Characterization of soil organic carbon fractions at Spartina alterniflora saltmarsh in North Jiangsu Effects of straw addition on decomposition, transformation and composition of soil organic carbon pool Effects of different conservation tillage measures on soil organic carbon pool in two sequence rotation systems of spring wheat and pease Soil organic carbon pool and its spatial distribution pattern in the Circum Bohai Region The size and characterization of biologically active organic carbon pool in soils