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 m easured in yellow soils (Ultisols) of the karst areas of southwest China. T he soils are relatively weathered, leached and impoverished, and have a low input of plant residues. The measurements were made for a 1-year period an d 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, re spectively. The cumulative CO2 efflux is 0.24 kg CO2-C m(-2) year(-1). Ther e is also a marked seasonal variation in the amount of soil microbial bioma ss carbon. but with the highest (644+/-71 mu g C g(-1) soil) and lowest (27 0+/-24 mu g C g(-1) soil) values occurring in the winter and summer, respec tively. 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. T he mean residence time of microbial biomass is estimated at 105 days, sugge sting that the carbon in soil microbial biomass may act as a source of the CO2 released from soils.