What is more important for enhancing nutrient bioavailability with biochar application into a sandy soil: Direct or indirect mechanism?

► Direct or indirect mechanisms accounting for the enhancing nutrient bioavailability with biochar application are evaluated in the present study. ► Direct release form biochar is important for cations (e.g. K, Ca, Na, Mg) bioavailability with biochar addition. ► The SEM-EDX analysis indicated that mineral sorption on biochar surface influenced P sorption. ► The direct P release altered soil pH and changes of Fe, Al oxides and accumulation of Ca controlled P bioavailability with biochar application. ► The long term effect of P bioavailability with biochar application deserves further attention. As a soil amendment and important bio-resources, biochar has been reported to increases in soil nutrient availability. However, the involved direct or indirect mechanisms are still not clear. In present study, we evaluated the effects of four biochar application rates (0, 1, 5 and 10%, w/w) on inorganic P fractions, available NO3−-N, NH4+-N, K, Na, Ca, Mg and of Fe, Al oxides in a sandy soil. To discern direct and indirect mechanisms, separately soil and biochar equivalent to the respective part in the mixture of soil and biochar were also studied. The results suggested that cations such as K, Ca, Na, Mg in mixture were mainly originated form biochar while inorganic N showed little effects form biochar regardless of direct or indirect mechanisms. P solubility with biochar application was more complex because they also affected by altering soil pH, changes of Fe and Al oxides, and direct P contributions from biochar. In addition, changes of Fe and Al oxides were complicated by altering soil pH, sorption on biochar, and direct release form biochar. In result, Fe and Al oxides showed no observable effects on P fractions in our soil type. Although P availability was enhanced due to the pH changes and direct release from biochar, the observed P availability was greatly lower than the predicted P availability because of the precipitation of large amount of Ca contained in biochar with soluble P in mixture. The results suggested that biochar application, resulting in pH changes and mineral sorption, can indirectly change nutrient bioavailability especially for P apart from direct release form biochar. The long term effect of P availability with biochar application deserves further attention for large-scale soil ecological restoration.