<p>The efficient treatment of biogas slurry poses a significant challenge for resource recycling and environmental protection in biogas engineering, as the discharge of nitrogen (N) and phosphorus (P) from biogas slurry can cause secondary pollution. Biochar, with its porous structure, high specific surface area, and surface functional groups, shows significant potential for adsorbing N and P from biogas slurry. Although modification and activation of biochar can effectively enhance its adsorption performance, practical application is often constrained by factors such as feedstock material, preparation conditions, operational parameters, and production costs. This study evaluated three types of biochar derived from apple wood branches (AWB), corn cobs (CCB), and reed straw (RSB). Initial single-factor experiments determined appropriate ranges for biochar dosage and solution pH. Then, the Box-Behnken Design (BBD) of the Response Surface Methodology (RSM) was used to develop a three-factor, three-level optimization model. The adsorption mechanisms were investigated using Scanning Electron Microscopy (SEM) for morphology, Fourier Transform Infrared Spectroscopy (FTIR) for surface chemistry, complemented by modeling with kinetic (pseudo-first-order and pseudo-second-order) and isotherm (Langmuir and Freundlich) models. The optimal adsorption conditions for each biochar were determined. AWB relies on its porous structure for physical adsorption, pore filling and π–π interactions, offering high capacity but weak binding strength. CCB removes ammonia nitrogen possibly through ion exchange, and total phosphorus via a “hydrolysis-complexation” mechanism involving chemical precipitation and ligand exchange. RSB exhibits strong chemical adsorption toward ammonia nitrogen via ion exchange and surface complexation with oxygen-containing groups, but shows limited capacity for total phosphorus.</p>