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 environments. These microorganisms release exopolysaccharides (EPS), which form the exopolymeric matrix (EPM), allowing them to bond soil particles together and survive long periods of dryness. Here we hypothesized that the biocrust EPM may constitute a passive filter that helps retain microbial metabolites. Methods: We report new methods for the investigation of metabolite sorption on biocrusts compared to the underlying unconsolidated subcrust fraction. A 13C-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 comparison of metabolite sorption on the biocrust EPM or EPS versus mineral sorption on the underlying soils. In these particular samples, the biocrust (and its extracted EPS) sorbed 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.