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Abstract "Heft 37"


Freiburger Bodenkundliche Abhandlungen

Schriftenreihe des

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


Heft 37


Ernst E. Hildebrand (Hrsg.)

Der Gashaushalt von Waldböden:
Messung, Modellierung und ökologische Bedeutung

Teil 1:
H. Schack-Kirchner und E.E. Hildebrand: Prozessmonitoring mit Gaskonzentrationsmessungen in Waldböden
Teil 2:
B. Metzler: Feinstwurzelgesundheit der Fichte in unterschiedlich durchlüfteten Bodenhorizonten

Freiburg im Breisgau 1998

ISSN 0344-2691


Summary:


Part 1: (Schack-Kirchner & Hildenbrand)

Soil-air samples were collected biweekly over 2-3 years by means of a convection free sampling device with high vertical resolution on three forest sites in southwest Germany. Soil-air composition was analyzed gaschromatographically in the laboratory. The three sampling sites (a gleyic cambisol in the pre-alpian morraine landscape, a cambisol on Buntsandstone in the northern black forest, and a cambisol on gneiss in the middle black forest) represent a wide range of properties influencing soil aeration.

Processes influencing soil-air composition were identified by comparing measured concentration relations of soil-air components with deterministic models based on the kinetic gas theory and the different diffusive mobilities. For this purpose, the relation between N2 and CO2 has shown to be particularly suitable, allowing to distinguish three types of soil aeration. In the case of optimal soil aeration, N2 decreases slightly with increasing CO2 concentration. When soil aeration is more restricted, i.e. CO2 is transported partially in the soil water, N2 concentrations in the soil air increase considerably. However, when soil respiration becomes anaerobic, i.e. CO2 is released without equivalent O2 consumption, the N2/CO2 relation culminates due to displacement of

N2. A qualitatve distinction between relative N2 enrichment and N2 production can be done by analyzing the relation between the noble gas Argon and N2. However, due to the high relative error of Ar analyisis, a quantification of denitrification by this relation is only possible in the case of low gas-diffusion coefficients and high denitrification rates.

If the qualitative process identification confirms the assumptions in Fick's law, a calculation of depth profiles of gas exchange is possible by combination of depth functions of gas concentrations and measured gas-diffusion coefficients. We found that quadratic spline regressions with defined bends of the 1. derivatives at the horizon boundaries are a suitable mechanistic model to obtain differentiable depth functions of gas concentrations. At the soil surface, we calculated gas fluxes additionally with a linearizing approach. In the forest stands of the black forest, the annual CO2 efflux ranged between 5 and 6 t CO2-C /ha/a, the N2O-N efflux was approximately 1kg N2O-N ha/a. The calculation of depth profiles for the plot in the northern black forest showed, that a major portion (72%) of soil respiration takes place in the Ah and AhBv. The variability of horizon-specific respiration rates can be explained by a Q,o model with r2 values between 45% and 91%. In contrast to soil respiration, only 30% of the total N2O efflux comes from the Ah and AhBv, but 45% is produced in the organic layer and 25% in the Bv.

The gradient method can be regarded as a cheap alternative to common open-bottom chambers with the advantage of minimal site disturbance and the unique possibility to localize the sources of gas efflux in the soil profile. However, precondition is the availability of of reliable time functions of soil gas-diffusion coefficients.



Part 2: Fine root health of Picea abies in differently aerated soil horizons (Metzler)

Soil oxygen concentration is a well known key factor for root growth and root health. However, only few information is available on the interdependence of the roots with the microbial community in the rhizosphere under different aeration. In a joint project with the department of forest soil science and forest nutrition root samples were taken from forest stands with differently aerated soil types and from different soil horizons of max. 60 cm depth.

Fine roots from less aerated soil horizons were more intensively colonized by minor pathogens like Mycelium radicis-atrovirens and with Cylindrocarpon destructans. In a pseudogley-soil some isolates of rather aggressive parasites like Pythium sp. and Cryptospohopis abiefma were obtained. The amount of mycorrhizal root tips on the other hand decreased with soil depth. It is concluded, that soil anaerobiosis will cause premature ageing of fine roots which both hinders mycorrhization and promotes susceptibility to root pathogens.



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