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In the past years, extracellular polymeric substances (EPS) produced by soil microorganisms received an increasing interest, as they not only protect microbes against environmental stresses, but seem to play a pivotal role in soil structure formation as well. Within soils, root deposits provide an important source of easily accessible energy and nutrients, stimulating microbial growth to produce EPS. Especially under semiarid climates, where a full vegetation cover cannot be sustained, large gradients in living conditions for microbes can be found between the root-soil interface and barren intercanopy spaces.In this study, we aimed to elucidate the plant-specific effects on microbes, EPS production and soil aggregation. At two sites in southern Spain, differing in carbonate and graphite content, legume shrubs of Anthyllis cytisoides and grass tussocks of Macrochloa tenacissima were selected. Soil samples were taken in the adjacent bare interspace, under the canopy and of the rhizosphere. From these samples the microbial community (here bacteria and archaea), EPS(-saccharide) content and soil aggregation (The rhizoplane microbial communities differed strongly between plant species and sites, whereby site was the most important factor shaping the communities. The plant species effect on microbial communities diminished strongly with distance to the root surface. At the carbonate-poor Rambla Honda site (site 1), plant species-specific effects were observed in the rhizoplane and rhizosphere, whereas in the carbonate-rich Alboloduy site (site 2) almost no plant species-specific effects were found at the genus level. The larger heterogeneity in microbial communities at site 1 was reflected in EPS-saccharide contents and subsequent soil aggregation, while no difference in soil aggregation was found at site 2. Both parameters increased strongest in the Anthyllis cytisoides rhizosphere at site 1.Despite the lack of a strong gradient with distance from

In the barren semiarid landscape individual plant species improve soil structure, reducing the erosion risks, whereby microaggregates form the most fundamental soil structural components. Extracellular polymeric substances (EPS) are considered an important glue determining aggregation in addition to inorganic binding agents such as carbonates. However, the role of the prokaryotic community in EPS formation and consequently for microaggregation in natural environments is not substantially clarified yet. EPS should be particularly important under semiarid conditions as it forms a protection mechanism of the prokaryotes against desiccation. Therefore, we examined the influence of the prokaryotic community on soil EPS content and subsequently on soil microaggregation in semiarid grasslands, depending on the parent material, common plant species and the distance to the plant. Soil samples were taken over a distance gradient from two major semiarid grassland plant species in Southern Spain, the legume shrub Anthyllis cytisoides and the grass tussock Macrochloa tenacissima, to the surrounding bare soil at two sites, rich and poor in carbonates. Total community DNA and EPS were extracted, followed by quantification of EPS-saccharide, bacterial abundance and examination of the prokaryotic community composition. Further, the particle size distribution of the microaggregate fraction was determined as an indication of microaggregation. We found that the overall prokaryotic community composition differed between the two sites, but not between plant species. Nonetheless, a link between the community composition and EPS content was established, whereby soil organic matter (OM) seems to be a regulating factor as increasing soil OM contents resulted in more EPS-saccharide. Furthermore, microaggregation was enhanced by the canopy, especially at the edge of Macrochloa tussocks. Contrary to the expectation that increasing inorganic C contents would diminish importance of EPS, the pa