Objectives We studied the effects of rice husk biochar on the P availability of compound fertilizer in soil. We aimed to provide scientific support for developing high efficiency and environmentally friendly fertilizer.

Methods Twelve composite fertilizer samples were prepared with monoammonium phosphate (MAP), diammonium phosphate (DAP), nitrophosphate (NP), ammonium polyphosphate (APP) as P fertilizer sources, plus urea, potassium chloride, stone powder, and other auxiliary components as raw materials, and rice husk biochar was added to the compound fertilizer at 0, 5%, and 10% levels. The 12 fertilizer samples were mixed with latosolic red soil and fluvo-aquic soil to carry out an indoor thermostatic incubation experiment. We measured the soil H₂O-P, NaHCO₃-P, NaOH-P and HCl-P content after 7, 14, 28, and 56 days of incubation. The soils mixed with the 12 fertilizer samples were loaded into 12 mm columns and stand for 7 and 30 days, then were leached with water, the contents of the five inorganic P forms were measured to determine the vertical diffusion of P from the compound fertilizers. The P recovery efficiency of water spinach (Ipomoea aquatica Forsk) was measured by plot experiments to explore the effect of rice husk biochar on the P utilization rate.

Results In the incubation experiment, the rice husk biochar (P<0.05) increased the H₂O-P and NaHCO₃-P contents, but did not increase the NaOH-P and HCl-P concents in both latosolic red soil and fluvo-aquic soil after 7, 14, 28, and 56 days of incubation, and adding 10% biochar showed better effect than adding 5%. In the latosolic red soil, the H₂O-P concentration was increased by 23.26% (MAP, 14 d), 4.99% (DAP, 7 d), 34.46% (NP, 7 d), and 12.17% (APP, 7 d), and in fluvo-aquic soil were increased by 25.53% (MAP, 56 d), 10.52% (DAP, 28 d), 21.69% (NP, 7 d), and 17.30% (APP, 14 d) by adding 10% rice husk carbon. The NaHCO₃-P concentration in the latosolic red soil increased by 15.01% (MAP, 28 d), 7.62% (DAP, 7 d), 25.15% (NP, 14 d), and 16.52% (APP, 7 d), and in the fluvo-aquic soil increased by 5.60% (MAP, 14 d), 3.30% (DAP, 7 d), 9.31% (NP, 7 d), and 5.95% (APP, 14 d) by adding 10% rice husk carbon. Rice husk biochar (10%) (P<0.05) influenced the diffusive movement of P, and the total P extracted from soil decreased by 13.88% (MAP), 16.86% (DAP), 9.37% (NP), and 14.82% (APP) in latosolic red soil after 30 d of incubation, but there was no significant effect on P diffusion in fluvo-aquic soil. The fresh matter yield and dry matter yield of water spinach applied with APP were increased by adding 5% rice husk carbon, and the utilization rate of p fertilizer was increased by 19.15%.

Conclusions Adding rice husk biochar (5% and 10%) could improve water spinach’s P efficiency and recovery of compound fertilizer in latosolic red soil. However, the effect of rice husk biochar addition on phosphorus release in fluvo-aquic soil was less than that in latosolic red soil. In sum, the effect of adding 10% rice husk biochar was better than 5%.