Carbon Stability of Engineered Biochar-Based Phosphate Fertilizers

Abstract

Converting abundant agricultural residues to valuable products, such as biochar, is a pressing challenge for society. Here, our objective has been to produce biochar-based fertilizers (BBFs) with high carbon stability, high yield, and characteristics favorable for their use in soils. Thus, H 3 PO 4 with and without MgO was co-pyrolyzed with coffee husk and poultry litter. Proximate analysis, total phosphorus and magnesium, biochar yield, carbon retention, and thermal and chemical carbon stability were assessed. The BBFs were slightly acidic (pH ∼6.0) and showed high contents of total P (146−206 g kg −1 ) and total Mg (96−98 g kg −1 ), similar to commercial fertilizers. Biochar yield increased up to 65% when treated with H 3 PO 4 and H 3 PO 4 −MgO, resulting in up to 78% greater carbon retention. BBFs showed higher thermal stability, by thermogravimetric analysis and muffle oxidation, when compared to corresponding pristine biochars. Conversely, the addition of H 3 PO 4 −MgO decreased the chemical stability (higher H 2 O 2 and K 2 Cr 2 O 7 oxidation), compared to the biochars, because of the increase in surface area and reactivity. We concluded that the co-pyrolysis of biomasses with H 3 PO 4 and MgO promoted thermal stabilization (increased yield) and increased chemical oxidation, because of increased surface area and reduced crystallinity of the BBFs.

Biochar association with phosphate fertilizer and its influence on phosphorus use efficiency by maize

“…These authors showed a much slower P release rate as compared to TSP and also a similar maize yield in a pot experiment. Additionally, Carneiro et al (2018) observed that co-pyrolysis of biomass with phosphate sources increased the yield of biochar and also increased the carbon retention during pyrolysis and carbon chemical and thermal stability. Future studies should be focused on testing novel binding agents during the granulation process, which can control the release behavior of P over time favoring the P uptake by the plant throughout the cultivation, thus improving the efficiency of use of soluble-P fertilizers.…”

Biochar Phosphate Fertilizer Loaded with Urea Preserves Available Nitrogen Longer than Conventional Urea

“…The pyrolysis of PL enriched with H 3 PO 4 + MgO caused a decrease in the pH of the PLB (from 8.3 in PL to 6.1 in PLB). Normally, pristine pure PL biochar tends to be highly alkaline (pH~11), and when it was impregnated with H 3 PO 4 in combination with MgO, the acidity of the P source was neutralized [17,21]. The total P content increased considerably with the impregnation of H 3 PO 4 and MgO (Table 2), reaching values as high as those in conventional P fertilizers.…”

“…The physical characteristics of biochar such as the number of pores, pore layout, and specific surface area are influenced by the raw material and affect the availability of nutrients in soil [38]. Furthermore, the co-pyrolysis of PL with H 3 PO 4 + MgO increases the surface area and small pores [17] that can potentially promote the link between urea and surface functional groups through chemical reactions. Pores are formed by the decomposition of organic macromolecules and give PLB the ability to accommodate urea molecules [37].…”

“…Biochar enriched with P sources was shown to greatly improve C stability [17,18] and showed increased plant P use efficiency in highly weathered soils [19,20]. The addition of MgO with acid P sources reduced the acidity and P water solubility but resulted in similar solubility in citric acid and neutral ammonium citrate + water (NAC + H 2 O) to triple superphosphate (TSP) [20], which has favorable use in P-fixing tropical soils [19].…”

“…The combination of poultry litter with H 3 PO 4 and MgO prior to pyrolysis increases the specific surface area [17] and increases the cation exchange capacity in the resulting biochar [21]. These characteristics favor biochar as a potential carrier of urea N to enhance the efficiency of N-P fertilizers for crops, which might allow a reduction in the frequency of urea application while increasing the N-use efficiency.…”