Straw biochar and compost can mitigate soil acidity and enhance carbon sequestration in acidic soils. However, their differential synergistic effects and underlying mechanisms remain poorly understood. To address this gap, an incubation experiment was conducted in which rice straw biochar (BC) and compost (DC) were incorporated into an Ultisol at rates of 1% and 3%. BC outperformed DC in elevating the soil pH (0.39 vs. 0.28 units), reducing the exchangeable acidity (69% vs. 62%), and decreasing the potential active aluminum pool (35.1% vs. 25.2%) due to its higher alkalinity. Additionally, BC enhanced the soil organic carbon more effectively than DC (83.7% vs. 64.0%). While 3% BC treatment reduced the readily oxidizable and dissolved organic carbon in the soil, DC increased these parameters. This contrasting effect is attributed to BC’s lower carbon reactivity, higher alkalinity, and greater C/N ratio compared to DC. Compared with the control, BC and DC also increased the soil exchangeable K⁺ (14.0-fold vs. 12.3-fold), Ca²⁺ (5.4-fold vs. 4.9-fold), and Mg²⁺ (3.7-fold vs. 5.2-fold). Overall, BC demonstrated superiority in mitigating acidity and sequestering carbon, while DC showed greater potential for improving fertility in acidic soils. Elucidating the distinct benefits of biochar versus compost provides valuable insights into the sustainable amelioration of acidic soils.

Contexts: Increasing the availability of phosphorus fertiliser is needed for better crop production in acidic soils. Aims: To observe how rice straw and its biochar affect phosphorus fertiliser recovery in an Ultisol and an Oxisol as a measure for the availability of phosphorus in acidic soils. Methods: A 60-day incubation study was conducted using treatments comprising of straw (1.5%, w/w) and biochar produced at 400°C (1.5%, w/w), a mixture of each at the same rate, and a control with two phosphorus rates of 0 and 100 mg kg−1. Key results: The application of both the straw and the biochar alone and in combination raised the soil pH by 0.12, 0.20, 0.32, 0.17, 0.31 and 0.38 units, and cation exchange capacity by 2.06, 3.02, 2.72, 2.56, 6.28, and 8.40% compared to control for both the Ultisol and the Oxisol, respectively. Similarly, dissolved organic matter also increased by 74.19, 55.28, 84.63, 24.39 and 136.44%, respectively compared to control for the corresponding treatments and soils except the straw amended Ultisol. With an increase in the soil pH, the rate of phosphorus recovery increased by 11.73, 12.99, 23.89, 2.72, 5.23 and 6.28% compared to control for the corresponding treatments and soils, respectively. Phosphate adsorption by the soils became hindered by increased cation exchange capacity and dissolved organic matter but decreased exchangeable trivalent aluminium cations, which led to a greater phosphorus recovery in both the amended soils. Conclusions: The combined application of rice straw and biochar would be the appropriate choice for increasing the applied phosphorus fertiliser availability in acid soils.

Biochar can inhibit soil acidification by decreasing proton release from nitrification and improving soil pH buffering capacity (pHBC). However, as a kind of complex materials, the role of different components of biochar in inhibiting soil acidification induced by nitrification is still unknown. To fill this gap, dissolved fractions and solid particles of biochar were separated and mixed thoroughly with an acid Ultisol before an incubation study following urea addition. Results showed that both the dissolved fractions and solid particles of biochar inhibited soil pH drop caused by nitrification. The dissolved fractions of biochar inhibited soil acidification by decreasing the proton input derived from nitrification, while the solid particles of biochar did it by enhancing the soil pHBC. The dissolved fractions from peanut straw biochar (PBC) and rice straw biochar (RBC) decreased the proton release by 16% and 18% at the end of incubation. The decrease in proton input was attributed to inhibition of soil nitrification and mineralization caused by the toxicity of phenols in biochar dissolved fractions to soil bacteria. The abundance of AOB and total bacteria decreased by more than 60% in the treatments with biochar dissolved fractions. Opposite effects were observed in the treatments with biochar solid particles. Soil pHBC increased 7% and 19% after the application of solid particles of RBC and PBC, respectively. The plentiful carboxyl on the surface of biochar solid particles was mainly responsible for the increase in soil pHBC. Generally, the mixed application of dissolved fractions and solid particles of biochar induces a superior effect on inhibiting soil acidification. The negative impacts of dissolved biochar components on soil microorganisms should be closely supervised.