This study aimed to study the effects of different proportions of cow manure and fertilizer on soil labile organic carbon components and carbon conversion related enzyme activities in apple orchard, and to reveal the mechanism of different fertilization methods on biological transformation of soil carbon pool, so as to provide the theoretical support for organic and inorganic scientific application and soil quality improvement in apple orchard.

Long-term positioning fertilization test was used as the platform. Six treatments were selected: no fertilizer (CK), 100% fertilizer (CF100), 25% cow manure with 75% fertilizer (CM25CF75), 50% cow manure with 50% fertilizer (CM50CF50), 75% cow manure with 25% fertilizer (CM75CF25), and 100% cow manure (CM100). Soil labile organic carbon components (particulate organic carbon, POC; microbial biomass carbon, MBC; readily oxidizing organic carbon, ROC) and carbon conversion related enzymes (α-D-glucosidase, AG; β-D-glucosidase, BG; Cellulase, CBH; Peroxidase, PER; Urease, UR) activity and other related indicators were measured.

(1) The content of SOC, POC and ROC in soil increased with the increase of the proportion of organic fertilizer applied. CM50CF50 had the highest MBC content, which was 139.7% higher than that under CK. In the non-fertilized area, compared with CK, the POC content under CF100, CM25CF75, CM50CF50 and CM75CF25 decreased by 32.8%, 28.4%, 21.6% and 14.7%, respectively. The ROC content under CM50CF50 and CM75CF25 treatments decreased by 31.5% and 17.4%, respectively. The content of labile organic carbon in fertilized area was significantly higher than that in non-fertilized area under the same treatment. (2) Compared with CK, the α-D-glucosidase activity under CM25CF75, CM50CF50, CM75CF25 and CM100 was increased by 87.7%, 68.4%, 278.1% and 331.6%, respectively. The β-D-glucosidase activity under CM25CF75 was the highest (39.00 µg·g^-1·h^-1). Urease activity first increased and then decreased with the increase of organic fertilizer application ratio. The α-D-glucosidase and urease activities of soil in the non-fertilized area were also significantly increased. (3) The combination of organic and inorganic application significantly increased the soil POC/SOC and carbon pool management index (CPMI) of the fertilization area, and the carbon pool management index under CM25CF75, CM50CF50, CM75CF25 and CM100 treatments increased by 19.7%, 38.3%, 56.2% and 103.5%, respectively. The carbon pool management index of organic and inorganic combined application in non-fertilized area decreased significantly. Soil carbon pool management index in fertilized area was significantly higher than that in non-fertilized area under the same treatment. (4) Correlation analysis and principal component analysis showed that there was a significant positive correlation between labile organic carbon components and α-D-glucosidase activity in the soil in the fertilization area, and the increase of organic fertilizer ratio contributed more to the increase of soil labile organic carbon. The effect of fertilization treatment on the fertilized area was greater than that on the non-fertilized area.

The effect of organic and inorganic combined application on soil improvement in fertilized area of apple orchard was greater than that in non-fertilized area. The combination of organic and inorganic application could increase the content of soil organic carbon and promote soil enzyme activity, which provided a theoretical basis for the sustainable management of soil ecological environment in apple orchard.