Effects of the combination of biogas slurry and chemical fertilizer over three years on soil aggregate stability in Cambisols

Purpose Biogas slurry serves as a high-quality organic fertilizer that enhances soil fertility. However, how the soil structure responds to BS is largely unknown. This research aimed to demonstrate the effects of biogas slurry application on soil aggregate stability in Cambisols on the Loess Plateau, China. Method A three-year field study was conducted to examine the influence of biogas slurry on soil aggregate stability under different fertilization regimes, including: no fertilizer application (CK), mineral fertilizer alone (NPK), and partial substitution of mineral nitrogen with biogas slurry at rates of 25% (25BS), 50% (50BS), 75% (75BS), and complete substitution (100BS). The Le Bissonnais method was employed to assess the resistance of soil aggregates to three distinct breakdown mechanisms: slaking (MWDFW), slow-wetting (MWDSW), and mechanical breakdown (MWDWS). Additionally, the study examined three major organic cementing agents: humus, calcium-bound humus (Ca-humus), and glomalin-related soil proteins (GRSP), and evaluated their effects on soil aggregate stability using path analysis. Result The application of biogas slurry (25BS, 50BS, 75BS, and 100BS) significantly increased the MWDFW, MWDSW, and MWDWS compared to the CK and NPK. This increase can be attributed to the substantial accumulation of humus, Ca-humus, and GRSP following the application of BS. However, it is important to note that excessive application of biogas slurry (75BS and 100BS) also led to significant increases in soil electrical conductivity value and dispersing agents such as exchangeable sodium ion. This excessive application also resulted in the decreases of the microbial contents and GRSP, ultimately weakening the aggregate stability. Consequently, the MWDFW, MWDSW, and MWDWS tended to increase first and then decrease as biogas slurry applications increased. Conclusion According to our results, excessive biogas slurry negatively impacts soil structure due to its high salt content, an optimal combination of BS and chemical fertilizer can effectively enhance soil aggregate stability in Cambisols.