Biologically active or labile fractions of soil organic carbon are vital in understanding decomposition potential of organic materials, nutrient cycling dynamics, and biophysical manipulation of soil structure. Proportions of dissolved organic carbon, microbial biomass carbon and mineralized carbon in total organic carbon are served as sensitive indices of soil organic carbon availability. However, most relevant work over the past decades has focused on forests and grasslands, with little emphasis on quantifying active carbon pool fluxes in human-dominated agricultural systems, especially on heavy metal contaminated subtropical rice field soils. We investigated different organic carbon fractions in paddy soils under two heavy metal contamination levels in Guangdong Province, South China, in order to compare and assess the availability of soil organic carbon substrates under long-term heavy metal stress. Our results showed that the contents of total organic carbon (TOC)，0.5mol•L-1 K_２ＳＯ_４ extractable organic carbon (K_２ＳＯ_４-C), microbial biomass carbon (MBC), mineralized organic carbon during １～7 d (CO_２-C_１～7d) and 8～28 d (CO_２-C8～28d) of laboratory incubation were consistently lower in highly contaminated paddy soils than those in slightly contaminated soils (p<0.001 in all cases). In contrast, availability indices of organic carbon presented different trends. Compared with slightly contaminated soil, highly contaminated soil were revealed greater K_２ＳＯ_４-C/TOC ratio and microbial metabolic quetient (CO２-C/MBC), whereas, lower MBC/TOC and mineralization rate of organic carbon during 8～28 d incubation (CO_２-C_8～28d•TOC-1 d-1). Differences in mineralization rate of organic carbon during １～7 d incubation (CO_２-C_１～7d•TOC^-1 d-1) of two soils were not remarked. Mineralization rate of organic carbon during initial 7 d of incubation was pronouncely higher than that during subsequent 21 d of incubation. Mutiple stepwise regression showed that CO２-C１～7d was significantly correlated with K_２ＳＯ_４-C and MBC (r2=0.83，p<0.001), while CO_２-C8～28d was remarkedly associated with TOC alone (r²=0.70，p<0.001). Our study indicates that K_２ＳＯ_４-C is readily decomposed active C pool.
However, long-term heavy metal contamination decreased micriobial population size and respiratory activity, inhibited microbial mineralizaiton efficiency of labile organic carbon. The accumulation of K_２ＳＯ_４-C could be attributed to the decreased mineralization rate because more labile organic carbons in paddy soils may directly derived from exudate of intensive fine rice roots rather than decompositon of raw litters. Moreover, elevated K_２ＳＯ_４-C in soil can enhance the solubility, mobility and biological uptake of metals, and thereby increase the biological availiablity of heavy metals. It may further restrained mineralization rate of active carbon pool.
On basis of the suppressed turnover process, relatively greater proportion of K_２ＳＯ_４ extractable organic carbon in highly contaminated paddy soil is not doomed to high availability of active carbon substrate supply. Thus, in order to assess the availablity of organic carbon of paddy soils polluted by heavy metals, the combined functioning of active organc carbon pools, mineralization intensity of microbes and supply potential of carbon substrates during different turnover stages should be taken into consideration.