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

Soil respiration (R-s) is overwhelmingly the sum of autotrophic respiration (R-a, root and rhizosphere) and heterotrophic respiration (R-h, microbes and soil fauna). Separating R-s into R-a and R-h 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 R-s 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 (T-soil) and soil water content (W-soil) 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 mu mol m(-2) s(-1), mean +/- s.e.) was significantly lower than that from untrenched plots (1.22 +/- 0.18 mu mol m(-2) s(-1)) (P < 0.001) during the study period. Compared with R-a, R-h made a major contribution to annual flux ofRs in Chinese fir forests. The relative proportion of R-h to R-s averaged 76 and 69% in 2007 and 2008, respectively. The seasonal changes of R-a to R-s ratio ranged from 13 to 56% with a mean of 33%. The annual mean R-s was 455 +/- 249 gCm(-2) year(-1) in the study site for the study period, of which R-h and R-a were 330 +/- 219 and 125 +/- 65 gCm(-2) year(-1), respectively. Both R-s and R-h was strongly correlated with T-soil at a 5-cm depth, while R-a had no relationship with T-soil. Temporal variation in W-soil had little effect on R-s and R-h. The results indicated that the fluxes of R-a and R-h 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.