Small-scale spatial heterogeneity of soil chemical and microbiological properties triggered by biocrusts and grasses in sand dunes of temperate climate

Soils in drylands form a mosaic of vegetated patches and interspaces, creating spatial heterogeneity in soil properties. While plants often generate ‘fertile islands’, interspaces are frequently colonized by biocrusts that also shape soil characteristics. To examine these processes in early successional stages in a sand dune ecosystem, we analyzed microbial activity, soil chemistry, and root colonization by arbuscular mycorrhizal fungi (AMF) and dark septate fungi (DSF) across five compartments: biocrust, below-crust soil, root system soil, rhizosheath soil, and bare soil. Biocrusts exhibited the highest levels of exopolysaccharides (EPS), organic C, and humus, as well as the highest dehydrogenase activity, which underscore their role in C fixation and soil organic matter accumulation. Rhizosheaths had the highest concentrations of loosely bound EPS fraction and contained the highest total mineral N and N-NH 4 + concentrations, exceeding those in remaining substrate compartments. The root system soil, in turn, acted as a broader nutrient mobilization zone, with particularly high available K and P-PO 4 − . Regarding fungal root colonization degree, higher frequency of occurrences and abundance of AMF than DSF mycelia were observed. The frequent occurrence of AMF and DSF in roots and DSF hyphae in biocrusts indicates a fungal network connecting plants and biocrusts, which is consistent with the fungal loop hypothesis described from arid climates. Biocrusts and vascular plants shape fine-scale soil heterogeneity in the initial successional stage on sand dunes, where microbial activity and nutrient enrichment show uneven distribution. • Soil heterogeneity on sand dunes is driven by grass and biocrust mosaic • Microbial activity and nutrient enrichment were unevenly distributed in soil • Grass roots showed a dual colonization strategy by both AMF and DSF • Root system soil acts as nutrient mobilization zone supplying P and K for plants • Rhizosheath is a hotspot for root–microbe interactions with high N concentration