Alternative strategies for nutrient cycling in acidic and calcareous forests in the Luxembourg cuesta landscape

In the forests of the Luxembourg cuesta landscape, nutrient cycling is affected by parent material, but in a different way than usually assumed. We challenge the ‘conventional wisdom’ that net N-mineralization is higher in calcareous than in acidic soils, due to higher biological activity and gross N-mineralization. In four separate laboratory incubation experiments, net N-mineralization was higher in acidic than in calcareous soil. Experiments with different tree species showed that soil type was even more important than litter quality. In acidic forests, high net N-mineralization may be due to dense organic layers, but also to differences in soil communities, which are dominated by fungi at low pH versus bacteria at high pH. Fungi have lower N-demand than bacteria, and may thus mitigate low activity and gross N-release. Model studies suggested that microbial immobilization was below 20% in acidic soil, and above 80% in calcareous soil, in both organic layer and mineral topsoil. Differences between fungi and bacteria were supported by selective inhibition. Microbial immobilization significantly decreased with the bactericide streptomycin, while respiration increased with the fungicide cycloheximide. This further supports that bacteria and fungi, and with them calcareous and acidic soils, show different strategies for N-nutrition. For P-nutrition, differences between calcareous and acidic soils are also important, as net P-mineralization mainly occurred in the organic layer, due to chemical sorption in the mineral soil. As a result, in the Luxembourg cuesta landscape, availability of both N and P may be higher in acidic than calcareous forests.