Seasonal dynamics of soil carbon dioxide efflux and simulated rhizosphere respiration in a beech forest

Respiration of the rhizosphere in a beech (Fagus sylvatica L.) forest was c alculated by subtracting microbial respiration associated with organic matt er decomposition from daily mean soil CO2 efflux. We used a semi-mechanisti c soil organic matter model to simulate microbial respiration, which was va lidated against "no roots" data from trenched subplots. Rhizosphere respira tion exhibited pronounced seasonal variation from 0.2 g C m(-2) day(-1) in January to 2.3 g C m(-2) day(-1) in July. Rhizosphere respiration accounted for 30 to 60% of total soil CO2 efflux, with an annual mean of 52%, The hi gh Q(10) (3.9) for in situ rhizosphere respiration was ascribed to the conf ounding effects of temperature and changes in root biomass and root and sho ot activities. When data were normalized to the same soil temperature based on a physiologically relevant Q(10) value of 2.2, the lowest values of tem perature-normalized rhizosphere respiration were observed from January to M arch, whereas the highest value was observed in early July when fine root g rowth is thought to be maximal.