Influence of Biochar Carbon Thermal Stability and pH on Phosphorus Sorption in Sandy Soils.

Phosphorus (P) eutrophication is a major pollution problem globally, with unprecedented amount of P emanating from agricultural sources. But, little is known about the optimization of soil-biochar P sorption capacity. The objective of this study was to determine how biochar feedstocks and pyrolysis condition influences carbon (C) thermal stability and pH and in turn influence the phosphorus sorption optimization. Biochar was produced from switchgrass, kudzu and Chinese tallow at 200, 300, 400, 500, 550, 650,750 oC. Biochar biogeochemical properties were determined and C thermal stability was determined by multi-element scanning thermal analysis (MESTA). Phosphorus sorption was determined using a mixture of 10% biochar and 90% sandy soil after incubation. Results indicated that biochar pH, high thermal stable C and P sorption increased but, available P, low thermal stable C and biochar recovery decreased with increasing temperature. The total biochar C increased with increasing pyrolysis temperature ranging from 52% at lowest temperature and 93% at highest temperature in the order of Chinese tallow> switch grass> Kudzu. Available P decreased with increased pyrolysis temperature and correlated negatively with P sorption (R2= -0.63; P<0.0001) for all feedstocks. There was a positive relationship between high thermal stable C (>400oC) and P sorption for Kudzu (R2= 0.62; P=0.0346) and Chinese tallow (R2= 0.73; P=0.0138), while an increase in the low thermal stable carbon indicated a negative trend on P sorption. pH increased with increasing pyrolysis temperatures for all feedstocks ranging from 7-12.3 in the order of kudzu>switchgrass>Chinese tallow. pH correlated positively with P sorption (R2= 0.7; P<0.0001) suggesting an influence of calcium on P sorption. In conclusion, optimization of biochar thermal C stability and pH is an important strategy for P eutrophication mitigation especially in acidic soils.