Joseph Osafo Eduah

Phosphorus Reactions in Three Contrasting Soils Amended with Biochar

The reactive nature of phosphate leads to the formation of insoluble Fe, Al and Ca bound P compounds in highly weathered tropical soils. Biochar amendments can change the surface chemical properties of highly weathered tropical soils, and hence affect phosphorus retention and distribution in soils. The overall objective of this study was to investigate P reactions with biochar and biochar amended soils of Ghana for effective P management. To achieve this, three major experiments were conducted: (1) phosphorus adsorptive characteristics of different biochar types namely cocoa pod husk, rice husk, corn cob and palm kernel shell biochar produced at two pyrolysis temperatures (300 oC and 650 oC); (2) phosphorus sorption and desorption studies on three constrasting soils amended with corn cob and rice husk biochar produced at 300 oC, 450 oC and 650 oC and  (3) phosphorus fractions in corn cob and rice husk biochar produced at three pyrolysis temperatures (300 oC, 450 oC and 650 oC) as well as P fractions in biochar amended soils. In experiment 1, P removal ability of four biochar types namely cocoa pod husk, rice husk, corn cob and palm kernel shell produced at two pyrolysis temperatures (300 oC and 650 oC) was investigated by using series of batch experiments. Sorption isotherms and kinetics models were used to assess the removal ability and mechanism of P adorption on the biochar types. The effect of equilibrium pH on P sorption was also examined. Biochar types attained maximum P adsorption within equilibrium time of 6 to 12 h. The maximum adsorption capacity calculated from the Langmuir isotherm was from 4.12 to 13.02 mg g-1 in an increasing order of CP300 < CC300 < RH300 < CP650 < PK300 < CC650 < RH650 < PK650 for the studied P concentration ranged of 25- 200 mg P L-1. Generally, biochar types produced at 650 oC had higher P adosprtion capacity than at 300 oC. The equilibrium pH for maximum P adsorption varied among biochar types occurring in a range of 2.8 to 4.8. Increasing equilibrium pH decreases P adsorption. Freundlich isotherm coupled with pseudo second order and Elovich models explained the adsorption data well indicating a chemisorption process on heterogeneous surface of biochar involving ligand exchange (PK300, PK650, CP650), electrostatic attraction (RH300, CC300) and surface precipitation (CP300, CC650, RH650). Palm kernel shell biochar at 650 oC with the highest P sorption capacity should be the preferred choice for removal of the anion from wastewater. The CP300 having the least P adsorption capacity and with its high total P content, alkaline pH and the presence of carbonate could be exploited for use as a liming material on acid soils. In experiments 2 and 3, incubation studies were conducted for 80 d to investigate the effect of corn cob and rice husk biochar produced at three pyrolysis temperature (300 oC, 450 oC and 650 oC) on P sorption characteristics and fractions in two acid and one neutral soil. The P sorption capacity of the two acid soils i.e. Kokofu Series (384.6 mg kg-1) and Ankasa Series (333.3 mg kg-1) were about three fold more than the neutral soil (Keta Series) (104.2 mg kg-1). Amending the acid soils with biochar increased the equilibrium P concentration in solution at decreasing pyrolysis temperature for the two biochar types. There was, however, an increase in P sorption with increasing pyrolysis temperature in the neutral soil. The amount of P desorbed increased in the acid soils but decreased in the alkaline soil. Biochar produced at 300 oC had more significant effect on both the decrease in P adsorption and increase in P desorbability in the acid soils. The results of modified Hedley P fractionation showed that the most labile P (Resin-P, NaHCO3-Pi) and organic P pool showed a decreasing trend with increasing pyrolysis temperature in the biochar types. However, the Ca-bound P (HCl-Pi) and residual P increased with increasing pyrolysis temperature. The interaction of biochar with soils resulted in an increase in the most labile P as well as moderately P (NaOH-Pi) fractions in the three soil types making P more available for plant uptake. The increase in the readily available P pool was more significant at relatively lower temperature (300 oC) than higher pyrolysis temperatures for both biochar types. However, the increase in calcium-bound P and residual P of the soils was more predominant when both biochar types produced at 650 oC were applied. The study thus showed that biochar pyrolysed at 300-450 oC could be used to reduce P sorption and increase P bioavailability in acid soils. The addition of biochar to near neutral soils might increase P retention possibly in the short-term, reducing P bioavailability.