Phosphorus Release from Various Biochar Feedstocks: Implications for Land Management.

Biochar is obtained by biomass heating in the complete absence or low concentrations of oxygen, through the process known as pyrolysis. The diversity in biochar feedstocks may confer different abilities in desorption of phosphorus (P). Therefore, biochar application at the same rate to soils would have different P release properties that are a function of the biochar source and the retention capacity of the soil. Our objective was to evaluate P associations in different biochars as well as its desorption when mixed with distinct soils. Mineralogical assessment (XRD) and elemental association (SEM-EDS) were performed in conjunction with desorption experiments in order to assess the mechanisms by which P is held. The desorption experiment was performed by mixing 1% (w/w) biochar from feedstocks of poultry litter (PLB), biosolids, mixed hard woods (HWB) and pure maple with two contrasting soils followed by 20 extractions using 0.01 M KCl. The total P concentrations were: 25615, 67330, 1898 and 730 mg kg^-1 for PLB, biosolids, HWB and maple biochars, respectively. Magnitude and pattern of P release depended on the biochar and soil type. The mineral whitlockite in PLB was indicated by XRD and corroborated by SEM-EDS analyses; solution analysis indicated a long-term P release consistent with the sparingly soluble mineral. Association between Mg and P in biosolids biochar was evident from SEM-EDS, consistent with XRD indication of struvite in the raw biosolids. No associations between P and other elements could be confirmed for HWB or maple biochars. All biochars released P rapidly initially, but levelled off after the first few extractions. Based on solid phase and solution chemistry, field application of biochar should take into consideration both the biochar and soil type to minimize environmental problems of over application of P.