Objectives The inherent physical and chemical properties of ordinary biochar render it incapable of exhibiting a notable phosphorus adsorption capacity. This study attempted to enhance the phosphorus adsorption capacity of biochar by iron modification, so as to enhance the phosphorus storage capacity and reduce the environmental risk caused by soil phosphorus loss.

Methods The biochars used in the study include walnut shell biochar (WSB), rice straw biochar (RSB), and wood biochar (WB). First, 10 g of each biochar was soaked in 1 mol/L HCl for 1 hour, washed with distilled water until the filtrate was neutral, and then dried. The biochar was then added to a 1 mol/L FeCl₃ solution, with mass ratios of iron to biochar set at 0.28, 0.56, and 0.84. After standing, filtering, drying, and calcination, the iron-modified biochars were obtained and labeled as WSB-0.28, WSB-0.56, WSB-0.84, RSB-0.28, RSB-0.56, RSB-0.84, WB-0.28, WB-0.56, and WB-0.84, respectively. The modified biochars were characterized using scanning electron microscopy, energy dispersive spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction, automatic surface area and pore size analyzer, and elemental analysis. The biochars were added to soil at ratios of 2%, 4%, and 6% (by mass) for phosphorus adsorption-desorption experiments. Biochars with strong phosphorus regulation capacity were selected for petunia cultivation experiments.

Results Fe₂O₃ was successfully loaded onto the surface of the three types of biochar, significantly increasing the number of adsorption sites, especially the phosphorus adsorption capacity of WSB-0.28 was significantly higher than that of WSB. The amount of phosphorus adsorbed by the soil increased with the addition of iron-modified biochar. At the same addition level, the phosphorus adsorption capacity of the soil followed the order: rice straw biochar>wood biochar>walnut shell biochar (except for WSB-0.28). Iron-modified wood biochar showed higher phosphorus desorption amount and desorption rate than the other biochars. Pot experiments indicated that phosphorus-enriced iron modified walnut shell biochar could promote the increase of root and leaf biomass of petunia seedlings, as well as enhance leaf chlorophyll content and flowering.

Conclusions Iron modification can enhance the phosphorus adsorption capacity of biochar, improving soil phosphorus retention, increasing phosphorus availability, and promoting plant growth. Therefore, modified biochar can be used as a novel carbon-based slow-release fertilizer in urban green space soils, contributing to efficient phosphorus retention and reducing the environmental risk of soil phosphorus loss.