Abstract: 【Objective 】The objective of this study was to clarify the effect of long-term fertilization on soil organic carbon (SOC) changes and carbon fraction differences, and to reveal C sequestration and soil fertility improvement model of fertilization, in Cumulic Haplustoll of Loess Plateau in China.【Method】Grain yields and SOC recorded from a 32-year dryland fertilization trial with six fertilizations were used to estimate C input and sequestration and SOC trends, and soil samples in the 32nd year taken from the experiments were separated into sand, coarse silt, fine silt, coarse clay, fine clay, and aggregates by physical way, and the content and distribution of SOC in these particle-size fractions and aggregates were analyzed. 【Result】Cumulative C input ranged from 8.10 t C?hm-2 for the unfertilized treatment (CK) to 69.40 t C?hm-2 for the treatment that received straw (S) plus inorganic N added annually and P fertilizers added every second year (SNP), and there was a difference of 1.82 times in SOC stock in the 0–20 cm soil layer among the fertilizer treatments and SOC stock was 21.76, 24.04, 16.01 t C?hm-2 in the M, MNP, SNP treatments in 2010. SOC stock increased with cumulative C input, C sequestration into SOC rate ranged from 25.80% to 36.05% but 8.20% from straw and root in the SNP, suggesting manure and root residues is more effective in building soil C than straw, SOC significantly built up with time except the CK, in which SOC remained almost stable by root resides C in input only, and annual soil-C sequestration rates (slopes for linear regression of SOC relate to time) were 0.246, 0.326, and 0.361 t C?hm-2?a-1 for the SNP, M, and MNP treatments respectively. SOC in the sand fraction for the MNP and SNP were 3.85 and 2.94 times higher than that for CK, and 2.41 and 1.84 times higher than that for the N. Compared with the CK and NP, SOC in total soil for the MNP increased by 32.50% and 18.10%, but SOC in the sand increased by 285.12% and 105.74%, implying SOC in the sand was the most sensitive fraction to the fertilization among the particle-size organic carbon fractions. However, fertilization increased the ratio of POC (particulate organic carbon) to MOC (mineral-incorporated organic carbon), and the ratio was up to 18.62%, 16.24%, 14.41% for the MNP, M, SNP，9.11% and 9.99% for the N and NP, resulting in improvement of soil active organic carbon and soil fertility. For the CK, SOC in macroaggregates (＞250 µm) was 9.14 times higher than that in microaggregates (＜53 µm), and fertilization increased the SOC by 15.83-23.84 times, and enhanced C/N ratio in macroaggregates, but had less effect on the ratio in microaggregates. SOC in aggregtaes increased with aggregate size, but the effect of organic C increrase in the ＞250 µm aggregates on soil C sequestration rate was higher significanly than that in the ＜53 µm aggregates, implying microaggregate plays an important role in soil organic C sequestration and physical protection. 【Conclusion】Long-term additions of organic materials and inorganic fertilizers, a friendly-environment soil fertility enhancement way for these dryland soils in the China Loess Plateau, significantly promoted C sequestration and sharply resulted in a substantial build-up of SOC due to C inputs from manure and straw and root residues, and the newly organic C was mainly accumulated in the sand and aggregates.

Key words: ong-term fertilizer , soil organic carbon , changs , fractions , Loess Plateau , Cumulic Haplustoll