Biochar enhances the use efficiency of guano de isla and could boost profitability and food security for smallholders in the global south

Abstract

Guano de isla (seabird guano) is a strategic organic fertilizer in Peru, reserved primarily for economically vulnerable smallholder farmers. However, its availability is declining due to ecological pressures on seabird populations, creating an urgent need to improve its use efficiency. This study tested the hypothesis that biochar can enhance the nutrient use efficiency of guano de isla, allowing reduced application rates without compromising yield or profitability. A randomized complete block experiment (n=10 replicates per treatment) was conducted in sandy soil in Lurín, Peru, to evaluate lettuce (Lactuca sativa) yield under eight fertilization treatments, including inorganic fertilizer (NPK), guano de isla (400 kg ha⁻¹), biochar (3 t ha⁻¹), and combinations thereof. The biochar+guano de isla (BG) treatment applied 200 kg ha⁻¹ of guano de isla (50% of the standard rate). Economic viability was assessed using the value/cost ratio (VCR) under conventional and organic market scenarios, with and without carbon credits. The highest lettuce yields were obtained in the full-rate guano de isla (G) and biochar+guano de isla (BG) treatments, representing a 40% increase compared to the control. Importantly, the BG treatment achieved statistically equivalent yields to the full-rate guano de isla treatment while using 50% less guano de isla. Under conventional market conditions, all treatments resulted in VCR values<1. However, under organic market conditions, the G, BG, and biochar treatments exceeded the profitability threshold (VCR>2), with the BG treatment reaching a maximum VCR of 12.9 when carbon credits were included. These findings demonstrate that biochar can significantly enhance the use efficiency of guano de isla in sandy soils, allowing reduced application rates while maintaining productivity. Integrating biochar with guano de isla could help conserve a strategic national fertilizer resource, improve farm profitability, and strengthen food security for smallholder farmers in the Global South.