Soil carbon sequestration efficiency of different particle-size fractions after long-term fertilization

【Objectives】 Our objective was to explore effects of various long-term fertilization practices on soil organic carbon distribution ratios and quantitate the relationship between organic carbon sequestered and additional organic carbon input in different particle-size fractions. 【Methods】 The designed treatments were selected to obtain a gradient of additional carbon input, including no fertilizer control (CK), chemical nitrogen, phosphorus and potassium fertilizers (NPK), straw return with chemical fertilizers (NPKS), chemical fertilizers combined with manure (NPKM), chemical fertilizer combined with manure under soybean rotation (NPKMR), manure (M) alone, and high application rate of NPKM (1.5NPKM). Soil samples were collected after twenty-year fertilization practices, and separated into sand (53-2000 m), coarse silt (5-53 m), fine silt (2-5 m) and clay (2 m) by the physical fractionation. 【Results】 Compared with CK, the total soil organic carbon (SOC) stocks and those of the particle-size fractions are significantly increased under the fertilization treatments. The long-term application of manure with or without chemical fertilizers is the most effective practice for increasing soil organic carbon. The SOC stock in the clay fraction (16.26 t/hm2) is the highest among the four particle-size fractions after the 20 years fertilization treatments. The treatments with manure and straw incorporation significantly increase the SOC distribution proportion in the sand size fraction (13.33%-25.36%), while the treatments reduce the proportions in the clay-size fraction, and there are no significant changes in both the coarse and fine silt fractions. However, the application of chemical fertilizer has less effects on the organic carbon distribution ratios of the fractions. The concentrations of SOC in the coarse silt, fine silt, and clay size fractions are significantly correlated with the mass proportion of sand size fraction and the mass ratio of sand to coarse silt, fine silt, and clay size fractions, respectively. The treatments with manure show higher carbon sequestration rates in the clay0.29-0.52 t/(hm2a), sand fractions0.30-0.40 t/(hm2a). Meanwhile, the coarse and fine silt fractions have low and equal sequestration rates0.09-0.16 t/(hm2a)after the 20 years fertilizations. Significantly positive linear correlations are observed between the SOC sequestered in total and all fractions with the gross accumulated organic carbon input during the 20 years. The sequestration efficiency of total organic carbon is 10.57%. The sequestration efficiencies in the clay (4.25%) and sand fractions (3.60%) are about double of those in the coarse silt (1.73%) and fine silt fractions (1.00%). 【Conclusions】 Our results indicate that in red soil, SOC accumulation mainly occurs in the clay and sand size fractions. Accumulation of SOC in the clay-size fraction benefits the sand-size fractions (aggregates) formation. Application of manure is not only an important practice for soil carbon sequestration, but also an optimal management to improve physical structure of red soil.