Relative influence of soil- vs. biochar properties on soil phosphorus retention

• Biochars can constitute a P source when applied to minimally-P-impacted sandy soils.
• Biochar enhanced P sorption at elevated solution concentrations may be reversible.
• Soil- rather than biochar properties dictate environmentally sound P loading rates.

Properties of biochar are thought to determine whether phosphorus (P) sorption or increased P availability occur following biochar application to sandy acidic soil; the effect of soil properties on P retention in biochar-amended acid soils remains largely unexplored. Our objective was to determine effects of hardwood biochar and poultry litter biochar on P sorption and release from two soils differing in P retention properties at three rates of biochar addition. Soils as well as soil + biochar mixtures were treated with 12 levels of solution P concentrations. Phosphorus concentration and the amount of P sorbed for each level of P addition at the end of the incubation showed that after P addition corresponding to the soil P storage capacity (SPSC) as determined prior to biochar application, P retention abruptly declined for all treatments irrespective of biochar type. Biochar did not diminish the capacity of the two soils to tenaciously bind P added as a soluble inorganic source. Also, the maximum P retention capacity of the soil (Smax) increased as amount of biochar applied increased. Biochar-enhanced P sorption at high solution concentrations would be environmentally beneficial only if the sorption were strongly hysteretic such that subsequent P release is minimal as concentrations approach background levels. X-ray diffraction analysis of the biochar after incubating at high P solution concentration did not reveal formation of a stable crystalline P phase. This study provides evidence that the amount of P that can be “safely” added to soils amended with biochar depends significantly upon the P retention property of the soil, and not only the biochar characteristics.

Figure optionsDownload as PowerPoint slide