A novel method to evaluate nutrient retention by biological soil crust exopolymeric matrix

Aims Biological soil crusts (biocrusts) are microbial communities commonly found in the upper layer of arid soils. These microorganisms release exopolysaccharides (EPS), which form the exopolymeric matrix (EPM), allowing them to bond soil particles together and survive long periods of dryness. The aim of this work is to develop methods for measuring metabolite retention by biocrust EPM and EPS. Methods We report new methods for the investigation of metabolite sorption on biocrusts compared to the underlying unconsolidated subcrust fraction. A C-13-labeled bacterial lysate metabolite mixture was incubated with biocrust, subcrust and biocrust-extracted EPS. Non-sorbed metabolites were extracted and analyzed by liquid chromatography/mass spectrometry. Results This simple and rapid approach enabled the comparison of metabolite sorption on the biocrust EPM or EPS versus mineral sorption on the underlying soils. Our results suggest that the biocrust (and its extracted EPS) sorb more metabolites, especially amino acids and organic acids, than the underlying subcrust. Conclusions This study demonstrates a useful method to highlight the essential role of biocrust (especially the EPM), which acts as a passive nutrient filter, sequestering metabolites released by microbes during wetting events. This may facilitate recovery of the community upon wetting and further enhance biocrust survival and nutrient retention.