Effects of arbuscular mycorrhizal fungi on inoculated seedling growth and rhizosphere soil aggregates

Arbuscular mycorrhizal fungi (AMF) inoculation have been shown to enhance host plant growth and rhizosphere soil aggregates largely due to the combined effect of extraradical hyphae and glomalin-related soil proteins but their effects on rocky desertified soils remain unknown. Extensive karst area of southwest China has undergone rocky desertification because of soil erosion and vegetation degradation. Karst soils in this area are mainly composed of silty clay loam developed from carbonate rock erosion under subtropical humid monsoon climate. We tested the hypothesis that AMF colonization of tree seedlings would improve water stable aggregates of degraded karst soil and enhance seedling growth. Seedlings of two tree species, Toona sinensis and Delavaya toxocarpa suitable for afforestation in karst area, were inoculated separately with three AMF, Funneliformis mosseae, Glomus versiforme and Rhizophagus intraradices in pots containing rocky desertified soil. Non-inoculated seedlings were used as control. After 12 months we found that the percentage of water stable aggregates (diameter > 0.25 mm; WSA (> 0.25)) in the rhizosphere soil of the inoculated seedlings was increased by 15.6-20.5% compared to control. AMF colonization significantly increased the quantity of aggregates with diameter > 2 mm compared to control but soil aggregates of size class 0.25 - 0.5 mm decreased significantly in inoculated seedlings, regardless of tree species. Shoot biomass of inoculated T. sinensis and D. toxocarpa seedling was increased by 15.7-21.2% and 8.5-20.4%, respectively compared to non-inoculated seedlings. In contrast, AMF inoculation minimally influenced the root biomass of the two tree species. The easily extractable glomalin (EEG) content and mean weight diameter (MWD) of rhizospheric soil of the inoculated seedlings increased by 28.9-75.1% and 31.7-48.1%, respectively compared to control. The AMF colonization was highly correlated with WSA (> 0.25), EEG and MWD. Overall results support our hypothesis that AMF colonization enhance the quantity of rhizospheric soil aggregates and improve the growth of host seedlings. We conclude that planting tree species inoculated with AMF has a potential for restoration of degraded karst ecosystem. In future research other locally adapted tree and shrub species should be tested following AMF inoculation to prepare a wider planting stocks to be used in restoration of degraded karsts.