Biochar-based phosphate fertilizers: solid-phase speciation and phosphorus solubility and potential as a support material for nitrogen

Abstract

The association of organic biomass and inorganic sources of phosphorus (P), to obtain biochar-based fertilizers (BBFs), has been suggested to increase the efficiency of use of P in acidic tropical soils and may, additionally, increase the efficiency of use of P nitrogen (N). Although the crop's response to fertilization with BBFs has already been investigated, it is imperative to understand the forms of P in the fertilizer and the transformations of such forms of P in the soil after fertilization. Therewith, triple superphosphate (TSP) with and without MgO, phosphoric acid (H3PO4) with MgO were pyrolyzed with chicken litter (PLB) to produce biochar-based phosphate fertilizers (BBFs). Aiming to evaluate the agronomic efficiency of BBFs (PLB-TSP, PLB-TSP-Mg, PLB-H3PO4-Mg) in relation to TSP and an organomineral fertilizer (OMF), an experiment was carried out in a greenhouse with the cultivation of coffee seedlings for seven months in Clay Oxisol. The original BBFs and soil samples collected after growing were analyzed for P by conventional chemical fractionation and by X-ray absorption speciation near the edge structure (XANES). BBFs with MgO increased the efficiency of the use of P in coffee trees when compared to TSP and OMF. The results of sequential fractionation in BBFs confirmed the dominance of P associated with Ca and Mg in low solubility compounds, such as monetite and pyrophosphate identified by XANES spectroscopy. There was an increase in the labile and moderately labile fractions of P after cultivation with PLB-TSP-Mg and PLB-H3PO4-Mg. The results confirm that BBFs can increase the efficiency of the use of P in an Oxisol in the medium to long term and promote a greater residual effect of fertilization due to the presence of higher organic compounds. BBFs also have suitable properties to increase the efficiency of N use while providing P for the plants. In this second work carried PLB-H3PO4-Mg was loaded with 4: 5 urea (BBF: urea, w/w) aiming to generate a 15-15% NP slow-release fertilizer (PLB -N) to be used in a single application to soil. PLB-N was characterized by pH, EC, nutrient content, and P solubility and analyzed by scanning electron microscopy (SEM) and X-ray dispersive energy spectroscopy (EDX). A greenhouse experiment was carried out in carried out with cultivation of common bean followed by maize to evaluate the agronomic efficiency and the residual effect of fertilization with PLB-N in an Argissolo Amarelo (Ultisol). Six treatments were tested, including four doses of N (100, 150, 200, and 250 mg kg-1) via PLB-N in single application plus a control with triple superphosphate (TSP) and urea (splitted 3 times) and a control without N-P fertilization. The pores and surface of the BBF were effectively loaded with urea but did not cause its slow-release, which increased the soil salinity and temporarily reduced the soil pH and negatively affected the yield of common bean when compared with conventional fertilization (TSP + urea). The greatest effect of PLB-N was the residual effect of fertilization where maize showed a linear response to N doses applied via PLB-N, but showed no response to the conventional TSP + urea fertilization. Therefore, PLB-N preserved available N (mainly as NH4+) that has been lost through conventional N fertilization. Biochar has potential as a loading matrix to preserve N availability and increase residual effect and N use efficiency by plants. The mechanisms that govern this process should be further evaluated to design increased efficiency N fertilizers.