Abstract: 【Objective】The objective of the experiment is to study the effect of straw incorporation on soil humus component content in the continuous cropping cotton field.【Method】This experiment was conducted in the long-term continuous cropping plots from the experiment station of Shihezi University. There were ten treatments in this continuous cropping plots, six treatments’ cotton straw were returned into the soil, and the other treatments’ were removed. There were three replicates per treatment, initial soil background values for each plot were similar. The six treatments which had the cotton straw included six continuous cropping plots, such as 5, 10, 15, 20, 25 and 30 years. The other four treatments were 1, 5, 10 and 15 years continuous cropping plots (they were marked CK1, CK5, CK10 and CK15, respectively). Cotton variety ‘Xinluzao 46’ in was grown cotton field, and the way of planting adopted was the ‘30+60+30’ configuration, and drip irrigation under mulch was used. The planting density was 198 000 plants per hectare. During the growth period, 11 times of drip irrigation were conducted, the total drip irrigation norm was 5 400 m³·hm^-2, total nitrogen application rate was 495 kg·hm^-2, 30% was used as basic fertilizer, others were fertilized with water. The other managements were the same as the general field management. 【Result】 Results showed that stalk returning could increase the soil humic acid content significantly in three soil layers. Besides, with the time of continuous cropping increasing, the humic acid content was remarkably raised, especially the humic acid content was most abundances in 20-40 cm soil layer. Compared with the 5, 10, 15, 20, 25 years continuous cropping, the soil humic acid content in 30 years increased by 139.90%, 86.68%, 93.33%, 58.60% and 22.86%, respectively. However, after continuous cropping and straw incorporation, the time of continuous cropping had no effect on fulvic acid content in soil, and the fulvic acid content was always (2.79 ± 0.19) g·kg^-1 in the 0-20 cm soil layer, (2.56 ± 0.10) g·kg^-1 in the 20-40 cm soil layer and (1.77 ± 0.15) g·kg^-1 in the 40-60 cm soil layer. Moreover, the fulvic acid content in the 0-20 cm soil layer and 20-40 cm soil layer was significantly higher than in the 40-60 cm soil layer. In addition, Straw returning could significantly increase the humin content in three different soil layers, humin content in the 0-20 cm soil layer was greater than in the 20-40 cm and in the 40-60 cm soil layers. In the different years of straw returning, C_HA /C_FA values＞1, and PQ＞0.5, C_HA/C_FA values and PQ values were increased as the time of continuous cropping increasing, besides, the 5-year continuous cropping was the minimum and the 30-year continuous cropping was the maximum. Compared with 0-20, 20-40 and 40-60 cm soil layers in the 5-year continuous cropping, C_HA/C_FA values of 30 years increased by 120.04%, 116.39% and 112.91%, respectively, and the PQ values of 30 years were increased by 37.15%, 36.44% and 36.81%, respectively. 【Conclusion】Straw returning increased the soil humic acid, humin and fulvic acid contents in the long-term continuous cropping cotton field, besides, it kept them in a stable dynamic level, but also significantly improved the C_HA/C_FA and PQ values, so that the quality of the soil humus transformation went far towards a good direction.

Key words: cotton, long-term continuous cropping , straw returning, soil humus, C_HA / C_FA, PQ values