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Trees growing in nutrient-limited temperate forest soils can gain nutrients by root proliferation into nutrient-rich hotspots and/or by forming mycorrhizal symbioses. In this study we investigated the effects of nutrient-rich hotspots (inhabited wood ant nests) on Norway spruce root biomass and occurrence of ectomycorrhizal (EcM) exploration types. Substrates were collected from the mineral soil layer in a temperate middle-European spruce forest (Nationalpark Harz, Germany) from four micro-regions within each of the five wood ant nests sampled, i.e. 1) centre of the belowground part of a nest, 2) nest's rim, 3) nest's run-off zone (ca. 20 cm from nest's rim), and 4) from the surrounding forest soil (>10 m from nest's rim). Root biomass, EcM exploration types, moisture and nutrient contents were determined in all substrates. Although naturally enriched, wood ant nests had neither root biomass nor occurrence of EcM exploration types higher compared to the surrounding forest soil. The probable main reasons were high phosphorus content and low moisture maintained inside inhabited nests, although the effect was not significant. Apparently, other substrate properties not determined in our study also affect the occurrence of exploration types in wood ant nest substrates. Inhabited wood ant nests thus seem unfavourable for root proliferation and their subsequent EcM colonization. However, roots can gain nutrients from the run-off zone around nests where low moisture is not maintained and nutrient contents are higher due to leaching from the nest substrate.

The creation of small-scale vegetation mosaics by mound building is an important aspect of ant activity in grasslands. The consequences of this process for the composition of the viable soil seed bank are poorly understood. In this study we quantified the impact of the yellow ant Lasius flavus on both aboveground vegetation and seed bank of a low-intensity pasture located in the Vogelsberg area (Hesse, Germany). A few species of the mound vegetation (mainly therophytes) were restricted to the mounds, but most species also occurred in the surrounding area. The seed bank of ant mounds differed from that of the pasture soil between the mounds, with the abundance of germinating seeds being twice as high in the mounds. This was mainly due to the very large number of seeds originating from winter annuals and Thymus pulegioides. Seed abundance of most other species was lower in the mounds. Our results showed that this is partly due to dispersule weight limiting dispersal of seeds from the surrounding vegetation onto the mounds.

Microbial biomass and activity as well as N-availability were measured in the mounds of three ant species strongly differing in foraging strategy and mound architecture: Myrmica scabrinodis, Lasius niger and L. flavus. Soil microbial biomass (Cmic) was significantly increased in the mounds of all three ant species. This positive effect was due to the accumulation of organic matter (Corg) within the mounds. Microbial activity was increased in M. scabrinodis mounds only. Available Nmin was accumulated in all mound types, independent of the feeding mode of the ants, with Lasius mounds having significantly higher Nmin content than M. scabrinodis mounds. It is hypothesised that the differences between the mound types are due to differences in nest architecture, especially the integration of grassy vegetation. Higher microbial activity in M. scabrinodis mounds may be a consequence of supplementary energy provided by root exudates. The amount of Nmin in M. scabrinodis mounds may be reduced by increased plant uptake and by immobilisation within the microbial biomass.