Objectives Biochar and arbuscular mycorrhizal fungi (AMF) are beneficial to the nutrient absorption of crops. We studied their individual and combined effect on the conversion of soil phosphorus existing forms and rice phosphorus uptake.

Methods A rice (Oryza sativa L.) pot experiment was conducted inside greenhouse. At the base of normal water and fertilizer management (CK), inoculation with AMF (A), addition of 10 g/kg biochar (B), and both inoculation and biochar addition (AB) were setup as treatments. At harvest of rice, soils were sampled for determination of microbial biomass, acid and alkali phosphatase activity (AP, ALP), and the contents of different phosphorus forms, rice root samples were collected for measurement of hyphal, arbuscular, and vesicle colorization rate and P uptake.

Results Compared to CK, biochar, AMF, and AB treatments significantly increased soil pH by 0.16, 0.17, and 0.23 units; biochar increased soil EC by 7.4%, while AMF reduced it by 6.4%; AB treatment increased soil microbial biomass carbon (MBC) and phosphorus (MBP) content by 39.1% and 139.7%, respectively. In paddy soil, sparingly labile phosphorus (HCl-Pi and Res-P) was dominant, accounting 75%−77% of the total P, following is moderate labile phosphorus (NaOH-Pi and NaOH-Po), accounting 16%−18%, the lowest was labile phosphorus (H₂O-P, NaHCO₃-Po, and NaHCO₃-Pi), accounting for only 7%−8%. All the biochar, AMF, and AB treatments increased the proportion of inorganic P fractions in labile and moderate labile phosphorus (H₂O-P, NaHCO₃-Pi, NaOH-Pi), and decreased the proportion of organic fractions (NaHCO₃-Po, NaOH-Po), and AB treatment was recorded significantly higher effect than biochar and AMF treatments. Compared with CK, all the biochar, AMF and AB treatments enhanced AP and ALP activities, and AB treatment was recorded significantly higher effect with an increment of AP and ALP activity by 25.1%, and 25.5% over those of CK. In addition, AB treatment was recorded 7.04 and 2.37 percentage points higher hyphal and arbuscular colonization rate than AMF treatment, but similar vesicle colonization rate. Both biochar and AMF treatments did not increase the shoot and root P uptake of rice significantly, while AB treatment increased the root P uptake by 200.0%, compared to CK.

Conclusions Combined application of biochar and AMF could enhance soil pH, and the acid and alkali phosphatase activities, promote the conversion of organic phosphorous to inorganic forms, thus optimizing the existing forms and availability of phosphorus in soil. The combined application of biochar and AMF also significantly improves the colonization rate of hyphal and arbuscular on rice root, so increase the P absorption directly. Therefore, the combined application of biochar and AMF is an effective measure to improve soil phosphorous availability and phosphorous uptake by rice.