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Abstract "Heft 31"

Freiburger Bodenkundliche Abhandlungen

Schriftenreihe des

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

Heft 31

Karl-Heinz Feger

Bedeutung von ökosysteminternen Umsätzen und Nutzungseingriffen
für den Stoffhaushalt von Waldlandschaften

Freiburg im Breisgau 1993

ISSN 0344-2691


Results from element cycling studies conducted in the ARINUS experimental watersheds Schluchsee and Villingen (Black Forest) are presented for the period November, 1987 through October, 1990. Sites are located within extended forest land with Norway spruce being the predominant tree species. Both the underlaying bedrock (granite and quartz-sandstone, respectively) and the acid nutrient-poor soils are typical of forest sites in Central Europe. An integrated approach is used combining measurements of element turnover in spruce ecosystems with input-output budgets of small experimental watersheds. The overall goal is to quantify the role of internal cycling on the levels both of the forest ecosystem (stand) and the landscape. Discussion focuses on the contribution of atmospheric deposition to the cycling of nutrients as well as its consequences for solid soil chemistry and drainage losses of elements. Special emphasis is put on proton transfers and their control by interacting biogeo-chemical processes. Based on the combined experimental approach, also the chemical changes of seepage water from beneath the rooting zone and stream/groundwater are delineated. Furthermore, the significance of forest management is addressed. Discussion includes the influence of land-use history and various intensities of biomass harvesting ranging from forms, which are common in current practical forestry, to highly intensified systems.

At the two sites, the rates of atmospheric deposition is low to moderate as compared to other forest regions in Central Europe. As a consequence, processes of internal cycling play a key role in the element budget of the ecosystem. Incorporation of cations in the biomass of the aggrading stand as well as excess mineralization in the soil results in a H+ production rate which clearly exceeds the external H+ load of 0.4 Kmol ha-1yr-1. The strong natural acidification of these base poor soils was considerably increased by the past forest management practices which mostly were detrimental to site fertility. At the Villingen site, forest management was characterized by an extremely high export of biomass. Starting as early as in mediaeval times these adverse practices resulted in a substantial loss of base cations and a disturbed decomposition of litter. At Schluchsee, in contrast, the change from the natural mixed stands to pure spruce plantations intensified subsoil acidification by increased rates of soil internal H+ production resulting from humus disintegration in the mineral soil. This unfavorable development was induced by extended clear felling followed by prolonged periods of pasturing on the scarcely stocked sites and the superficial rooting of the subsequent spruce stands.

Nitrogen deposition amounting to 15 kg ha-1yr-1 is nearly equal at both sites and fairly low as compared to other regions. There is evidence for N deposition affecting element cycling of the studied spruce ecosystems. However, N nutrition of the stands and N drainage losses are primarily controlled by the pattern of microbial turnover which is highly contrasting between the two sites. The microbial transformations reflect a strong influence of site management history. In spite of the current N input from the atmosphere, the ecosystem at Villingen is still in the state of N deficiency which coincides with the minimal export of this element. At Schluchsee, in contrast, N supply is optimal with N output being in the magnitude of the input. There, N output is in the same order of atmospheric deposition. The internal cycling of N resulting from high mineralization rates is a 'relic' of the original mixed forest. Elevated drainage losses of elements from that part of the mineral soil, which is only scarcely utilized by the root system of Norway spruce, indicate that a new equilibrium between mineralization and nutrient uptake has not yet been established.

Nutrient deficiencies exist with respect to Mg at Schluchsee and K at Villingen, respectively. At both sites, nutrient deficiency is related to a tight element supply in the soil which likely has been deteriorated even by a weak proton load. At the Schluchsee site, Mg deficiency is primarily due to the extremely low Mg content of the granitic parent material. At the Villingen site characterized by a periodic perched water table in the soil K deficiency is related to chemical disequilibria inherent to soil structure. Furthermore, enhanced forest growth and nutrient imbalances both originating from a higher N deposition, as well as interactions between nutrient supply, mineralization, and soil water regime are discussed as additional causes. Both the distribution of available element pools in the ecosystem and the measured turnover rates reveal a very tight cycling for the most of the nutrients. Because Norway spruce naturally develops a rather shallow fine root system, the stands are not able to fully utilize the anyhow marginal nutrient supply in the soil. According to the low absolute concentrations of Al in the soil solution and its chemical speciation an impact of direct Al toxi-city effect is not likely. Nevertheless, Al may antagonistically impede the uptake of base cations and by this mechanism contribute to the evolution of nutrient deficiencies.

Atmospheric deposition obviously affects the hydrosphere. This impact, however, is distinctly mediated by the hydrological and biogeochemical processes interacting differently in each watershed. Processes in the non-rooted subsoil, as well as in the debris zone and bedrock fissures are crucial in the buffering of protons originating from both atmospheric deposition and ecosystem-internal sources. The acid buffering capacity depends on base cation release. Besides the mineral composition, it is highly controlled by different hydraulic pathways within the watershed. If vertical pathways are predominant, e.g. in the Schluchsee watershed, drainage water stays in contact with the mineral interface for a considerably long time. In contrast, if vertical drainage is impeded (e.g. at Villingen) and, hence, water and element transport occurs laterally along rather superficial pathways, the importance of interaction processes in the subsoil and bedrock is significantly reduced. As a consequence, the chemical composition of streamwater in such watersheds is strongly dominated by soil processes, especially those in the organic upper layers.

The most pronounced consequence of atmospheric deposition is the provision of 'mobile' anions controlling cation transport in the terrestrial system. This applies both to the output of base cations and ionic Al with the latter being toxic to the aquatic biota. In the studied watersheds, SO4-2 is the dominant anion. Its mobility is controlled by complex transformations in the ecosystem. In this respect, the interactions between solution SO4-2 and organic S forms in the solid soil are more important than processes of formation/dissolution of AlSO4-type minerals.

Forest management in these highly instable spruce stands must intend to reduce the ecosystem internal H+ load. This can be realized by a restricted export of base cations when forest biomass is harvested. Furthermore, excess mineralization owing to a disintegration of humus reserves has to be avoided. A balanced nutrient supply can be achieved - in the short-term - by addition of the specific elements being deficient. Large-scale liming operations on biologically active sites bear considerable risks. Under such conditions, liming accelerates mineralization and enhances the superficial arrangement of fine roots. For the long-term management of these problematic spruce sites, silviculture should prefer site-adapted tree species developing deeper root systems.

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