Long-term effect of biochar-based fertilizers application in tropical soil: Agronomic efficiency and phosphorus availability

Incorporation of phosphorus (P) into an organic matrix may be an effective strategy to increase plant P use efficiency in high P-fixing soils. The objective of this work was to evaluate the effect of biochar-based fertilizers (BBFs), produced from poultry litter (PLB) and coffee husk (CHB) enriched with phosphoric acid and magnesium oxide, in combination with triple superphosphate (TSP) on plant growth and soil P transformations. Treatments were prepared as: TSP, CHB, PLB, CHB + TSP [1:1], CHB + TSP [3:1], PLB + TSP [1:1] and PLB + TSP [3:1]; with numbers in brackets representing the proportion of BBF and TSP on a weight basis. Cultivations were: Mombasa grass, maize, and common bean interspersed with fallow periods. After cultivations, a sequential extraction procedure was employed to determine P distribution among different P pools. A kinetic study was performed and revealed that TSP released approximately 90% of total P, and BBFs less than 10% in the first hour. BBF alone or in combination with TSP presented higher or similar biomass yields, relative agronomic effectiveness, and P uptake when compared with TSP. As for the soil, BBFs increased non-labile P fractions, which can be due to pyrophosphate formed during pyrolysis. According to these results, BBFs could totally or partially replace conventional soluble P fertilizers without compromising crop yield either in the short and long-term.