Long-term biochar addition significantly decreases rice rhizosphere available phosphorus and its release risk to the environment

Abstract Phosphorus (P) availability, diffusion, and resupply processes can be altered by biochar addition in flooded rice rhizosphere, which controls the risk of P release to the environment. However, there are few in-situ investigations of these rhizospheric processes and effects. To explore the effects of biochar addition on soil P availability, high-resolution dialysis (HR-Peeper), diffusive gradients in thin films (DGT), and zymography techniques were used to provide direct evidence in the rice rhizosphere at the sub-millimeter scale. Long-term (9-years) field and greenhouse pot experiments demonstrated that biochar addition notably decreased the soluble/labile P and Fe concentrations in rice rhizosphere (vs. no biochar addition; CK) based on the results of Peeper, DGT, and two-dimensional imaging of labile P fluxes. DGT-induced fluxes in the soil/sediment (DIFS) model and sediment P release risk index (SPRRI) further indicated that biochar addition decreased the diffusion and resupply capacity of P from soil solid to the solution, thereby decreasing P release risk to the environment. These processes were dominated by Fe redox cycling and the hydrolysis of Al (hydro)oxides that greatly increased the unavailable P (Ca-P and residual-P). Additionally, greenhouse pot experiments (without additional biochar) showed that the previous long-term biochar addition significantly increased soil phosphatase activity, due to an adaptive-enhancing response to P decrease in the rhizosphere zone. The in-situ study on the biogeochemical reactions of P in the rice rhizosphere may provide a new and direct perspective to better evaluate the biochar addition and potential benefits to agricultural soils. Graphical Abstract