Phosphorus Dynamics in a Ferralsol Amended with Acai Waste Biochar and Lime

Acidic tropical soils act as strong sinks for phosphorus (P) due to the high adsorption capacity of this nutrient by iron (Fe) and aluminium (Al) minerals. In this study, we investigated the effects of applying acai waste biochar (Euterpe oleracea Mart.) in combination with dolomitic lime on the P dynamics of a Ferralsol with cowpea (Vigna unguiculata (L.) Walp) test crop. Application of 12 t ha−1 of biochar and 75% of the recommended lime rate increased soil pH by 1.77 units, reaching 6.77, and resulted in the lowest C:N ratio (18.99) at 0–5 cm depth. Inorganic P concentrations increased in Resin-Pi (2-fold), NaHCO3-Pi (2-fold), NaOH-Pi (2.89-fold) and HCl-Pi (4-fold) fractions relative to the corresponding treatments without biochar, while NaHCO3-Po decreased markedly, declining from 68% to 9% of the organic P fraction, NaOH-Po showed a less consistent response among treatments. In addition, P recovery in the Resin-Pi fraction increased, reaching 34.91% and 37.36% in the treatments with 12 t ha−1 of biochar and both 75 and 100% liming, respectively. Combined use of alkaline biochar and lime is a practical strategy to raise pH and increase labile and moderately labile inorganic P, and improve P use efficiency in acid Ferralsols. These responses are consistent with a redistribution of P among the assessed fractions and with the absence of detectable short-term effects on arbuscular mycorrhizal fungi (AMF) colonisation and easily extractable glomalin-related soil protein (EE-GRSP) production.