Effects of plant-microbes interaction on geotechnical properties of landfill topsoils

Abstract

Insufficient ecological restoration of man-made habitats on restored landfill sites is commonly observed. It would be advantageous to promote better plant growth, focusing on the roles of symbiotic arbuscular mycorrhizal (AM) fungi and soil amendments (biochar). Restoration, such as plant, bacterial and fungal establishment, imposes considerable effects on soil physical stability that is related to safety. The objectives of this research were to 1) investigate the ecological performance in both the restored South East New Territories (SENT) landfill in Hong Kong, and the Xiaping (XP) landfill (with biochar) in Shenzhen, China, and 2) study the effects of AM fungi on the relevant geotechnical properties. At the two restored landfill sites, plant communities were surveyed. Soil bacterial and AM fungal communities were quantified using 16S and 18S rRNA genes, respectively. Three grass species were used as the hosts of AM fungi in three different experiments in the laboratory. The effects of AM fungi on soil matric suction, soil hydraulic conductivity and root tensile strength were investigated. At the SENT landfill, plant diversity was higher, while bacteria and AM fungi was lower, compared with an adjacent natural site. Less native species (50% less) were observed in the restored site, which consisted of different plant, soil bacterial and AM fungal communities to the natural site (P < 0.05, analysis of similarity), 15 years after restoration. For the XP landfill, biochar amendments significantly increased plant diversity, number of species, number of individuals and evenness. The diversity and number of species for bacteria and AM fungi were also increased. Mycorrhizal treatments (MT) in the root zone retained a lower soil suction (~10 kPa less) at a depth of 30 mm, but a higher suction (~5 kPa more) at a depth of 75 mm (root zone), compared with non-mycorrhizal treatment (NM), during 30 min of wetting. During drying, the soil of MT tended to dry more slowly than NM. Root biomass was a key factor in retaining suction. Soil hydraulic conductivity did not significantly correlate with the concentration of AM fungal glomalin. Root tensile strength was enhanced (species-dependently) by AM fungi. This enhancement ranged from 17 to 64% was associated with higher cellulose and hemicellulose content. Soil cohesion would be increased by enhanced root tensile strength (~45% more) and enhanced root biomass due to AM fungi, contributing to the resistant force, making the plant-soil system more stable. The present research points to limitations of current practice in ecological restoration, and proposes a method for restoring plant, bacterial and AM fungal communities. The initial selection of plant species and adjustment of soil properties to be similar to the natural environment are essential. The interaction between plant and AM fungi can impose considerable effects on the physical stability of the plant-soil system, especially through enhancing plant biomass and root tensile strength.

Date of Award	2017
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology