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Soil labile organic carbon fractions are important in ensuring soil productivity and improving soil quality. Plantations offer an opportunity to sequester carbon (C), but fundamental information is needed on labile fraction dynamics in soils over long periods. Two replicate sites of three stand ages (10, 20 or 30 y) of citrus in the Three Gorges Reservoir area of China were studied. The results showed that soil organic C (SOC) concentration and storage, and readily oxidisable C (ROC) content increased in the surface soil with stand age, but dissolved organic C (DOC) and the proportion of the SOC present as ROC and DOC showed no such significant difference. The microbial biomass C (MBC) and water-soluble carbohydrate (WSCh) increased gradually, and tended to be highest in the 20-y-old stand; thereafter MBC decreased again and WSCh became constant. Basal soil respiration (BSR) measured over a 24-h period at 25°C was lower under mature stands (after 20 y) than under younger (10 y) stands. BSR and qCO 2 (expressed as BSR per unit microbial biomass) were negatively correlated with SOC fractions except DOC. The rate increase with age was highest in WSCh among the labile fractions measured and it was closely correlated with stand age, SOC and other labile organic C fractions; this suggested that WSCh could be the most sensitive fraction for detecting changes in organic C due to the long-term growth of citrus.

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