Soil-released carbon dioxide from microbial biomass carbon in the cultivated soils of karst areas of southwest China

Soil microbial biomass and the emission of CO2 from the soil surface were measured in yellow soils (Ultisols) of the karst areas of southwest China. The soils are relatively weathered, leached and impoverished, and have a low input of plant residues. The measurements were made for a 1-year period and show a reciprocal relationship between microbial biomass and surface CO2 efflux. The highest (42.6+/-2.8 mg CO2-C m(-2) h(-1)) and lowest (15.6+/-0.6 mg CO2-C m(-2) h(-1)) CO2 effluxes are found in the summer and winter, respectively. The cumulative CO2 efflux is 0.24 kg CO2-C m(-2) year(-1). There is also a marked seasonal variation in the amount of soil microbial biomass carbon. but with the highest (644+/-71 mu g C g(-1) soil) and lowest (270+/-24 mu g C g(-1) soil) values occurring in the winter and summer, respectively. The cumulative loss of soil microbial biomass carbon in the top 10 cm of the soil was 608 mu g C g(-1) year(-1) soil over 17 sampling times. The mean residence time of microbial biomass is estimated at 105 days, suggesting that the carbon in soil microbial biomass may act as a source of the CO2 released from soils.