Divergent effects of a Bacillus biofertilizer immobilized in guinea pig manure biochar: Boosting productivity vs. enhancing nutritional quality in maca monocultures

Abstract

Intensive monoculture of maca (Lepidium meyenii) in the Peruvian Andes induces edaphic degradation, requiring sustainable amendments. We evaluated the agronomic and nutritional effects of a Bacillus subtilis biofertilizer immobilized in guinea-pig manure biochar, contrasting it with its individual components. A field experiment was conducted in Junín, Peru (4449 m a.s.l.) using a randomized complete block design with four treatments: control, biochar alone (3 L m⁻²), liquid B. subtilis (125 L ha⁻¹, 10⁷ CFU mL⁻¹) and biochar + B. subtilis. Plant establishment, yield, nutritional quality, and hypocotyl nutrient content were evaluated. Results revealed a functional divergence between treatments. Biochar-based treatments (alone and combined) significantly increased plant density (up to 49 vs. 29 m⁻² in the control) and yield (1290–1500 vs. 550 kg ha⁻¹), associated with greater K accumulation (up to 22,252 mg kg⁻¹; p < 0.001). In contrast, B. subtilis alone primarily enhanced nutritional quality, resulting in the highest protein (3.88 g 100 g⁻¹) and fat (0.24 g 100 g⁻¹) contents. The combined treatment achieved the highest individual plant weight (10.1 g) and high yields; however, it did not retain the quality-related benefits, exhibiting the lowest protein content (3.34 g 100 g⁻¹). Redundancy analysis (RDA) confirmed this functional separation: biochar was associated with a mineral–productivity gradient, whereas B. subtilis aligned with a proximate–nutritional gradient. These findings indicate that biochar primarily acts as edaphic conditioner driving productivity, while B. subtilis enhances nutritional quality. The absence of full synergism in the combined treatment suggests independent modes of action, resulting in a yield–quality trade-off. This enables the design of differentiated fertilization strategies tailored either toward maximum productivity (biochar alone) or premium nutritional quality (B. subtilis alone).