Organic Substitution Thresholds for Sustainable Nutrient Management in Tibetan Alpine Agroecosystems: Trade-Offs Among Phenological Synchrony, Rhizosphere Stability, and Carbon Sequestration

Alpine agroecosystems face fertilizer-induced nutrient imbalances, including early nitrogen accumulation and late-season nitrate residue. While organic substitution is promoted as a sustainable practice, different substitution ratios may generate contrasting responses among nutrient availability, rhizosphere function, and SOC temporal stability. We established a field experiment in Tibetan highland barley with five treatments at equivalent N, P, and K rates: 100% NPK, 75% NPK + 25% cattle manure (CM), 50% NPK + 50% CM, 25% NPK + 75% CM, and 100% CM, plus an unfertilized control. Soils were sampled from 0–10 cm bulk soil, 10–20 cm bulk soil, and rhizosphere soil (0–10 cm) at booting, filling, and maturity. Stoichiometric ratios in the 0–10 cm layer changed mainly at maturity, whereas responses in the 10–20 cm layer and rhizosphere were weaker, indicating stage- and compartment-dependent treatment responses. The treatments showed three response modes: mineral fertilizer-dominated nutrient pulses, manure-dominated late-season nitrate residue and surface SOC stability, and partial substitution patterns with comparatively balanced nutrient responses. No single substitution ratio optimized all measured indicators. We therefore propose these modes as a framework for organizing treatment-specific nutrient responses and as candidates for further multi-year and multi-site testing, rather than as ready-to-apply fertilization prescriptions.