| Summary: | Carbon sequestration is the primary function of biochar. Hence, it is necessary to design biochar with high carbon (C) retention and low C loss. In this study, three P compounds, including KH<sub>2</sub>PO<sub>4</sub>, Ca(H<sub>2</sub>PO<sub>4</sub>)<sub>2</sub>, and NH<sub>4</sub>H<sub>2</sub>PO<sub>4</sub>, were premixed with corn stalk (1:4, <i>w</i>/<i>w</i>), aiming to produce biochars (CSB+K, CSB+Ca, and CSB+N) with high C sequestration and slow release of P at three temperatures (300, 500, and 700 °C). The addition of all P sources obviously increased C retention, with the order of NH<sub>4</sub>H<sub>2</sub>PO<sub>4</sub> (65.6–83.5%) > Ca(H<sub>2</sub>PO<sub>4</sub>)<sub>2</sub> (60.4–78.2%) > KH<sub>2</sub>PO<sub>4</sub> (50.1–76.1%), compared with the pristine biochar (47.8–73.6%). The addition of Ca(H<sub>2</sub>PO<sub>4</sub>)<sub>2</sub> and KH<sub>2</sub>PO<sub>4</sub> led to an increase in aromaticity and graphitization, as evidenced by H/C, FTIR, Raman and XPS analysis, whereas an opposite result occurred on CSB+N. Furthermore, all three phosphates reduced C loss of biochars with H<sub>2</sub>O<sub>2</sub> oxidation, and CSB+Ca showed the best effect. Ca(H<sub>2</sub>PO<sub>4</sub>)<sub>2</sub> and KH<sub>2</sub>PO<sub>4</sub> pretreated biochars had higher resistance to K<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub> oxidation and thermal treatment. In contrast, the C loss of NH<sub>4</sub>H<sub>2</sub>PO<sub>4</sub>-added biochar at 500 and 700 °C with K<sub>2</sub>Cr<sub>2</sub>O<sub>7</sub> oxidation was increased by 54% and 36%, respectively. During the pyrolysis process, Ca(H<sub>2</sub>PO<sub>4</sub>)<sub>2</sub> was transformed into insoluble Ca<sub>2</sub>P<sub>2</sub>O<sub>7</sub>, leading to the lowest P release rate of CSB+Ca. This study indicates that co-pyrolysis of corn stalk and Ca(H<sub>2</sub>PO<sub>4</sub>)<sub>2</sub> is optimal for increasing C retention, enhancing C stability and improving slow-release performance of P regardless of pyrolysis temperature.
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