Magnesium-enriched poultry manure enhances phosphorus bioavailability in biochars

Pyrolysis of calcium-rich feedstock (e.g., poultry manure) generates semi-crystalline and crystalline phosphorus (P) species, compromising its short-term availability to plants. However, enriching poultry manure with magnesium (Mg) before pyrolysis may improve the ability of biochar to supply P. This study investigated how increasing the Mg/Ca ratio and pyrolysis temperature of poultry manure affected its P availability and speciation. Mg enrichment by ∼2.1% increased P availability (extracted using 2% citric and formic acid) by 20% in Mg-biochar at pyrolysis temperatures up to 600 °C. Linear combination fitting of P K-edge XANES of biochar, and Mg/Ca stoichiometry, indicate that P species, mainly Ca–P and Mg–P, are altered after pyrolysis. At 300 °C, adding Mg as magnesium hydroxide [Mg(OH)2] created MgNH4PO4 (18%) and Mg3(PO4)2.8H2O (23%) in the biochar, while without addition of Mg Ca3(PO4)2 (11%) predominated, both differing only for pyrophosphate, 33 and 16%, respectively. Similarly, the P L2,3 edge XANES data of biochar made with Mg were indicative of either MgHPO4.3H2O or Mg3(PO4)2.8H2O, in comparison to CaHPO4.2H2O or Ca3(PO4)2 without Mg. More importantly, hydroxyapatite [Ca5(PO4)3(OH)] was not identified with Mg additions, while it was abundant in biochars produced without Mg both at 600 (12%) and 700 °C (32%). The presence of Mg formed Mg–P minerals that could enhance P mobility in soil more than Ca–P, and may have resulted in greater P availability in Mg-enriched biochars. Thus, a relatively low Mg enrichment can be an approach for designing and optimize biochar as a P fertilizer from P-rich excreta, with the potential to improve P availability and contribute to the sustainable use of organic residues. [Display omitted] •Phosphorus bioavailability increases in Mg-enriched biochar up to high pyrolysis temperatures.•OH− anions provided with Mg(OH)2 could be a mechanism by which Mg-doped biochars have higher P availability.•Adding Mg to poultry manure decreased the formation of Ca–P minerals in low temperatures.