Pore Structure Reorganization and Effective Porosity Regulation in Grey Desert Soil Under Biogas Slurry Drip Irrigation

Degraded grey desert soils are characterized by severe nutrient deficiencies and structural compaction. This study elucidated how biogas slurry drip irrigation regulates the micro-pore architecture, fertility, and macroscopic hydraulic properties. A one-year field experiment was conducted using a completely randomized design with three replications. The experimentation included three irrigation levels (W1: 70% W, W2: 85% W, and W3: 100% W, where W is full irrigation) and three slurry ratios (S1: 60% S, S2: 80% S, and S3: 100% S, where S is the annual nitrogen application rate of 93 kg ha−1), with undisturbed (CK) and chemical fertilizer (CF) controls. Surface soil samples (0–20 cm) were analyzed based on treatment averages using scanning electron microscopy and the van Genuchten (vG) model. The results indicated that W3S2 increased the total porosity to a peak of 42.39% compared with the CK baseline of 25.25%, while expanding the mean pore diameter to 9.24 μm. Concurrently, the application minimized the morphological pore fragmentation, reducing the fractal dimension from 1.82 under CK to 1.61 under W3S3. Although the macroscopic porosity expanded, the effective saturated water content decreased. We hypothesize that this reduction is driven by partial micropore clogging by organic coatings. This mitigated the excessive near-saturation water retention and accelerated drainage, while significantly increasing the specific water capacity at 100–1000 kPa suctions to delay moisture depletion. W2S3 (85% W, 100% S) performed favorably with regard to soil fertility and water retention stability. The W2S3 treatment optimized soil fertility and water retention stability by achieving peak concentrations of 17.69 g kg−1 for SOM and 1.31 g kg−1 for TN. Path analysis suggested that physical microstructural traits dominate macroscopic hydraulic regulation. In conclusion, biogas slurry drip irrigation provides a sustainable framework to optimize structural and hydraulic resilience in dryland agriculture.