Effect of biochar on P uptake from two acid soils

Biochar may increase P availability in acid soils via direct P addition from biochar and indirect effects through soil processes. A pot trial was carried out to examine the effects of incubated and non-incubated poultry litter (Pl) and rice husk (Rh) biochars on maize ('Zea mays') growth and P uptake from acid soils (Tenosol and Ferrosol). Biochar was mixed with soil at a rate of 10 ton/ha and P was applied at 0, 5, or 50 mg P kg⁻¹ as superphosphate such that rates were part way up a pre-determined P response curve for maize. Appropriate, non-P limited controls were also established for each soil type. Compared to Rh biochar, Pl biochar treatments on both soils resulted in higher plant biomass and P uptake but lower P recovery as a proportion of P applied. Phosphorus uptake on non-incubated Pl-amended Ferrosol and Tenosol respectively was 9 and 7 times greater than on both Rh-amended soils. Incubation resulted in sorption of applied P and reduced bioavailability, and there was no indication that biochar reduced P sorption in Ferrosols. Interestingly, P uptake was greater from a combination of P and Rh biochar than when either was applied alone, providing evidence of synergistic benefits of biochar application. These benefits may have arisen due to the liming effect of biochar, to competitive inhibition of P sorption, and/or modification of the soil physical environment and further research is warranted.