Continuous and cumulative acidification and N deposition induce P limitation of the micro-arthropod soil fauna of mineral-poor dry heathlands

Phosphorus content of mineral-poor sandy soils is steadily decreasing due to leaching caused by continuous and cumulative acidification and N deposition. Sod-cutting as a traditional restoration measure for heathland vegetation appears to increase P limitation, as most of the P present is in the organic matter being removed by sod-cutting. Mineral weathering, the natural inorganic source of P, becomes limiting or has even ceased as a result of the depletion of minerals. Previous investigations indicate a P limitation of the macrofauna under these circumstances. If this also holds for the soil fauna, hampering of decomposition may occur. To test experimentally whether soil fauna is indeed limited by the amount of P in the system, we set up an experiment in sod-cut heathland in which we added P or Ca (as Dolokal), resulting in: P + Ca+, P + Ca-, P-Ca+ and P-Ca- (control) treatments and an extra reference block in the original grass encroached heathland vegetation. The Ca treatment was added because liming is used to recover from acidification effects, but as a side effect Ca may also bind P. Three growing seasons after the addition of P and Ca, we found a significant increase in herbivores and predators among the soil fauna, with herbivore numbers higher in the P+/Ca-plots than in the P+/Ca + plots, indicating a lower availability of P in the presence of added Ca. Predators increased in all P+ plots. Fungivorous browsers responded negatively to the treatment after three growing seasons, both to P and to Ca addition. Phoretic species responded rapidly either to fewer numbers (when these are fungivorous browsers) or to greater numbers (when these are herbivorous browsers) to P addition. P addition induced also an allometric effect, via the medium-sized species increasing in greater numbers than both the larger and smaller species.