Abstract: The aim of this study was to determine the effects of the flax rotation sequence on the soil carbon pool in a dry?land oil flax cropping area. A field experiment was conducted in a typical semiarid gully area of the Loess Plateau from 2012 to 2016 to study the effects of six rotation sequences with different flax frequencies (fallow; PWFW: 25% flax, potato?wheat?flax?wheat; WFWP: 25% flax, wheat?flax?wheat?potato; FWPF: 50% flax, flax?wheat?potato?flax; WPFF: 50% flax, wheat?potato?flax?flax; FFFF: 100% flax, flax?flax?flax?flax). We measured soil aggregates, total organic carbon (TOC), particulate organic carbon (POC), microbial biomass carbon (MBC), and nitrogen (MBN) in all the treatments. A continuous cropping treatment served as the control. The results showed that the fallow treatment and a lower frequency of flax significantly increased the <0.25 mm soil aggregate content, TOC content, and POC content in the 0-30 cm soil layer. All of these parameters decreased with increasing flax frequency. The soil aggregate contents in the 25% flax treatments were 2.02%-4.04% higher and 9.56%-11.73% than those pre?sowing and in the fallow treatment, respectively. The SOC was 5.95%-7.48% higher in the 25% flax treatments than in the continuous cropping treatment. In the 50% flax treatments, the timing of the flax crop in the rotation sequence significantly affected soil SOC. The SOC content was significantly higher in the FWPF treatment than in the WPFF treatment. The treatments could be ranked, from highest soil SOC content in the 0-60 cm layer to lowest, as follows: fallow>pre?sowing≈25% flax>50% flax>100% flax. Compared with continuous cropping, rotation significantly increased the soil TOC and POC contents in the 0-10 cm soil layer, consistent with the phenomenon of surface enrichment. The treatments could ranked, from highest soil POC content to lowest, as follows: 25% flax≈fallow>50% flax>100% flax. In addition, crop rotation treatments significantly increased the soil MBC content. The soil MBC content in the 0-30 cm layer decreased gradually with increasing flax frequency. Compared with pre?sowing, fallow, and crop rotation treatments, continuous cropping significantly reduced the ratio of soil microbial C/N. The SOC and microbial biomass were significantly affected by the interaction between the 50% flax rotation sequence and soil depth. These results indicated that a fallow treatment could significantly improve the soil physical and chemical properties. The 25% flax rotation sequence could maintain the stability of soil aggregates and increase soil TOC, SOC, and POC contents. The 50% flax rotation sequence could improve the soil microbial biomass and the ratio of soil microbial C/N. Because the rotation sequence with 25% flax frequency could maintain the stability of SOC, it was identified as the ideal rotation sequence for dry?land oil flax crops.

Key words: dry?land oil flax, crop rotation sequence, soil organic carbon, microbial biomass, soil aggregate