Biologically Available Phosphorus in Biocrust-dominated Soils of the Chihuahuan Desert

In desert soils, phosphorus (P) is an important limiting nutrient and its cycling characteristics are less understood compared to nitrogen and carbon. Phosphorus cycling is controlled by both geochemical and biological factors. Traditionally, P availability has been characterized via sequential extraction analyses such as Hedley fractionation, but newly developed extraction methods allow for the examination of more biologically relevant P fractions, providing insight on mechanisms of P acquisition by plants and microbes. We examined these P cycling features in the context of biological soil crusts (biocrusts), which have been found to be important drivers of nutrient cycling and have the potential to release bound labile P for uptake by soil biota and plant roots. We adopted the biologically-based P (BBP) method that incorporates four extractions: calcium chloride (CaCl2), citric acid, phosphatase enzymes, and hydrochloric acid (HCl) that mimic different P acquisition strategies. We coupled the extractions with a 33P-labeled orthophosphate addition and incubation to assess the fate of freshly available phosphate (PO43-). Low P concentrations in the CaCl2 extraction suggests that drylands lack easily accessible P in soil solution, while higher amounts in the citric acid- and enzyme-extractable pools suggest dryland soil biota may acquire P through the release of organic acids and phosphatases. Radiolabel addition results showed that added PO 43- is, within 24 hours, quickly adsorbed onto mineral surfaces or incorporated into hydrolysable organic compounds, instead of remaining in the soil solution as very little recovery was observed in the CaCl 2 pool. Compared to areas of disturbance or with no intact biocrust, areas with biocrusts showed overall lower P concentrations across all four extractable pools. This suggests that biocrust organisms may prevent P adsorption onto mineral surfaces by incorporating P into their biomass. Overall, our results indicate that while P is not immediately available in the soil solution, organisms have several viable strategies, including organic acid and enzyme production, to access P in dryland soils