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Abstract "Heft 34"

Freiburger Bodenkundliche Abhandlungen

Schriftenreihe des

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

Heft 34

Astrid Wölfelschneider

Einflußgrößen der Stickstoff- und Schwefel-Mineralisierung auf
unterschiedlich behandelten Fichtenstandorten im Südschwarzwald

Freiburg im Breisgau 1994

ISSN 0344-2691


Controlling factors of nitrogen and sulfur mineralization on spruce sites in the southern Black Forest as affected by different treatments

At two forest sites in the southern Black Forest (SW Germany), the N- and S-mineraliza-tion was examined in their seasonal dynamics by means of field and laboratory incubation tests. The aim was to quantify the effects of fertilization and liming as well as experimentally increased N- and S-inputs on the mineralization of the two elements. Besides that, influences of the abiotic factors temperature and soil moisture were determined. Furthermore, potential influences of small-scale spatial variability were examined. The gross-mineralization and microbial activity (microbial biomass and CO2-production) were measured parallel to net mineralization. The main objective was to elucidate in detail the nitrification process and its regulating factors,.by determining the number of heterotrophic and chemolithoauto-trophic nitrifying microorganisms. Main interest in this respect was drawn to potential changes resulting from fertilization and liming, respectively.

At the ARINUS-experimental sites Villingen and Schluchsee, soil samples were taken on several occasions during the period July 1992 until June 1993, in order to determine the rates of N- and S-net-mineralization. At Schluchsee, four soil depths (humus layer: Of and Oh; mineral soil: 0-10 cm and 30-40 cm) were sampled on the different treatment-plots. Additionally, the sampling locations were differentiated small-scale site variability resulting from differences in crown density. In Villingen, samples were taken only from the humus layer (Of and Oh) and the upper mineral soil (0-10 cm). The soil samples were directly prepared in the field. The field incubation tests were carried out on the site by burying the soil samples in polyethylene bags during a incubation period of 30 days. The laboratory incubation was carried out during a period of 30 days with constant temperature and moisture conditions. Simultaniously, CO2-production rates as well as microbial biomass were determined. Futhermore, counts of heterotrophic and chemoautotrophic nitrifying microorganisms were determined by means of the MPN-method.

As to the N-mineralization, there is a clear difference between the two sites. The Schluchsee site reveals high N-mineralization rates in the organic humus layer and the upper mineral soil. A considerable part of mineral nitrogen is supplied as nitrate, in the course of which the nitrification degree increases according to soil depth. In Villingen, in contrast, reproduction rates are significantly lower in the humus layer. In Villingen, nitrification could not be found under field conditions, only in laboratory incubations an extremely low nitrate production rate was observed. Incubation tests under field conditions show a close relationship between the temperal variation in N-mineralization, soil moisture and temperature for both sites, whereas laboratory incubations yielding constantly higher mineralization rates do not indicate any seasonal differences. In Schluchsee, a significant influence of spatial variability was observed. At this site in all horizons in higher counts of soil microorganisms and heterotrophic nitrifying organisms were found. In the humus layer, this slevated microbial biomass is partly associated with chemolithoautotrophic nitrate oxidizing bacteria. In the mineral soil, chemolithoautrotrophic NH4+- and NO3--oxidizing microorganisms as well as heterotrophic ratifying microorganisms can be found on almost all sampling dates. In Vil-lingen, microbial biomass is comparably lower in the humus layer. Heterotrophic nitrifying organisms are only present in the mineral soil, while chemolithoautotrophic nitrifying microorganisms cannot be proved in any soil depth.

The effect of dolomite liming in Schluchsee after two years is so far confined to the Of-humus layer. An increase of the N net production rates was not observed, whereas a nearly complete nitrification took place. With distinctly higher pH-values, the gross-mineralization is markedly higher. Besides high counts of total microorganisms and heterotrophic nitri-fiers, chemolithotrophic bacteria were found in the Of-humus layer after liming. Because the Schluchsee soils are well-drained, the nitrification rate, stimulated by liming, increased nitrate concentration in the soil solution down to a depth of 80 cm. The (NH4)2SO4-treatment at both sites in the humus layer and in the upper mineral soil led to an increased NH4+-N-production. At Schluchsee, the N-treatment did not show an increase of microbial biomass; obviously, at this site the microbial N-demand is almost saturated. The increased NH4+-N-supply appears to be a "priming" -effect due to an easily available N-source. On the N-deficient site Villingen, the NH4+-N-production after the N-treatment increased to a considerably higher extent compared to Schluchsee. In addition, an increase of soil microorganism populations is also observed after N-treatment in Villingen. The higher NH4+-N-production restricted to the deeper mineral soil in Schluchsee resulted in an increased nitrification. However, the Mg SO4 -fertilization, did not affect N-mineralization.

The S-mineralization in the dystric cambisol in Villingen is slightly higher compared to the Schluchsee podzol. By nearly equal S-deposition, S-storage and a similar sulfate adsorption capacity in the topsoils of Villingen and Schluchsee the low S-production in Schluchsee is apparently due to immobilization in the increased microbial biomass. Liming in Schluchsee resulted in an increase of SO42--production in the Of-humus layer under field incubation conditions. Under laboratory conditions, SO42- is immobilized to a great extent in the increased microbial biomass. In Villingen and Schluchsee, the (NH4)2SO4 -treatment resulted in a higher S-mineralization in the Of-layer. In Schluchsee, S-retention apparently is enlarged as a result of a higher nitrification. In Schluchsee, the Mg SO4  fertilization increased S-production in the Of-humus layer. The high S-mineralization rates after the treatment with  (NH4)2SO4 and MgSO4 respectively are probably due to an increased mineralization of estersulfate-sulfur and C-bound sulfur. These two organic sulfur fractions were obviously formed to a greater extent according to the increased S-supply. Interrelations between N- and S-mineralization can be seen from the temporal variation of mineralization rates, notably in Schluchsee. However, significant correlations between N- and S-mineralization rates are only found occasionally, since both processes are controlled by different factors.

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