the litter fall also contributes more Pb than rain and canopy drip. The inputs of Fe and P, increased through the fixation of aerosoles, get as much as 80 % from the litter fall.

The process of leaching is obvious for the elements K, Mn, Ca and Mg. At the forested upslope stands the input with litter fall is, therefore, less than the input with the canopy drip. The higher acidity and lower pH value in the tree crowns of the old stands favors the leaching from the crown. The lower element content in the needles of these stands points to a stronger leaching. An additional increase of the concentration in the canopy drip is observed by washing the adherent aerosoles out of the tree crown. For the trace elements Zn, Cd, Ni and Co the plant content is low and, therefore, also the turnover in the biocycle. Consequently, for these elements the atmospheric input is dominant. For Pb atmospheric and biogenic influences depend on the interception (chapter 7.14) but are obviously dominated by the processes in the atmosphere. Cu and Be have an equivalent input from the litter of the spruce trees and ground vegetation on the one hand and from the atmospheric input on the other. Higher Be turnover in the biocycle of the spruce stands is caused by the high available Be amount in the soil (chapter 7.17). The ground vegetation of the unforested sites is by far less able to turn over high Be amounts.

Also the soil input differs for the majority of the elements between the ecosystems. In spite of this fact, the quantitative element turnover is mainly dominated by the biocycle. Relatively independent of the element flows, in all topsoil compartments the seepage for nutrients and trace elements is significantly less than the input (chapter 7). The correlations between the transported element amounts in the topsoil seepage show a relatively uniform mobility (chapter 6). Most of the elements are highly mobile in the very acid topsoils. The element amounts which are not taken up by the roots and fed into the biocycle are leached out because of the high percolation rate. This feature levels the different conditions in the ecosystems under observation. The main elements Si, Al, Na and the trace elements Co and Be which are not important for the plant supply show more differences between the sites due to different substrate, redox and moisture conditions. The high Si and Al export from the podsol site demonstrates the intensive weathering. The ions liberated by weathering under acid conditions are mobile. The very low pH values (below 3,8) hinder mineral new-formation because free ions are leached from the system. From the podsols Si and Al is moved laterally with the slope water. Therefore, the transport rates in the topsoil of the stagnogley show the strong slopewater flow (chapters 7.2 and 7.3). The advanced iron translocation in the Podsol Zweiseenblick (KEREN, 1978) is consistent with the low pH values. An acidification stress by the high mobilization of organic matter can be deduced from the high iron transports under the clearcut site (chapter 7.10). The prevailing slopewater flow, relatively close to the surface, transports the total iron amount to the lower slope. The high lateral transport rates in the Stagnogley and the enrichment in the Go of the "Ockererde" (chapter 2.3.3) makes this obvious. In the Braunerde profiles the acidification is less, advanced. The Al translocation in the forest site Erstaufforstung already shows the beginning podsol ization (KEREN, 1978). For all sites the exchange capacity in the mineral soil is low (chapter 2). Favourable exchange conditions are limited to the humous topsoils and to the enriched horizons.

Also for the subsystem soil it is still a problem to judge which influence the different parameters have on the element turnover. Throughout the Bärhalde catchment the soil cover is very varied. In spite of the homogeneous granitic parent material the soil types change within very short distances. The microvariability of different soil types shows a zonation along the slope. Raw-soils and Podsols at the upper slope grade into acid Braunerden. At the lower slope and in the hollows hydromorphic soils such as Stagnogleys, bog soils and "Ockererden" are found. The valley is dominated by Gley soils. With such a heterogeneous soil cover it is evident that no general behaviour of elements in the whole catchment can be elaborated.
Beside the conditions within each ecosystems, the transport between the systems, e.g. the waterflow moving siopewards, is of importance for the whole export from the catchment.

The effect of low pH values in the topsoils of increasing liberation by weathering was already mentioned, as well as the high mobility of all elements under acid conditions. Consequently, a transport in vertical direction and also a lateral transport of iron can be stated. Al and Co, p.e., are transported in large quantities with the seepage. Therefore, translocation into the subsoils and enrichment at the lower slope is observed.

In the subsoils, again, the properties of site and soil types change the flow rates of the elements. The correlative interrelations differ even more than in the topsoils. Al, the alkali elements (K, Na) and the earth alkalies (Ca, Mg, Be) are closely related where the pH values rise above 4 - 4.7 (H20). In all ecosystems, because of the relatively high mobility and the missing plant uptake, the outputs of these elements is higher than from the topsoil. A relative difference is observed in the Stagnogley at reducing conditions and high amounts of lateral flow. Therfore, the Al transport in the subsoil is relatively low (chapter 7.3). The increased pH value (at 10 cm soil depth already 4.2) lowers the'mobility of Al. The observed Al enrichment at this depth bears out the influence of pH on the mobility. For Mn, however, no enriched horizons are found. But the fact that Mn is 100 % silicate-bound shows that the mobile Mn at existing (low) pH values and a high water saturation, is quickly exported. In the "Ockererde" with lower flow rates (higher pore volume and higher flow bulkhead) together with oxidative conditions, the governing process is element enrichment. The reddening of the color shows the high amount of iron oxides in the Go horizons. High content of pyrophosphate-soluble iron shows the high percentage of organic iron compounds.

The immobilization of heavy metals and trace elements along the lower slope is obvious from the smaller transport amounts and the decreasing outputs together with the run-off from the catchment. Both the forest bogs and the "Ockererde" with humus-rich topsoils and high sorption capacity function as sinks for the elements. A comparatively low fixation is stated for the elements Cd, Ni and Co. The decrease of the transported amounts form the profiles in the lowest slope to the transport with the streamlet is less than for the other trace elements. Their low affinity of Ni and Cd to sorption was described, especially where the concentration of the solution increases (HERMS and BROMMER, 1984). The export of trace elements from the Bärhalde is consistent with their statements. Be is mainly liberated from the Bärhalde granite by weathering. An immobilization at the lower slope is not observed. Similar to the other alkaline and earth-alkaline elements, Be has a high mobility in all profiles and is, therefore, enriched in the runoff from the catchment.

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