Abstract
The addition of exogenous organic matter (straw) directly affects the turnover of organic carbon in the topsoil, and soil organic carbon sequestration and influencing factors of continuous return of maize straw under shallow buried drip irrigation is of great significance for the sustainable production of maize and the improvement of farmland fertility in the irrigation area of the Xiliaohe Plain. The study spanned seven consecutive years of field positioning experiments, during which we assessed 0 (0a), 3 (3a), 5 (5a), and 7 years (7a) of the full return of straw to examine the effects of the number of straw returning years on soil structure, extracellular enzyme activity, and carbon component content, storage, sequestration, and pool management indices. Compared with 0a treatment, the generalized soil structure index in the 0−30 cm soil layer of 3a, 5a and 7a treatments increased by 1.65%, 1.99% and 3.33%, respectively, and there were corresponding reductions of 5.55%, 6.65% and 12.7%, respectively, in the soil three-phase structure distance. Furthermore, the activities of β-1,4-N-acetylglucosaminidase and β-glucosidase in the 0–30 cm soil layer of 3a, 5a and 7a treatments increased by 4.34%, 13.0%, 15.9% and 12.1%, 16.3%, 20.9%, respectively, and similarly, the activities of cellulase and sucrase in the 0−30 cm soil layer of 3a, 5a and 7a treatments increased by 5.86%, 13.4%, 19.2% and 12.6%, 21.3%, 34.1%, respectively. We also detected significant differences in the geometric mean enzyme activity and total enzyme activity in response to different treatments, and the geometric average enzyme activity and total enzyme activity recorded in the 0−30 cm soil layer of 3a, 5a and 7a treatments, which increased by 8.63%, 15.90%, 22.25% and 9.12%, 17.19%, 25.09%, respectively. In addition, we found that the content and storage of soil organic carbon, easily oxidized organic carbon, water-soluble organic carbon, and microbial biomass carbon in 3a, 5a and 7a treatments were all significantly higher than the corresponding values measured in 0a treatments, with the content of soil organic carbon and easily oxidized organic carbon in the 0−30 cm soil layer of 3a, 5a and 7a treatments increasing by 13.4%, 32.7%, 42.7% and 17.5%, 27.5%, 42.5%, respectively, and the content of the soil water-soluble organic carbon and microbial biomass carbon increased by 13.2%, 18.5%, 28.5% and 33.9%, 45.3%, 56.1%, respectively, Similarly, the storage of soil organic carbon and easily oxidized organic carbon in the 0–30 cm soil layer of 3a, 5a and 7a treatments were found to have increased by 11.6%, 29.5%, 36.2% and 14.9%, 23.8%, 35.5%, respectively, and the storage of water-soluble organic carbon and microbial biomass carbon had increased by 10.7%, 15.2%, 22.3% and 31.0%, 41.0%, 48.5%, respectively. Compared with 3a treatment, we detected significantly higher soil carbon sequestration in the 0–30 cm soil layer of 5a and 7a treatments. Moreover, compared with 3a treatment, there were significant differences in the carbon pool index and carbon pool management index obtained for the 0–30 cm soil layer, with the carbon pool index for 5a and 7a treatments increasing by 17.0%, 26.1% and 7.00%, 19.9%, respectively. Redundancy and correlation analyses of influencing factors and soil organic carbon sequestration revealed that the soil β-1,4-N-acetylglucosaminidase activity, liquid volume fraction, soil three-phase structure distance, soil cellulase activity, and soil β-glucosidase activity were the main factors influencing soil organic carbon sequestration during different straw returning years, with explanatory rates of 76.0%, 4.10%, 3.30%, 1.70% and 3.40%, respectively. Under shallow buried drip irrigation, the continuous return of maize straw over 7 years was found to improve the stability of the soil structure and activity of soil carbon-related extracellular enzymes, and promoted the decomposition and transformation of returned straw by soil microorganisms, thereby enhancing soil organic carbon sequestration and the carbon pool index.
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