Tree girdling effect on bacterial substrate utilization pattern depending on stand age and soil microclimate in Eucalyptus plantations

Abstract

It has been increasingly recognized that soil microorganisms and plants are tightly linked, however, the effects of belowground plant-carbon allocation on soil microbial communities has not been fully understood, particularly in subtropical and tropical forest ecosystems. In the present study, we conducted a tree-girdling experiment to explore the relationships between plant belowground carbon allocation and the functional diversity of soil microbial communities in two subtropical Eucalyptus plantations (2-year-old and 24-year-old). The functional diversity of soil culturable bacterial communities was characterized with community level physiological profile (CLPP) 1 year after tree girdling. We found that carbon substrate utilization patterns of soil culturable bacterial communities were significantly affected by both tree girdling and sampling season; but microbial metabolic activity and functional diversity were only affected by sampling season. Microbial metabolic activity and substrate utilization pattern, as well as soil moisture and temperature, did not show any apparent variations among stand ages. Pearson's correlation analysis indicated that both microbial metabolic activity of all 95 carbon sources (r=0.869; P<0.01) and the specific substrate metabolic activities (amides and amines: r=0.838, P<0.01; polymers: r=0.884, P<0.01; miscellaneous: r=0.895, P<0.01; carboxylic acids: r=0.910, P<0.01; amino acids: r=0.806, P<0.01; carbohydrates: r=0.808, P<0.01) were positively correlated with soil temperature. Metabolic diversity (r=0.806, P<0.01) and metabolic evenness (r=0.819, P<0.01) was positively correlated with soil temperature but metabolic richness (r=−0.517, P<0.05) was negatively correlated with soil moisture. Metabolic evenness was positively correlated with soil temperature (r=0.416, P<0.05). Seasonal changes in soil bacterial community were greater than those induced by girdling and stand age, and were tightly linked to seasonal soil microclimatic differences, such as soil moisture and temperature. The girdling treatment exerted a greater effect on soil bacterial communities during the dry season than in the rainy season. Our study illustrated that the response of bacterial substrate utilization profiles to changes in plant belowground carbon supply is seasonally dependent in subtropical Eucalyptus plantations.