Soil microbial and enzymatic signatures decode ancient millet fertilization strategies in Guanzhong arid farmlands

Agricultural archaeology faces limitations in differentiating manure sources via stable isotopes (e.g., δ¹⁵N). This study establishes a microbial-functional identification framework that deciphers prehistoric manuring strategies in Guanzhong dryland millet systems by linking microbial taxa to soil biochemistry. Pig manure (ZH) significantly enriched Actinobacteria (27.12%), elevating nitrate-N (10.54 mg·kg−1) and Olsen P (151.13 mg·kg−1), evidencing a “fast nutrient-Actinobacteria synergy” that archaeologically explains spatial co-occurrence of pig husbandry and millet storage. Sheep manure (YH) drove peak urease activity (1.64 mg·g−1).) but depleted P ( ↓ 37.72% vs ZH), clarifying its rarity in archaeological records within calcareous soils. Cattle manure (NH) showed slow-release properties, aligning with modern topdressing. Chemical fertilizer (CF) reduced microbial richness by approximately 4.4% compared to organic manure treatments (based on ACE/Chao indices). Our manure identification framework re-orients agricultural archaeology from isotope tracing toward comprehensive integration of soil biochemical processes and microbial functional networks.