| In order to ascertain the characteristics of organic carbon pools in cropland of China under long-term fertilization, Red soil,Medial black soil, Thick-bedded black soil, Grey desert soil, Black loessial soil and Fluvo-aquie soil were collected from different cropland of climatic region. The particle-sized organic carbon fractions, aggregate organic carbon fractions and oxidative labile organic carbon were separated by supersonic and centrifugal way, wet sieve and density flotation, oxidation of KMnO4 respectively. The content, distribution, and temporal and spatial variations of these soil carbon pools under long-term fertilization were analyzed. Then the correlation between different soil organic carbon pools and soil fertility was determined. The main results and conclusion are given as follows:(1) Compared with no fertilization, there were significant increases in soil organic carbon pools under application of organic fertilizer (M, 2M, NPM, NPKM and 1.5NPKM). The S-OC and cfPOC fractions show the highest increases with 36.4%～397.5% and 138.7%～538.4%, respectively, indicating that they were the more sensitive organic pools to agricultural practice. The amplitudes of the two pools in different soil were in an order of Grey desert soil> Red soil >Fluvo-aquie soil>Medial black soil>Black loessial soil >Thick-bedded black soil. Application of straw (NPKS) caused decreases of ffPOC in Red soil (by 9.6%) and S-OC in Grey desert soil (by 34.3%) while no cultivation (CK0) resulted in remarkable decrease of ffPOC in red soil (58.8%). However, the other carbon pools were increased in both CK0 and NPKS. As a result, the two treatments show increases of organic carbon pools as whole. Application of inorganic fertilizer (N, NP, NPK) show the lowest amplitudes, suggesting that applying inorganic fertilizers only has little effects on increase of soil organic carbon pools.(2)The soil organic carbon was dominated by mineral associated organic carbon (MOC), counting 56.9%～84.7% of TOC, so it was regarded as the main pools for sequestrating carbon in soil. Because the CC-OC pool counting 44.2%～58.9% of MOC, so it was the biggest organic carbon pool in different sized silt and clay carbon pools that are associated with mineral. The mineral associated with organic carbon was significantly decreased (10.0%～19.6%) due to fertilization, especially by applying organic fertilizer. The active organic carbon pools separated by physical fractionation include cfPOC, ffPOC and iPOC in soil aggregate. These carbon pools count 18.8%～31.8% of TOC, which was close to that of labile organic carbon from oxidation of KMnO4 (19.8%～33.1% of TOC). The active carbon pools in TOC were obviously increased by applying various fertilizers, especially organic fertilizer that resulted in increase of 44.0%～63.4%.(3)During 17 years of application of organic fertilizer, organic carbon pools in Grey desert soil and Medial black soil were significantly increased with time. But the rate of annual change was obviously different among the organic carbon pools. The annual change of S-OC was the highest with 0.24 g·kg-1·a-1, and FC-OC was the lowest with 0.02 g·kg-1·a-1. The treatments of straw back to cropland (NPKS) and no cultivate (CK0) showed the same level or increase of organic carbon pools. For the Medial black soil, the S-OC,CS-OC,CC-OC and ffPOC were increased by 0.08,0.12,0.07 and 0.03 g·kg-1·a-1 respectively. However, applying fertilizer did not show significant effect on different particle-sized organic carbon fractions in Grey desert soil, except for cfPOC and LOC that were increased by 0.07and 0.05 g·kg-1·a . Applying inorganic fertilizer (N, NP, NPK) was not effective to increase organic carbon pools, only S-OC, CS-OC and CC-OC was increased obviously in medial black soil by 0.05, 0.10 and 0.06 g·kg-1·a-1 respectively. However, in Grey desert soil, the CS-OC, FS-OC and FC-OC were decreased by 0.01, 0.03 and 0.01g·kg-1·a-1, respectively, after application of inorganic fertilizer (NPK). Applying nitrogen only caused decreases of the iPOC (0.01 g·kg-1·a-1) and MOC (0.02 g·kg-1·a-1).(4)Relative rate of S-OC,CS-OC and MOC in six kinds of cropland was increased with increase of mean annual temperature whereas the rate of FS-OC was increased with increase of annual precipitation. The distribution percentage of fPOC (cfPOC+ffPOC) in TOC and the increased extent with organic fertilizer of LOC were enhanced with increase of annual precipitation and mean annual temperature. Moreover, the correlations(r>0.888**,n=6)between change of organic carbon pools and temperature and precipitation were significant. These results indicate that higher temperature and humidity was effective to accumulate and transform organic carbon, and lower temperature and humidity was effective to maintain. But the changes of CC-OC and FC-OC were more related with their contents in soil.(5)Within particle-sized organic carbon fractions, different aggregate organic carbon fractions, oxidative labile organic carbon and with TOC could increased by each other, and the relationship between organic carbon fractions from different way also was significantly correlated(r>0.412**,n=46). The ratio between active and inert organic carbon pool was changed after application of fertilization, especially with organic fertilizer, the ratio was increased from 35.0% to 200.0%. The amplitude of the ratio in different soil under applying organic fertilizer was in an order of Grey desert soil> Red soil >Fluvo-aquie soil>Medial black soil>Black loessial soil >Thick-bedded black soil. Active organic carbon pool had significant correlation with content of total N, total P, available N, available P and available K (r>0.372*,n=32), indicating that active organic carbon pools have close relationship with soil fertility. The analysis of stepwise regression shows that the S-OC is the optimal organic carbon pool that can indicate the change of soil organic carbon and fertility under agricultural practices, because it have the closest relationship with soil fertility and more sensitive to agricultural practices.
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