Objectives We studied the effects of carbon and phosphorus additions on phosphorus uptake of rape and the bio-availability of soils under different phosphorus P levels, to explore the pathway of improving phosphorous fertilizer use efficiency.

Methods A rape pot experiment was carried out, the high-P purple soil was taken from vegetable field, where available and total P content were 163.23 mg/kg and 1.72 g/kg, and the low-P purple soil was taken from a maize growing field, with available and total P content 3.93 mg/kg and 0.31 g/kg. Four treatments were set up, including: no carbon or phosphorus added (CK), adding phosphorus (+P), adding carbon (+C), and added carbon and phosphorus (+C+P), glucose was used as carbon source. After 30 days of growth, rape plants were harvested for measurement of aboveground biomass, and P content, the soil was sampled for measurement of microbial biomass P content (MBP), and phosphatase activity. Additionally, we analyzed the contents of four phosphorus components in the soil—CaCl₂-P, Citrate-p, Enzymatic-P, and HCl-P, using the bioavailability-based phosphorus classification method (BBP).

Results In low-P soil, adding P favored the growth and P uptake of rape, the rape biomass in +P and +C+P treatment was 6.48 and 4.21 g/pot, which was 2.39 and 1.20 times higher than the CK, and the P contents were increased by 71.43% and 58.33%. In high-P soil, addition of C and P (+P、+C、+C+P) did not significantly increase rape growth, but enhanced the plant P content, with increment of 30.55%, 33.45%, and 42.52% in +P, +C, and +C+P treatment, respectively. In low-P purple soil, the addition of carbon and phosphorus significantly increased Enzyme-P levels by 2.76 times on average, +P and +C+P treatment increased CaCl₂-P content by 24.76% and 27.72%, and +C enhanced Citrate-P content, but no any treatment showed significant effect on HCl-P. In high-P purple soil, all the treatments decreased CaCl₂-P content, especially +C treatment resulted in 34.45% of drop, relative to that of CK; and compared to CK, +P, +C, and +C+P treatment increased Citrate-P significantly by 12.50%,17.43% and 24.47%. In low-P soil, +P, +C, and +C+P treatment increased soil MBP and phosphatase activity by 28.80%−59.39% and 5.96%−6.69%, while in high-P soil, only +C and +C+P treatment increased MBP by 35.65% and 37.08%, phosphatase by 5.33% and 7.76%.

Conclusions In low-P soil, adding P could increase the growth and P uptake of rape, while in high-P soil, adding P could only lead to the luxurious P uptake of rape. Adding phosphorus and carbon could improve phosphorus bioavailability of low-P soil. However, the response of soil phosphorus bioavailability to external carbon is more significant in high-P soil. Therefore, in low phosphorus soils, both carbon and phosphorus should be added to enhance the effect of fertilizer, while in high phosphorus soils, organic carbon should be supplemented to improve the bioavailability of phosphorus.