Herein, biochars from 6 different feedstocks (taro straw, corn straw, cassava straw, Chinese fir straw, banana straw, and Camellia oleifera shell) were produced using magnesium chloride (MgCl2) as a modifier due to their sorption behavior toward NH4+-N and phosphorus in an aqueous solution. The biochar characteristics were evaluated, including pH, pHPZC, biochar magnesium content, and total pore volume (PVtot). The experimental results in terms of the kinetics and equilibrium isotherms showed that the cassava straw and banana straw biochars exhibited the theoretical maximum saturated adsorption capacities of 24.04 mg.g(-1) (NH4+-N) and 31.15 mg.g(-1) (TP), respectively. Biochar produced from these feedstocks had higher magnesium contents and greater total pore volumes, reflecting the significant contributions from magnesium and steric effects. FTIR, XRD, and SEM/EDS analyses demonstrated that NH4+-N and TP sorption mechanisms predominantly involved surface electrostatic attraction, Mg2+ precipitates and complexation with surface hydroxyl functional groups.