Actual and potential carbon sequestration in topsoils of eastern China

Evaluating the current storage of soil organic C (SOC) and its future sequestration potential helps to develop goal-oriented strategies to enlarge the soil C pool. Here, we investigated the contents of particulate and mineral-associated organic C (POC and MAOC) and assessed the C sequestration potentials as stable mineral-associated fractions in 240 triplicated topsoils collected from adjacent woodlands, uplands, and paddies across four climate zones in eastern China. SOC content was generally lower in warmer climates (subtropics and tropics) than in cooler ones (mid- and warm temperate), with a generally descending order of paddies > woodlands > upland croplands in each climate zone. The SOC contents of all soils were dominated by MAOC, whereas with a lesser proportion of MAOC in warmer climates. Compared with woodlands, cropland use reshaped SOC compositions: paddies had larger POC and MAOC pools but with reduced SOC stability, whereas upland croplands had a lower total SOC content yet with higher stability. The average C/N ratios of particulate organic matter were comparable among the three ecosystems and four climates (15 ± 3.4), while those of mineral-associated organic matter were lower in subtropics (10 ± 2.0) than other climatic zones (13 ± 1.7). These differences in the C/N ratios of mineral-associated fractions are explained by the fast organic matter turnover, intensive N fertilization and strong adsorption of N containing organic compounds by Fe/Al oxides in subtropics. Regardless of climate and land-use, MAOC accumulation in all ecosystems was not saturated as assessed by the 95^thquantile regression between the mass proportion of the clay + silt fraction and the current MAOC content. Although the average MAOC saturation degrees exceeded 60%, its wide variation within and between individual soils suggested a further substantial C sequestration potential, particularly in the mid-temperate. These findings underscore that SOC management strategies must be tailored to local land-use and climate conditions to raise stable SOC pools.