Biological soil crusts of temperate forests: Their role in P cycling

- The ecological role of biological soil crusts (BSCs) in the P-cycle was elucidated.
- P concentrations, -pools and -species were determined in BSCs and soil samples.
- P pools indicated weathering of Fe/Al-P species by BSC organisms.
- Vegetation, soil horizon and forest management intensity affected the P-cycle of BSCs.
- BSC green-algae richness and life form depended on forest management intensity.

To elucidate the ecological role of biological soil crusts (BSCs) in the P-cycle and to disclose vegetation (pine vs. beech), soil horizon (A horizon vs. C horizon) and land use intensity effects, we examined BSCs collected from a temperate forest in Schorfheide-Chorin (Germany). Total C, N and P of the three sample compartments crust, crust-adhering soil underneath and crust-free soil were quantified and inorganic and organic P (Pi, Po) contents in five sequentially extracted P fractions of the different compartments were determined. In addition, P species were characterized using P K-edge XANES (X-ray absorption near edge structure) and 31P NMR (nuclear magnetic resonance) spectroscopy. BSC biodiversity of algae was morphologically determined using enrichment cultivation. Results showed an accumulation of total P in the BSC with high shares of Po. Proportions of labile and moderately labile Po pools were higher in BSC than crust-free soil in expense of residual P indicating weathering of Fe/Al-P species by BSC organisms. Vegetation affected the C/N ratio and proportion of labile Pi in total P of BSC; they were significantly higher under beech compared with pine. XANES results revealed BSC weathering of mainly Fe-P under beech and Al-P under pine. Soil horizon affected the composition of BSC organisms; there were more filamentous algae in BSCs on C horizon than A horizon. Moderately labile Pi concentrations were higher in BSC on C horizon compared with A horizon while the share of the labile Po pool in total P was lower on C horizon than A horizon. Increasing forest management intensity decreased the share of moderately labile P and Po in total P as well as the monoester-P/diester-P ratio in pine BSC. While in BSC under pine changes occurred in microbial diversity, under beech changes occurred in algal richness and life form. We conclude that BSCs in Central European forests are particularly involved in the transformation of Pi to Po fractions and respond differently to management intensity depending on the predominating tree species.