Litter quality effects of beech and hornbeam on undergrowth species diversity in Luxembourg forests on limestone and decalcified marl

Question: Do beech and hornbeam differ in undergrowth species diversity, and could these differences be explained by mass of the organic layer, pH or soil moisture? Could species richness under beech and hornbeam be explained by differences in N dynamics? Location: Ancient forests dominated by beech and hornbeam in Luxembourg, on decalcified and lime‐rich parent material. Methods: Species composition and soil characteristics, i.e. mass of the organic layer, pH, moisture content, bulk density and air‐filled pore space, were determined under beech and hornbeam on decalcified marl and limestone, and analysed with cluster analysis, correspondence analysis and ANOVA, with tree species and parent material as independent factors. Net N mineralization, nitrification and N in soil and microbial biomass were determined in a laboratory incubation experiment over a pH gradient, and correlated with species richness. Results: Species richness was lower under beech than under hornbeam on both decalcified marl and on limestone. Mass of the organic layer was higher under beech in all cases, but pH and soil moisture only differed between tree species on decalcified marl. Species richness increased with soil N, microbial N and nitrification in the mineral topsoil, but was not correlated with net N mineralization, and was negatively correlated with nitrification in the organic layer and with efficiency of N mineralization per unit microbe. Conclusion: Litter quality affected species richness on both limestone and decalcified marl via the organic layer, but only on decalcified marl in terms of pH or soil moisture. Species richness did not seem to be stimulated by high overall net N mineralization or high nitrate availability in the organic layer, but only by high nitrate in the mineral topsoil.