Biological soil crusts as key player in biogeochemical P cycling during pedogenesis of sandy substrate

Little is known about phosphorus (P) in biological soil crusts (BSCs) and their role in biogeochemical P cycling. The present study evaluated P in BSCs with an array of methodological approaches including sequential P fractionation, solution P-31 nuclear magnetic resonance (NMR) spectroscopy, synchrotron-based P K-edge X-ray absorption near-edge structure (XANES) spectroscopy, elemental mapping (mu-XRF) combined with mu-XANES and P-lipid quantification. BSCs (light algal crusts) were collected at seven sites along a sediment weathering gradient in north-eastern Germany (based on feldspar weathering indices) from non-weathered dune sands at the Baltic Sea coast to more strongly weathered Weichselian glacio-fluvial sands near Berlin. The total P (P-t) concentrations of BSCs ranged from 93 to 389 mg kg(-1) and were not significantly correlated with the change in feldspar weathering index. While concentrations of stable P (H2SO4-extractable) strongly decreased, labile P (resin- + NaHCO3-extractable P) in BSCs increased with increasing sediment weathering. Based on P-31 NMR spectra, 20 to 62% of NaOH-EDTA extracted P was orthophosphate monoesters and 0 to 9% was diesters. For BSCs, P K-edge XANES showed that Ca-P species decreased with increasing weathering. Heterogeneity of the BSCs was exemplarily shown at the micrometre scale by element mapping mu-XRF and mu-XANES, using four 10 x 10 mu m spots of a vertical cross-section within a coastal dune BSC. While only the P-lipid class phosphatidic acid (PA) increased with increasing weathering, analyses of BSC phospholipid fatty acids (PLFAs) revealed decreasing contributions of Gram-positive bacteria with increasing coastal sediment weathering but different taxa were independent of any investigated P parameter. Proportions of lipid-P varied between 0.02 and 0.1% of Pt, indicating a constant share of living biomass in BSCs along the gradient. In conclusion, this multi-method study of P speciation in BSCs showed that these communities play a key role in the biogeochemical P cycle, especially by transforming stable P into labile, easily bioavailable P.