Monday, 10 July 2006

Mechanisms of Solute Transport Modify Small-Scale Abundance and Function of Microorganisms in Soil.

Ellen Kandeler1, Christian Poll1, Joachim Ingwersen2, Thilo Streck2, Esther Enowashu1, and Sven Marhan1. (1) Institute of Soil Science, Soil Biology Section, University of Hohenheim, Emil-Wolff-Str. 27, Stuttgart, 70599, Germany, (2) Institute of Soil Science, Biogeophysics Section, University of Hohenheim, Emil-Wolff-Str. 27, Stuttgart, 70599, Germany

Particulate organic matter at the interface between soil and litter (detritusphere) offers new sites for microorganisms and releases soluble substrates into the adjacent soil. The aim of our study was to trace the translocation of litter-C into SOM and microbial biomass under different solute transport conditions. We hypothesise that the mechanism of solute transport as well as the soil water content trigger differing gradients of microbial activity and differing substrate utilization by the microbial community in the detritusphere. In two consecutive experiments, transport was restricted either to diffusion or to diffusion and convection. Two levels of soil water content were established in each experiment. When diffusion was the exclusive transport mechanism, the addition of maize litter induced distinct gradients in enzyme activities, soil organic C content and microbial biomass to a depth of 1.5-2.8 mm. Convection enlarged these zones of enhanced microbiological activities to 3.3-9.3 mm. The moisture regime modified the temporal pattern of diffusive C transport, microbial growth and enzyme release by inducing faster transport at high water contents. Convective transport seemed to be unaffected by soil moisture. Using a convective-diffusive transport model with first-order decay, it was possible to simulate the observed activity profiles. The results show that the spatial dimension of the detritusphere is governed by the ratio between decay rate of available substrates and transport rate. A higher decay rate tends to downsize the detritusphere, whereas an increased transport rate tends to enlarge the detritusphere. Moreover, such a mechanism should stratify available substrates in soil. With increasing distance from the soil-litter interface, the ratio between easily and heavily degradable substrates should continously decrease. This, in turn, should affect the vertical abundance and function of soil microorganisms. In addition, provided that the effective diffusion/dispersion coefficient is known, the measured steady-state enzyme activity profiles could be used to calculate the substrate decay rate. Bacteria and fungi showed differing utilization strategies as revealed by coupling PLFA analysis with stable isotope techniques. Fungi assimilated C directly in the litter, whereas bacteria utilized the substrates from the soil and therefore depended more on transport processes than fungi. The spatial variability of the detritusphere depends on transport and soil water conditions. Our results also underline the importance of spatial modelling for understanding C cycling in soils. Present studies test the impact of microbial community composition on C and N transport in the detritusphere.

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