The importance of soil organic carbon(SOC) under forests in the global carbon cycle depends on the stability of the soil carbon and its availability to soil microbial biomass.We investigated the effects of successive rotations of Chinese fir(Cunninghamia lanceolata(Lamb.) Hook) plantations on the stability of SOC and its availability to microbes by adopting the two-step hydrolysis with H2SO4 and density fractionation.The results showed that successive rotations of Chinese fir decreased the quantity of total SOC,recalcitrant fraction,and carbohydrates in Labile Pool I(LP Ⅰ),and microbial properties evidently,especially at 0-10 cm horizon.However,cellulose included in Labile Pool II(LP Ⅱ) and the cellulose/total carbohydrates ratio increased in successive rotations of Chinese fir.The non-cellulose of carbohydrates included in LP I maybe highly available to soil microbial biomass.Hence the availability of SOC to microbial biomass declined over the successive rotations.Although there was no significant change in recalcitrance of SOC over the successive rotations of Chinese fir,the percentage of heavy fraction to total SOC increased,suggesting that the degree of physical protection for SOC increased and SOC became more stable over the successive rotations.The degradation of SOC quality in successive rotation soils may be attributed to worse environmental conditions resulted from disturbance that related to "slash and burn" site preparation.Being highly correlated with soil microbial properties,the cellulose/total carbohydrates ratio as an effective indicator of changes in availability of SOC to microbial biomass brought by management practices in forest soils.
