Phosphorus Sorption Behavior in Biochar-Amended Soils.

Biochar as a soil amendment is reported to improve soil health and crop yield. However, if biochar addition to a soil results in release of nutrients in excess of plant uptake, the nutrient could be lost from the soil to nearby aquatic systems. This could particularly be a problem with phosphorus (P) that causes eutrophication. Phosphorus is known to be released from the soil when the threshold P saturation ratio, or PSR, (the molar ratio of P to [Fe+Al]) reaches 0.1.  We evaluated P dynamics in two soil types, one from Florida, USA, and the other from Karnataka, India, following addition of biochar prepared from different raw materials.  These included hardwood biochar from Casuarina equisetifolia inoculated with microorganisms (BCX) and also without added microrganism, procured from Bangalore, India, and a commercially available biochar from USA. Three soils of varying P content from Florida and three soils from coffee agroforestry systems from Koppa, Karnataka, India were used. The soil and soil-biochar (2% w/w) mixtures were incubated with P concentrations ranging from 0 to 100 mg L^-1 for two weeks. Post incubation, the equilibrium P concentrations were determined and plotted against the externally added P. Irrespective of the biochar source, P began to be released from the soil when the threshold PSR was reached. Biochar-amended soil samples with the highest level of P addition were analyzed by x-ray diffraction to identify the specific mineralogy after the two-week incubation. Biochar-amended soils showed significantly lower P release at higher levels of externally added P. The X ray diffraction patterns did not identify any major stable phosphate minerals. This indicates that P held with biochar addition could be a temporary phase, possibly related to its moisture holding capacity. The results suggest that, for enhancing plant uptake of P and reducing environmental problems, the soil type needs to be taken into consideration while deciding on field application of biochar.