Phosphorus availability changes in acidic soils amended with biochar, fly ash, and lime determined by diffusive gradients in thin films (DGT) technique.

The lime, biochar, and fly ash are widely used to ameliorate acidic soils. The purpose of our pot experiment was to understand whether these soil amendments affected the availability of phosphorus in soils using diffusive gradients in thin films (DGT) technique. Three soil amendments, biochar (B), fly ash (F), and lime (L), were applied alone or combined to Ultisol and Alfisol at the rate of 40, 40, and 6 g/kg, respectively. The DGT and chemical extraction techniques were used to quantify the soil available (phosphorus) P dynamic in the rice growth period. The addition of amendments can effectively increase the phosphorus content extracted by 0.01 M KCl solution. The biochar amendment showed the best effect on increasing the contents of DGT-P and Olsen-P in the soil. Combined with the DGT-induced fluxes in sediments (DIFS) model, it was found that soil amendments mainly increased the adsorption rate (k) and desorption rate (k-1), and reduced soil reflect time (Tc) of phosphorus released to the pore water. The P contents accumulated in rice grains correlated more closely with the effective P concentration measured by DGT technique, compared with those with soil total P, pore water P, and Olsen-P. These results suggested that the DGT technique had significant advantage over conventional chemical extraction techniques when assessing the bioavailability of phosphorus to rice growth. Application of soil amendments increased the contents of Al-P and Ca-P of both Ultisol and Alfisol, and decreased the content of Fe-P in soils. The addition of amendments could change the affinity of Fe and phosphorus decreasing the phosphorus fixation in soils. The increase of available phosphorus in soil is mainly due to the change of pH value and the direct diffusion of phosphorus in the amendments (biochar and fly ash). In conclusion, soil amendment can affect the availability and adsorption ability of phosphorus in soil, and biochar has the best effect on the availability of soil phosphorus.