Phosphorus Fractionation and Sorption Characteristics of Biochar Amended Soils of Ghana
Joseph Osafo Eduah1, Henrik Breuning-Madsen2, Mark Abekoe1, Mathias Neumann Andersen3
1University of Ghana, Dept. of Soil Science, Ghana
The high phosphorus fixing capacity of tropical soils including Ghanaian soils has restrained the development of economically sustainable crop production. The application of biochar can change surface chemical properties of highly weathered tropical soils, and hence affect phosphate sorption and distribution in soils. However, little information is available on the effect of biochar at varying levels of carbonisation on the retention, bioavailability and fractions of phosphate in tropical soils of Ghana. In the present study, incubation studies were conducted for 60 days to investigate the effect and mechanism of corn cob and rice husk biochar on P sorption and fractionation in two acid and one alkaline soil. The biochars were produced at varying pyrolytic temperatures (300°C, 450°C and 650°C) and applied at a rate of 1% (w/w) to the soils. Phosphorus fractionation of biochars and soil-biochar mixtures was assessed by a modified Hedley method whiles sorption of P was studied by fitting the equilibrium solution and adsorbed P concentrations using Langmuir and Freundlich sorption isotherms. Amending the acid soils with biochar increased the equilibrium P concentration in solution significantly with decreasing pyrolytic temperature for the two biochar types. There was however, an increase in P sorption with increasing charring temperatures in the alkaline soil. The interaction of biochar with soils resulted in an increase in the readily available P (Resin-P & NaHCO3-Pi) making P more available for plant uptake. The increase in the readily available P pool was more significant at relatively lower temperature (300°C) than higher charring temperatures for both biochar types. Calcium-bound P (HCl-P) of the soils increased sharply upon biochar addition but the Al & Fe-bound P (NaOH-Pi, moderately labile P) decreased. These changes suggest that the increase in P sorption in the alkaline soil with biochar addition could be due to Ca-induced P sorption or precipitation and was less affected by Fe and Al oxides. Biochar effects on soil phosphorus were aligned with characteristics of biochar and biochar-soil mixtures (PAS-FTIR, XRD etc.). The study thus showed that biochar pyrolysed at 300-450°C could be used to reduce P sorption and increase P bioavailability especially in acid soils.
Keywords: Acid soils, adsorption, akaline soil, biochar, desorption, fractionation, phosphorus
Contact Address: Mathias Neumann Andersen, Aarhus University, Dept. of Agroecology and Environment, Aarhus, Denmark, e-mail: MathiasN.Andersenagrsci.dk