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

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 (similar to 15.5 kN m(-3)) and high (similar to 19.0 kN m(-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 correlated with soil aggregate stability. Simple linear regression models revealed a significant positive effect of root length per soil volume on soil aggregate stability. Compared to the non-inoculated control plants, mycorrhized White Alder produced significantly more roots and, consequently, higher soil aggregate stability. Furthermore, the combined biological effect of plant roots and mycorrhizal mycelia on aggregate stability in soil with low density (similar to 15.5 kN m(-3)) was comparable to the compaction effect of the pure soil from 15.5 to similar to 19.0 kN m(-3). Literature data on the effect of vegetation on the angle of internal friction Phi' of the same moraine showed similar correlations, i.e. that Phi'of low density soil material (similar to 15.5 kN m(-3)) increased by the same amount whether by planting with White Alder or by compaction to similar to 19.0 kN m(-3). Based on this coincidence and from a soil mechanical perspective, soil aggregate stability is suitable to estimate the joint effect of plants and mycorrhizal fungi with respect to their contribution to soil and slope stability in the near-surface layer. (C) 2013 Elsevier B. V. All rights reserved.