BOTTOM-UP CONTROL OF THE SOIL MACROFAUNA COMMUNITY IN A BEECHWOOD ON LIMESTONE: MANIPULATION OF FOOD RESOURCES

Abstract

Food resources of soil animals of a beechwood (Fagus sylvatica L.) on limestone in northern Germany were manipulated by adding carbon (glucose), nitrogen (as NH4NO3), and phosphorus (as NaH2PO4) separately and in combinations to experimental plots for 15 mo. By adding glucose and nutrients we intended to manipulate microbial biomass in litter and soil which is assumed to form a major part of the food resources of saprophagous soil animals. Manipulation of the microbial part of the diet of saprophagous soil animals allowed us to study the importance of “bottom-up” forces without changing spatial habitat characteristics. Addition of glucose and nutrients caused an increase in microbial biomass with a maximum in the CNP treatment (+52.3%, +51.1%, and +44.0% for the litter layer, for 0–3 and 3–6 cm soil depths, respectively). Separate additions of carbon and nutrients indicated that microorganisms in the litter layer were primarily limited by nitrogen, whereas soil microorganisms were primarily limited by phosphorus. In soil the content of carbon decreased by addition of phosphorus, and in litter the content of nitrogen increased by addition of nitrogen, also indicating that soil microorganisms were limited by phosphorus and litter microorganisms by nitrogen. Nutrient (nitrogen and phosphorus) concentrations in leaves of ash seedlings growing in experimental plots indicated that competition between microorganisms and plants for nutrients was severe in glucose treatments with microorganisms being the superior competitor. None of the animal groups studied (Lumbricidae, Diplopoda, Isopoda, and Chilopoda) exhibited a response that was parallel to that of microorganisms. Rather, earthworms appeared to be limited by availability of carbon, and in contrast to microorganisms, earthworm numbers and biomass increased strongly with application of glucose only. Maximum biomass of earthworms in the glucose-only treatment indicates that earthworms and microorganisms competed for carbon resources with microorganisms being the superior competitor when carbon and nutrients were available. Earthworm species differentially responded to experimental manipulations, and the density of one species (Aporrectodea rosea) even declined due to glucose applications. The responses of millipedes and isopods were generally less pronounced, but some species increased in numbers under certain treatments. Density of centipedes, particularly that of the soil-dwelling Geophilidae, was strongly reduced due to glucose applications. In contrast to our expectations, earthworms appear not to form a major part of the diet of centipedes in the studied beech forest.