Survival of two introduced plant growth promoting micro-organisms in green roof soil in southern Finland

Glomus intraradices and Bacillus amyloliquefaciens are two commercially used plant growth promoting micro-organisms. They associate with plant roots to facilitate host plants to absorb nutrients, induce resistance against pathogens and pests, and regulate growth through phytohormones. Growth conditions for plants on green roofs are often unfavorable. In order to test whether growth and development of green roof plants could be enhanced via improving the microbial interface, G. intraradices and B. amyloliquefaciens were inoculated on experimental plots on a green roof in the summer of 2012. The experimental plots were marked as R (inoculated with B. amyloliquefaciens from Rhizocell), M (inoculated with G. intraradices from MYC4000), and C (control). The green roof was made of sedum-herb-grass mats. The plants included e.g. stonecrops, bluegrasses, yellow rockets, white clover, mullein, pennycress, and moss. The survival and development of G. intraradices and B. amyloliquefaciens were studied respectively from Poa alpina roots and soils in the summers of 2012 and 2013. G. intraradices was not detected in P alpina roots according to root staining and microscopy. Probable reasons for the lacking of G. intraradices include high phosphorus content in the soils, high soil temperature, and low soil moisture. PCR and qPCR were used to detect Bacillus content in green roof soils. The abundance of B. amyloliquefaciens was related to soil water content and soil temperature. During the last two measurements in 2012, 4 weeks of high moisture content in the soil resulted in large increase of B. amyloliquefaciens content in both M and R groups, but then decreased substantially due to drought and heat in 2013. In 2013, Only R group increased from the third to the last measurement, indicating probable resistance of the B. amyloliquefaciens strain from Rhizocell additive. The synergistic effect of B. amyloliquefaciens and G. intraradices might be responsible for the thousand-fold increase of Bacillus content in M group in 2012.