Linking above-ground and below-ground effects in autotrophic microcosms: effects of shading and defoliation on plant and soil properties

Although factors affecting plant growth and plant carbon/nutrient balance - e.g., light availability and defoliation by herbivores - may also propagat e changes in below-ground food webs, few studies have aimed at linking the above-ground and below-ground effects. We established a 29-week laboratory experiment (similar to one growing season) using autotrophic microcosms to study the effects of light and defoliation on plant growth, plant carbon/nu trient balance, soil inorganic N content, and microbial activity and biomas s in soil. Each microcosm contained three substrate layers - mineral soil, humus and plant litter - and one Nothofagus solandri var. cliffortioides se edling. The experiment constituted of the presence or absence of two treatm ents in a full factorial design: shading (50% decrease in light) and artifi cial defoliation (approximately 50% decrease in leaf area in the beginning of the growing season). At the end of the experiment a range of above-groun d and below-ground properties were measured. The shading treatment reduced root and shoot mass, root/shoot ratio and leaf production of the seedlings, while the defoliation treatment significantly decreased leaf mass only. Le af C and N content were not affected by either treatment. Shading increased NO3-N concentration and decreased microbial biomass in humus, while defoli ation did not significantly affect inorganic N or microbes in humus. The re sults show that plant responses to above-ground treatments have effects whi ch propagate below ground, and that rather straightforward mechanisms may l ink above-ground and below-ground effects. The shading treatment, which red uced overall seedling growth and thus below-ground N use and C allocation, also led to changes in humus N content and microbial biomass, whereas defol iation, which did not affect overall growth, did not influence these below- ground properties. The study also shows the carbon/nutrient balance of N. s olandri var. cliffortioides seedlings to be highly invariant to both shadin g and defoliation.