Biochar-based phosphate fertilizer as an alternative to improve phosphorus use efficiency in tropical soils

Abstract

Phosphorus (P) is rapidly sorbed on highly weathered tropical soils. Most phosphate fertilizers used in Brazil have high solubility, providing fast P release and greater susceptibility to P fixation in soils. Thereby, we sought to produce a biochar-based organomineral fertilizer aiming to increase the P use efficiency in acidic tropical soils. For this purpose, monoammonium phosphate (MAP), triple superphosphate (TSP) and phosphoric acid (H3PO4) with and without the addition of magnesium oxide (MgO) were mixed with poultry litter to produce biochar-based fertilizers (BBFs) by pyrolysis at 500 °C. Three studies were carried out to characterize and evaluate the agronomic efficiency of the BBFs. In the first part of the study, it was evaluated mineral composition, X-ray diffraction, Fourier transform infrared spectroscopy, P release kinetics in water of the BBFs and a bioassay with maize in a greenhouse using a clayey Oxisol. The addition of MgO dramatically reduced the release kinetics of P in water from BBFs due to the formation of low solubility compounds. However, these BBFs promoted maize biomass production close to TSP and also raised the pH and soil Mg content after cultivation. In the second part of the study, it was evaluated two BBFs (biochar poultry litter + TSP + MgO, identified as PLB-TSP-MgO, and biochar poultry litter + H3PO4 + MgO, identified as PLB-H3PO4-MgO) in relation to (i) the soluble fractions, (ii) diffusion of P in the soil using a visualization technique over time, and (iii) the agronomic efficiency in granular and powder form in the maize crop. The P water solubility of BBFs was dramatically reduced, but the solubility of citric acid and neutral ammonium citrate + water is similar to the TSP. The method of visualization showed that the P diffusion process of BBFs in soil is slower than the TSP, characteristic of slow-release fertilizers. BBFs have agronomic efficiency similar to TSP when applied in powder. The PLB-H3PO4-MgO, applied as a granule, promoted maize biomass production equivalent to TSP, despite the smaller proportion of the granule that dissolved during maize cultivation (~ 16% versus 40% of TSP), preserving P available in the granule for future harvests. In the third part of the study, the BBFs were applied at different doses in an Oxisol and incubated for a period before cultivation of Urochloabrizantha for three growth cycles to evaluate the agronomic efficiency and thereafter, a sequential extraction procedure was used to determine the P distribution among different P pools. The shoot dry matter yield in the first cropping cycle was higher at the highest P rate for TSPplanting (no incubation). However, in subsequent crop cycles, all BBFs showed higher biomass yield as compared to TSPplanting. The PLB-TSP addition positively influenced the dynamics of the P fractions in the soil with an increase in the labile and moderately labile P fractions in soil after cultivation.