Contribution of autotrophic and heterotrophic respiration to soil CO2 efflux in Chinese fir plantations

Soil respiration (Rs) is overwhelmingly the sum of autotrophic respiration (Ra, root and rhizosphere) and heterotrophic respiration (Rh, microbes and soil fauna). Separating Rs into Ra and Rh components is a major challenge but necessary for understanding the implications of environmental change on soil C cycling and sequestration. In this study, a trenching method was employed to partition Rs sources in Chinese fir plantations in Southern China. Soil CO2 efflux (FCO2) rates were measured using an infrared gas analyser system with soil chambers at the trenched and untrenched (Control) plots from January 2007 to December 2008. Soil temperature (Tsoil) and soil water content (Wsoil) were also measured at the plots during the study period. The results showed that the mean soil FCO2 rate from trenched plots (0.88 ± 0.12 µmol m–2 s–1, mean ± s.e.) was significantly lower than that from untrenched plots (1.22 ± 0.18 µmol m–2 s–1) (P < 0.001) during the study period. Compared with Ra, Rh made a major contribution to annual flux of Rs in Chinese fir forests. The relative proportion of Rh to Rs averaged 76 and 69% in 2007 and 2008, respectively. The seasonal changes of Ra to Rs ratio ranged from 13 to 56% with a mean of 33%. The annual mean Rs was 455 ± 249 gC m–2 year–1 in the study site for the study period, of which Rh and Ra were 330 ± 219 and 125 ± 65 gC m–2 year–1, respectively. Both Rs and Rh was strongly correlated with Tsoil at a 5-cm depth, while Ra had no relationship with Tsoil. Temporal variation in Wsoil had little effect on Rs and Rh. The results indicated that the fluxes of Ra and Rh were controlled by different factors and the microbial communities, compared with roots, were likely more sensitive to global warming in affecting soil C fluxes in Chinese fir ecosystems in subtropical regions.