Iron-modified biochar: from phosphate adsorption capacity to fertilizing potential evaluated in ryegrass pot trials

All of the 3 main plant macronutrients (nitrogen, phosphorus and potassium) have seen a rapid increase in their fertilizer use in food production and each of them comes with specific challenges. Focusing on chemical phosphorus fertilizers implies pondering the environmental challenges they pose: ranging from phosphorus deficiency where their application is lacking, to soil leaching and eutrophication where their input is too high, and resource depletion, given phosphate rocks being a nonrenewable source. Solving these challenges requires up-to-date policies (which the EU is trying to adopt) facing limitations in accessibility and reusability of phosphorus and adopting better fertilizers in the agronomical sector. Biochar is explored as a possible new type of phosphorus fertilizer: produced from pyrolysis, it is a very stable and porous material offering potential for strong phosphorus adsorption and its slow release in soil, giving a possible solution for phosphorus soil leaching. To achieve this, biochar hardly can be used as is: iron doping was explored to obtain a strong chemical bond between the material surface and phosphorus. To study the iron doping process of biochar and the difference in adsorption potential between the raw and doped versions of the material, a series of analyses were used. These included Scanning Electron Microscopy, Energy-Dispersive Spectrometry, Fourier Transform Infrared Spectrometry, X-Ray Diffraction, Thermogravimetric Analysis, Inductively Coupled Plasma Spectroscopy – Optical Emission Spectroscopy, and phosphorus adsorption tests carried out both on raw and iron doped biochars. A greenhouse plant test using Ryegrass in pot was used to verify the agronomical behavior of raw and iron doped, phosphorus adsorbed biochars. The results showed potential for applying the doped material as a slow-release phosphorus fertilizer. However, longer-term studies are needed such crop-field tests to have a better picture of biochars’ behavior.