Belowground carbon allocation by trees, understory vegetation and soil type alter microbial community composition and nutrient cycling in tropical Eucalyptus plantations

We studied the influence of plant functional groups on soil microbial community composition and nutrient cycling in a tropical Eucalyptus forest ecosystem with different plantation age and soil types by means of stem girdling (SG) and understory removal (UR). Fungal and bacterial communities were characterized using phospholipid fatty acids, and ectomycorrhizal fungi (EMF) colonisation was estimated visually. Total bacterial and saprotrophic fungal biomass was highest in soils treated with SG, followed by UR, through modification of plant belowground C allocation and N supply. EMF root colonisation, biomass of EMF, arbuscular mycorrhizal fungi (AMF) and the fungal-to-bacterial ratio were lowest in these soils. We found that EMF, AMF and the fungal-to-bacterial ratio were highest in sandy loam soils with a high C/N ratio and low pH. The fungal-to-bacterial ratio was higher in 5-year-old than in 15-year-old plantation. We propose that girdling of trees and removal of understory plants are important ecological components, due to their modification of plant belowground C allocation and N supply as key determinants of microbial community composition. Our results highlight the fact that soil abiotic factors play an important role in shaping the microbial community and nutrient cycling in tropical forest ecosystems.