Heterogeneous co-supplies of phosphorus, sulfur and carbon enhance phosphorus availability by improving rhizosphere processes in a calcareous soil

Background and aims: Phosphorus (P) availability in calcareous soils is limited. Acidifying the soil by adding sulfur (S) or enhancing microbial activity by adding carbon (C) can increase P bioavailability. However, understanding of the impact of the combined application of P, S and C on P availability in calcareous soils is lacking. In this study, we investigated the effects of heterogeneous (co-localized) supply of different combinations of P, S and C on the maize (Zea mays L.) shoot and root growth and soil P availability in calcareous soil with low available P. Methods: A rhizo-box experiment with heterogeneous supply of P, S and C was conducted to assess the responses of maize growth, root morphology and physiology, as well as soil P availability. Results: Heterogeneous low P (P1) with S supply significantly increased shoot and root biomass, shoot P content and P-use efficiency (PUE) compared to homogenous or heterogeneous P1 supply alone. Application of S decreased the rhizosphere soil pH and stimulated the proliferation of maize roots. By contrast, heterogeneous P1 and C supply did not change shoot biomass, P uptake or PUE but decreased root biomass. The addition of C, however, enhanced acid phosphatase activity and carboxylates concentration in the rhizosphere, and increased NaHCO₃-extractable P, implying enhanced P availability. Conclusion: Adding S combined with P is an effective approach to engineer the rhizosphere by increasing maize growth and PUE in calcareous soil, whereas adding C increased NaHCO₃-extractable soil P but did not influence maize shoot growth.