| In this study, a 27-year long-term fertilization field experiment was chosen to research soil aggregates, which was at the Gaoping Agronomy Farm, Pingliang, Gansu, China. Native sod (NS) and cultivated soils which were unfertilized control (CK), nitrogen fertilizer (N), farmyard manure (M), nitrogen and phosphorus (NP), manure plus N and P fertilizers (MNP), straw plus N and P fertilizers (SNP), were studied in Calcic Kastanozems of the dryland, Loess Plateau. The article tested the contents of air dry aggregates (ADA) and water-stable aggregates (WSA), soil organic carbon (SOC), soil dry bulk density (Db), researched effects of tillage and long-term fertilization on soil aggregates in Calcic Kastanozems, analyzed the size distribution and stability of soil aggregates, the correlation between soil aggregates and SOC contents, Db, total soil porosity (St), aimed at offering basis of theory for establishing effectual methods and key measures to develop soil fertility and productivity and maintain soil quality stability. The main results were obtained as follows:1. The contents of > 0.25 mm air dry aggregates increased by 22.21% and 22.73% in 0～10 and 10～20 cm soil layers after cultivation, respectively; and that of > 0.25 mm water-stable aggregates decreased by 21.35% and 38.82% in the two depths, individually, especially the > 5 mm and 5～2 mm sizes WSA content declined the most. Long-term fertilization (LTF) improved the amount of > 0.25 mm air dry aggregates in 0～10 cm layer, however, slightly influenced at 10～20 cm depth. There was no remarkable regularity on the composition of ADA, but applications of manure, or manure with chemical fertilizer greatly developed the content of big aggregates; the main effects on WSA were increased > 5 mm and 5～2 mm sizes WSA content. The > 2 mm and > 1 mm sizes WSA content of M, MNP and SNP treatments remarkably more than no fertilization control whereas that of adding N and NP was slightly lower than CK in both soil layers. It is concluded that organic matters enhance the formation of bigger soil aggregates but chemical fertilizers were disadvantages to form soil aggregates. Long-term applications of organic matters or it with chemical fertilizers changed effectively the composition of soil aggregates, increased significantly the big sizes WSA content and improved greatly the soil structure. Adding manure, organic inorganic fertilizers assorted use and straw return back to fields were good method to improve soil fertility.2. Tillage and LTF strongly influenced soil aggregate mean weight diameter (MWD), destruction rate (Dr) and fractal dimension (D). MWD of ADA and Dr were significantly higher but MWD of WSA was remarkably lower in cultivated soils compared to native sod in 0～10 and 10～20 cm soil layers. MWD of ADA and WSA greatly changed by LTF at both depths. MWD of ADA was positively correlated with the amount of > 5 mm, > 1 mm, > 0.5 mm, > 0.25 mm ADA and MWD of WSA was also positively correlated with the amount of > 2 mm, > 1 mm, > 0.5 mm, > 0.25 mm WSA, and the bigger the aggregates sizes, the more the correlation between them. Contemporarily, the LTF treatments MWD of WSA was bigger than that of no fertilization, and that of farmyard manure and straw with fertilizer were more than that of chemical fertilizer applied in this study. Dr was negatively correlated with the amount of > 0.25 mm WSA and SOC content, respectively. Wether in dry-sieve or wet-sieve method, it indicated that the higher the amount of soil aggregates, the lower the fractal dimension of the aggregates size distribution. After wet-sieving, fractal dimension (D) in cultivated soils was more than in native sod and it was bigger in topsoil than in 10～20 cm depth. D was negatively correlated with the amount of > 0.25 mm WSA, MWD of WSA, total soil porosity (St), respectively, but it was positively correlated with Db. The results also showed that the more the amount of > 0.25 mm WSA and the greater MWD, the lower the D. It was greatly improved the soil structure by increasing stability of soil aggregates.3. The amount of SOC decreased by 63.17% in the 0～10 cm layer, but increased by 6.17% in the 10～20 cm depth after cultivation. The results also indicated that fertilization had remarkable effects on SOC. It declined by using N fertilizers, and improved by adding M, NP, MNP and SNP. The increased order was M > MNP > SNP > NP. The contets of SOC was negatively correlated with the amount of > 0.25 mm ADA, and was no correlation with the amount of > 0.25 mm WSA, but was positively correlated with the amount of > 2 mm, > 1 mm > 0.5 mm WSA, and the bigger the aggregates size, the more the correlation between them. 4. Db and St were almost not altered when native sod was changed into cultivated soil in 0～10 cm layer. Otherwise, Db in cultivated soils indecreased by 19.24% and St decreased by 12.95% in 10～20 cm depth after cultivation. The study also showed that Db improved and St declined by using NP fertilizers and SNP in 0～10 cm layer, and Db increased and St decreased by adding N, M, and MNP. LTF strongly influenced Db and St in 10～20 cm depth relative to 0～10 cm layer, and the order of Db decreased and St increased was M > SNP > NP > MNP > N. Db was negatively correlated with the amount of > 0.25 mm WSA, and St was positively with it. 5. The aggregate size distribution of surface soil was influenced greatly by tillage and LTF. Higher stratification ratio of MWD, SOC and Db generally meaned that less soil disturbance. In all, the ratio of MWD, SOC and Db in 0～10 cm layer was higher than in 10～20 cm depth, but St were opposite to others. Ratio of Db and St in native sod were about 1.0, however, Ratio of Db in cultivated soils were 0.8～0.98.
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