Soil aggregate stability related to soil density, root length, and mycorrhiza using site-specific Alnus incana and Melanogaster variegatus s.l

•Ectomycorrhiza fungus Melanogaster variegatus increases root length of Alnus incana.•Mycorrhized Alnus incana (White Alder) increases soil aggregate stability (SAS).•Effects of mycorrhized roots on SAS correspond to an increase in dry unit weight.•SAS is correlated with dry unit weight as is known for the friction angle Φ’.•SAS may be used to add biological effects to Φ’ in slope stability calculation. Eco-engineering aims at stabilising soil and slopes by applying technical and biological measures. Engineering structures are commonly well defined, immediately usable and operative, and their stability effects quantifiable. Differently, the use of plants requires more restrictive boundary conditions and the protection potential is rarely easily calculable and is developing as a function of growth time. Soil aggregation processes play a crucial role in re-establishing soil structure and function and, conclusively, for successful and sustainable re-colonisation. Mycorrhizal fungi are key-players that foster the development of a protective vegetation cover. They accelerate and increase plant growth and, additionally, contribute to soil aggregate stability which, on its part, was recently proposed as an appropriate indicator with regard to the quantification of biological effects on soil and slope stability. The objective of this study was to determine the effects of mycorrhizal fungi on the host's root system as well as on soil aggregate stability. Furthermore, the biological contribution to soil aggregate stability was compared to mechanical stabilisation effects due to soil compaction. The site-specific plant-fungus symbiosis Alnus incana and Melanogaster variegatus s.l. of a recently stabilised steep catchment on moraine was used for laboratory experiments. Aggregate stability tests were performed with samples of differently treated moraine, including soil at low (∼15.5kNm−3) and high (∼19.0kNm−3) dry unit weight, soil planted with A. incana (White Alder) as well as the combination of planting with alder and inoculating with the mycorrhizal fungus M. variegatus s.l. After a 20 week growth period in a greenhouse, a total of 100 samples was tested and evaluated. Positive correlations were found between the soil aggregate stability and the three variables dry unit weight, root length per soil volume, and degree of mycorrhization. Based on robust statistics it turned out that over all samples dry unit weight and degree of mycorrhization were strongest correlat