Introduction

1. Residual of CO2 Fig. 1: Soil CO2 evolution as dependent on soil moisture and faunal composition soil water content [% WHC] IFZ - Department of Animal Ecology Justus Liebig University Giessen Research Centre for Bio Systems, Land Use and Nutrition Effects of water content fluctuations on organic matter transformation vary with the functional type of soil fauna present Introduction We analysed whether changes in soil microclimate modulate in a different way the impacts of two faunal groups on soil organic matter transformation. Earthworms were chosen as representative of those parts of the soil fauna that excrete stable pellets with a relatively high carbon content. Nematodes, in contrast, were chosen as a model for soil fauna that creates liquid excreta with high nitrogen content. Fig. 2: Nitrogen concentration in the soil solution at the end of the experiment as dependent on soil moisture and faunal composition Methods 32 microcosms (10 cm diameter, 23 cm height) were filled with soil substrate and inoculated with earthworms and nematodes in 3 densities and in combination according to a D-optimised incomplete design. The microcosms were kept outside for 58 days and were subjected to near natural weather conditions. Temperature and moisture were partially manipulated to create a wider spectrum of microclimates. CO2 was measured daily by means of gas chromatography. Soluble nitrogen was measured at the end of the experiment. At the end of the experiment, nitrogen concentration in the soil solution was strongly reduced at higher soil moistures in the presence of lumbricids, while nematodes induced a slight increase (Fig. 2). Soluble carbon and nitrogen concentrated within earthworm casts are more readily accessible in a water filled pore space because diffusion is largely facilitated in the thicker water films present in such pores (Fig. 3a). In contrast, nitrogen dissolved in liquid nematode excreta diffuse further away from the inner pore surface colonised by the micro-organisms, if pores are water filled (Fig. 3b). As a consequence of their opposing effects, the combination of lumbricids and nematodes exhibited less dependence on water content. This was particularly visible under dry soil conditions in our experiment. Results Fluctuations in soil moisture affected the impact of earthworms and nematodes on soil CO2 evolution as well as the nitrogen availability in the opposite way (Tab. 1). Temperature effects were not significant. Discussion We interpret the diverging trends of CO2 evolution and nitrogen concentration remaining at the end of the experiment to originate from the same consumption process: Carbon is respired, and nitrogen is assimilated by the micro-flora. We assume that the opposing effects of earthworms and nematodes on microbial soil organic matter transformation can be largely explained by opposing effects of increased soil water content on the accessibility of carbon and nitrogen from animal excreta for micro-organisms. Tab. 1: GRM analysis (n.a. = not available; n.s. = not significant) Conclusions The results illustrate how different functional groups of soil organisms can create complementary effects and thereby stabilise soil processes in the presence of a diverse soil community. Higher moisture strongly augmented soil CO2 evolution in the presence of earthworms. In contrast, soil CO2 evolution nearly ceased under higher moisture in the presence of nematodes. Both faunal groups together exhibited an additive superimposition of the single effects (Fig. 1). Fig. 3a: Diffusion of soluble nutrients out of an earthworm cast is largely facilitated if water films are thick or pores are water saturated in the surrounding soil. Fig. 3b: Nutrients from liquid nematode excreta are diluted and removed away from the microbial film residing at the inner pore surface (orange) in water saturated pores. Acknowledgement: This research was conducted within the Priority Programme “Soil as sink and source of CO2”, funded by the Deutsche Forschungsgemeinschaft. O. Fox, S. Vetter, K. Ekschmitt and V. Wolters Justus Liebig University,Department of Animal Ecology Heinrich-Buff-Ring 26-32 (IFZ), D-35392 Giessen, GermanyE-mail: Klemens.Ekschmitt@allzool.bio.uni-giessen.de