Surface soil mixing is more beneficial than the plough layer mixing mode of biochar application for nitrogen retention in a paddy system

The benefits of biochar as a soil amendment have been investigated extensively, but few studies have considered the effects of different application modes on nitrogen (N) dynamics. In the present study, a pot experiment was conducted to evaluate the effects of two different biochar application modes [plough layer mixing (PLM) and surface soil mixing (SSM)] on the N dynamics in a paddy system. It was found that biochar application significantly reduced the total N (TN), NH₄⁺-N, and NO₃^--N contents of the paddy surface water after fertilization, and that the SSM mode of application was more effective in doing this, particularly for NH₄⁺-N, reducing TN by 11-76%, NH₄⁺-N by 31-77%, and NO₃^--N by 31-60% compared with the control at 7 days after fertilizer dressing. By contrast, the effect of biochar application on soil N varied with biochar application mode, N form, and rice growth period. In general, there was no significant effect of biochar type on soil N content, with both types of biochar resulting in a higher TN content of the soil after the tiller stage compared with the control. In addition, the SSM mode of application led to a higher TN content but lower NH₄⁺-N content of the soil than the PLM mode, while the two application modes had varying effects on the NO₃^--N content depending on the growing period of the rice. The rice grain yield increased by 25-36% with the SSM application mode and 11-14% with the PLM mode. These findings indicate that the SSM mode of biochar application in paddy soils is a more promising strategy for both reducing the risk of N loss and improving rice yield than PLM mode.