Variations of total organic carbon and particulate organic carbon in paddy soils contaminated by heavy metals.

Six soil profiles were sampled from a paddy field contaminated by heavy metals in the downstream area of Hengshihe River in Dabaoshan Mine in the north of Guangdong Province.Vertical distributions of total organic carbon(TOC), particulate organic carbon(POC), and the ratio of POC to TOC in the soil profiles were studied.Soil organic carbon(SOC) content and stability under different levels of heavy metal contamination were discussed.Compared to slightly and moderately contaminated sample plots, TOC decreased drastically in severely contaminated soil profiles, and the vertical distribution of TOC changed obviously due to severe heavy metal contamination.POC varied similarly as TOC among different soil layers.However, the variation of POC was more distinctive than that of TOC.Heavy metal contamination affected the vertical distribution characteristics and allocation proportion of different organic carbon fractions, thus had an effect on soil organic carbon stability and availability.POC increased distinctively in the moderately contaminated soils, leading to acceleration of SOC degradation and loss.TOC decreased distinctively in the severely contaminated soils, reduced fixation and accumulation of SOC.Different heavy metals potentially had different effects on stability of SOC, and there was an antagonism between Pb and Cu, Cd.Pb enhanced stability of SOC, while Cu and Cd had adverse effects.However, the associative effect of multiple heavy metals totally inclined to reduce stability of SOC.