To challenge the "conventional wisdom" that rates of net N-mineralization increase with pH, we measured net N-mineralization,
respiration and/or microbial C and N in four Luxembourg beech forests with similar litter input, but different soil types,
using laboratory incubation experiments. Litter input and fungal/bacterial colony ratios were also measured. To test whether
the results could be explained by existing theoretical models, equations of C and N dynamics were reformulated to allow estimation
of microbial growth efficiency, gross C and N release and microbial uptake, based on measured values of net N-mineralization,
respiration and C:N ratios of substrate and microbes.
Instead of an increase, net N-mineralization rates showed a significant
sevenfold decrease from acid to calcaric soil in the organic layer, and a fourfold decrease in the mineral topsoil. At the
same time, microbial N-demand increased with pH, as indicated by the significant decrease in net N-mineralization per unit
microbe or unit C respired. These results could be explained by theoretical models. In organic layer and mineral topsoil,
despite high gross N-release, net N-mineralization rates decreased with pH because of higher microbial immobilization. Increase
in microbial N-demand was associated with a decrease in fungal/bacterial colony ratio: the more the bacteria, the higher the
Acid and calcaric soils seem to have different strategies to sustain ecosystem N-fertility. In calcaric
soil, N-availability to the vegetation seems indeed supported by high biological activity and gross N-release, which is needed
to compensate for the potentially high immobilization by bacteria. In acid soil, however, despite low gross N-release, N-availability
to the vegetation may not be lower than in calcaric soil, due to high amounts of fungi and low microbial N-demand.