Improvement and Maturation of Liquid Biofertilizers in Series-Connected Biodigesters: Comparative Analysis of Guinea Pig Manure and Vermicompost Leachate

The recovery of livestock waste through multistage anaerobic digestion represents a key strategy for producing high-efficiency liquid biofertilizers within circular economy frameworks. This study compared two underexplored substrates—guinea pig manure and vermicompost leachate (VL)—processed in series biodigesters to evaluate their nutrient composition and agronomic performance. The guinea pig manure biol exhibited higher macronutrient concentrations (N = 1.09–3.74 g L−1; P = 0.06–0.64 g L−1; K = 1.85–3.20 g L−1) and electrical conductivity (14.1–26.5 mS cm−1), while VL presented a more balanced nutrient profile (N = 0.65–0.71 g L−1; P = 0.04–0.09 g L−1; K = 2.46–3.76 g L−1) and slightly lower salinity (15.0–17.2 mS cm−1). Micronutrient levels (Fe, Mn, Zn, B) exceeded the reference thresholds established by EU Regulation 2019/1009 for liquid fertilizers, suggesting the need for dilution prior to field application. In maize field trials, VL diluted 1:7 increased above-ground biomass by 28%, and guinea pig biol diluted 1:10 achieved a 22% increase compared to the control, confirming their biostimulant potential. However, the high sodium content (848–1024 mg L−1) may limit application on saline or poorly drained soils, requiring adaptive agronomic management. These findings demonstrate that multistage anaerobic digestion effectively transforms unconventional organic waste into nutrient-rich biofertilizers, expanding the scientific foundation for alternative substrates and reinforcing their potential to enhance Andean smallholder agriculture, nutrient recycling, and food security within a sustainability-oriented bioeconomy.