Enhancing phosphorus availability in biochar: Comparing sulfuric acid treatment to biological acidification approaches

BackgroundThe use of sulfuric acid (SA) to acidify biochars is known to enhance their phosphorus (P) fertilizer value. Potentially, biological approaches such as lowering the pH of biochar by lactic acid co‐fermentation or applying biochar with a nitrification inhibitor (NI) to reduce rhizosphere pH are an alternative to SA.AimThis study aimed to evaluate the two methods for increasing plant P availability from two biochars and compare them with SA‐treated biochars (as a reference) in a pot experiment.MethodsMeat and bone meal biochar (MB‐C) and digestate solids biochar (DS‐C) were bio‐acidified (BA) by lactic acid fermentation with organic waste. The untreated, SA‐treated, BA biochars, and biochars co‐applied with a NI (3,4‐dimethylpyrazolephosphate) were tested in a pot experiment with maize.ResultsThe fermentation reduced the pH of the organic waste biochar mixtures to <4.3 and increased water‐extractable P (WEP) to 30% of total P. The untreated biochars had a mineral fertilizer replacement value of >50% and SA increased replacement values to ≈100%. The application of NI did not affect rhizosphere pH or P uptake. The BA MB‐C increased soil solution P concentration, but P uptake did not significantly increase. The application of the BA DS‐C raised soil pH and reduced plant P uptake and biomass.ConclusionThe untreated biochars showed considerable P fertilizer effectiveness, suggesting that acidification may not always be necessary. Rhizosphere acidification and the bio‐acidification of biochars were not effective in further increasing P uptake, despite higher levels of WEP.