Conversion of a natural evergreen broadleaved forest into coniferous plantations in a subtropical area: effects on composition of soil microbial communities and soil respiration

In this study, we examined how the conversion of native broadleaved forests into plantations of more productive forest species for timber use affects soil microbial community composition at 0–10-cm depth and soil respiration by comparing two 36-year-old plantation forests of Chinese fir (Cunninghamia lanceolata, CF) and Pinus massoniana (PM) with an adjacent relict natural forest of Castanopsis carlesii (NF, ~200 years old) in Sanming, Fujian, China. The soil microbial community composition was determined by phospholipid fatty acid (PLFA) analysis. The monthly in situ soil respiration rate was measured from October 2010 to September 2012. Results showed that the abundance of Gram-negative bacterial PLFAs, actinomycetal PLFAs, and total PLFAs did not vary significantly with forest conversion. The CF soil was characterized by higher abundance of fungal PLFA and lower abundance of Gram-positive bacterial PLFA compared with NF and PM soils. Redundancy analysis (RDA) indicated that the significant change in the composition of soil microbial community was mainly due to fine root biomass and soil pH. Annual soil respiration rate averaged 161.7 mg C m−2 h−1 in NF, 95.1 mg C m−2 h−1 in CF, and 103.2 mg C m−2 h−1 in PM. The NF showed significantly higher mean annual soil CO2 flux (1421 g C m−2 year−1) than CF (837 g C m−2 year−1) and PM (907 g C m−2 year−1). After forest conversion, the apparent temperature sensitivity of soil respiration (Q10) increased from 1.75 in NF to 2.04 and 1.98 in CF and PM, respectively. The mean annual soil respiration was significantly correlated with soil organic C (SOC) content and abundances of microbial PLFAs except for abundance of fungal PLFA, but not significantly correlated with fine root biomass (<2 mm in diameter) across the different forest soils. The latter behavior may be due to the higher fine root biomass in the CF than in the NF and PM. Our results suggest that studies incorporating the microbial community composition with soil respiration may enhance our understanding of the mechanisms of soil C dynamics.