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 KH₂PO₄, Ca(H₂PO₄)₂, and NH₄H₂PO₄, were premixed with corn stalk (1:4, w/w), 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 NH₄H₂PO₄ (65.6–83.5%) > Ca(H₂PO₄)₂ (60.4–78.2%) > KH₂PO₄ (50.1–76.1%), compared with the pristine biochar (47.8–73.6%). The addition of Ca(H₂PO₄)₂ and KH₂PO₄ 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 H₂O₂ oxidation, and CSB+Ca showed the best effect. Ca(H₂PO₄)₂ and KH₂PO₄ pretreated biochars had higher resistance to K₂Cr₂O₇ oxidation and thermal treatment. In contrast, the C loss of NH₄H₂PO₄-added biochar at 500 and 700 °C with K₂Cr₂O₇ oxidation was increased by 54% and 36%, respectively. During the pyrolysis process, Ca(H₂PO₄)₂ was transformed into insoluble Ca₂P₂O₇, leading to the lowest P release rate of CSB+Ca. This study indicates that co-pyrolysis of corn stalk and Ca(H₂PO₄)₂ is optimal for increasing C retention, enhancing C stability and improving slow-release performance of P regardless of pyrolysis temperature.

Animal manure application is widely recognized for its agronomic benefits in enhancing soil fertility and crop productivity through organic matter enrichment and nutrient supply, but the critical application time governing its greenhouse gas emission trade-offs remains unresolved. The objective of this study was to investigate the effects of pig manure compost application timing on nitrous oxide (N₂O) emissions and maize yields in Northeast China through a four-year field experiment. The treatments included: (1) inorganic fertilizers (NPK); (2) NPK plus pig manure compost applied in spring (NPK-MS); and (3) NPK plus pig manure compost applied in autumn (NPK-MA). The N₂O fluxes, NH₄⁺-N contents, NO₃⁻-N contents, and maize yields were analyzed. The results showed that compared with NPK, NPK-MA increased N₂O emissions by 44.4%. Applying pig manure compost in autumn promotes N₂O emissions during the freeze–thaw period. However, there was no significant effect of NPK-MS on N₂O emissions compared with NPK (p > 0.05). Spring-applied manure compost (NPK-MS) resulted in an 11.9% increase in maize yield compared to NPK. In contrast, autumn-applied manure compost (NPK-MA) did not significantly affect maize yield (p > 0.05). Furthermore, yield-scaled N₂O emissions were significantly increased in NPK-MA (p < 0.05). Overall, spring application of pig manure compost is recommended for increasing maize yield without significantly increasing N₂O emissions while in Northeast China.

Biochar is widely used in composting to reduce nitrogen loss; however, the application of acid-modified and alkali-modified biochar in composting remains insufficient. We hypothesize that acid-modified maize straw biochar can simultaneously reduce NH₃ and N₂O losses during the composting process. To test this, a composting experiment was conducted with four treatments: a control with pig manure and corn straw only (CK), adding 5% corn straw biochar (BC), adding 5% HNO₃-modified corn straw biochar (BCN), and adding 5% NaOH-modified corn straw biochar (BCNa). The results showed that, compared to CK, NH₃ emissions were not decreased by BC, but significantly reduced (p < 0.05) by 32.6% in BCN and 36.8% in BCNa, respectively. N₂O was significantly decreased (p < 0.05) by 27.6% in BC and 30.9% in BCNa, respectively. However, BCN significantly increased N₂O emission (p < 0.05) by 368.7%, compared to CK. Compared to CK, the global warming potential (GWP) in the BCNa treatment was significantly reduced by 35.2% (p < 0.05), while the GWP in BCN was significantly decreased by 10.3%. Overall, although BCN treatment may increase N₂O emissions, it can still reduce the GWP. In comparison, BCNa treatment achieves the most significant reduction in GWP during pig manure composting.