Response of soil organic carbon and its particle-size fractions to different long-term fertilizations in red soil of China

Through collecting the red soil under 24 years different fertilization, changes in total organic carbon (SOC) and its fractions in red soil were studied by physical partial size fractionation method. The effects of various fertilizations on soil organic carbon in total soil and its particle-size fractions were analysed based on historical recorded data. Results showed that manure combined with mineral fertilization(NPKM) had the highest SOC content (10.33g/kg) compared with other treatments. Also, organic carbon concentration in sand (2000～53m), fine silt (5～2m), and clay (2m) size fractions under the NPKM treatment was the highest of all treatments. Compared with the CK treatment, application of mineral NPK fertilizations (NPK, 2NPK) and the organic fertilizations (M,NPKM) significantly increased the distribution proportion of organic carbon in sand and clay size fractions, whereas significantly reduced the distribution proportion in coarse-silt and fine-silt size fractions. Averaged soil carbon sequestration rate under the treatments of mineral fertilizaters (NPK,2NPK), M and NPKM were 0.05 t/(ha?a), 0.18 t/(ha?a), 0.26 t/(ha/a), respectively. There was a strong positive linear correlation between carbon sequestered and carbon input (R2=0.909, P0.01) at this site, indicating that the conversion rate of carbon input to SOC was 8.1%. With the increase in carbon inputs , the carbon stocks in coarse silt and fine silt fractions decreased gradually. However, carbon stock in sand and clay increased with higher increment in clay than that in sand. We concluded that organic carbon was mainly sequestrated in clay fraction, which was the key component for soil carbon sequestration. Soil organic carbon did not reach the saturation in red soil and have the potential to sequester more carbon.