Rhizodeposition and its impact on microbial community structure and function in trees

Rhizodeposition is a key process influencing nutrient availability through its stimulation of microbial activity in the rhizosphere. Microbial communi ty structure and function in the rhizosphere has been hypothesised to be a consequence of variation in the quantity and quality of rhizodeposition. Th is paper discusses the character of carbon loss from trees and describes th e use of community level physiological profiles (CLPP) to characterise micr obial communities from the rhizosphere and rhizoplane of hybrid larch (Lari x eurolepis), Sitka spruce (Picea sitchensis), sycamore (Acer pseudoplatanu s) and non- rhizosphere (bulk) samples from a farm woodland site. Canonical variate analysis (CVA) was used to analyse the carbon utilisation data and discriminate treatment effects. CVA discriminated the rhizosphere and rhiz oplane microbial communities from the bulk soil. The carbon profiles of the rhizosphere microbial communities from the three tree species were similar , but there was significant discrimination of the communities from the thre e tree rhizoplanes. These differences were mainly attributable to differenc es in carboxylic acid utilisation. Isolation and enumeration of culturable microorganisms from these soils confirmed the stimulatory effect of the rhi zosphere on microbial growth, and in particular on pseudomonad proliferatio n, and that there are qualitative and quantitative differences in microbial communities associated with these tree species. The possible relationship between microbial diversity and carbon availability in the rhizosphere of d ifferent tree species is discussed.