Effects of magnesium-modified biochar on soil nitrogen leaching and growth of Chinese cabbage

The overuse of nitrogen fertilizers, most commonly urea, is ultimately having a serious impact on the environment through nitrogen leaching. Biochar addition is considered to be a promising measure for mitigating nitrogen leaching. However, whether biochar has a long-term effect is unclear. Moreover, Mg-modified biochar (Mg-biochar) has more excellent physicochemical and surface properties than original biochar, and it has been widely used in water pollution remediation; however, its effect on soil nitrogen leaching is still uncertain.

The optimal Mg-biochar was selected by adsorption test. In the subsequent soil column simulation experiment, a total of five treatment groups: nitrogen fertilizer (CK), nitrogen fertilizer + fresh biochar (BC), nitrogen fertilizer + Mg-biochar (MGBC), nitrogen fertilizer + freeze-thaw aged biochar (DBC), and nitrogen fertilizer + freeze-thaw aging Mg-biochar (DMGBC) were set up to explore the distribution of nitrogen in the soil-leaching solution-plant system.

In this study, biochar modified by 2 mol/L MgCl₂ was chosen to be the optimal Mg-biochar. Compared with the CK, BC reduced the soil nitrogen leaching by 16.96% and increased the soil nitrogen content and cabbage nitrogen absorption by 1.65% and 114.19%, respectively. Mg-biochar had higher nitrogen retention ability than original biochar, and after freeze-thaw aging, biochar maintained or even improved nitrogen retention ability.

The biochar addition had good nitrogen retention ability, and Mg-biochar performed better, and they showed long-term effect on mitigating soil nitrogen leaching. Meanwhile, based on the law of conservation of matter, biochar addition also reduced gaseous nitrogen loss.