Four-year continuous residual effects of biochar application to a sandy loam soil on crop yield and N2O and NO emissions under maize-wheat rotation

Biochar application has been reported to effectively mitigate N₂O emissions and increase crop yield; however, its long-term residual effects remain largely unknown. A four-year field experiment was designed in the North China Plain (NCP) to evaluate the residual effect of biochar on N₂O and NO emissions and crop yield under maize-wheat rotation. The study included five treatments: no N fertilization (Control), N fertilizer application (CN), and N fertilizer plus maize-straw biochar application at 3 (NB3), 6 (NB6) and 12 t ha⁻¹ (NB12). Biochar amendment had no effect on maize yield during the four-year rotation, but increased maize grain N uptake by 12.9 − 14.1 % and fertilizer N use efficiency by 13.8 % in the fourth year. Meanwhile, NB12 treatment decreased wheat yield by 11.7 − 15.5 % in each year, except the second, while decreased average wheat yield. Biochar application significantly reduced N₂O emissions by 31.5 − 42.4 % during the first maize season, and by 6.9 − 21.3 % in the third and fourth years primarily due to reduction in NH₄⁺ availability for nitrification. Furthermore, this mitigation effect was positively related to soil moisture content and decreased with time after biochar application. Cumulative N₂O emissions during the four wheat seasons were 0.25 − 0.85 kg N₂O-N ha⁻¹, and only decreased by 8.9 − 9.9 % under biochar amendment in the third and fourth years probably because low soil temperature suppressed N₂O production. Biochar addition in the third and fourth years significantly reduced NO emissions by 9.0 − 20.1 % during the maize growing seasons and 10.5 − 19.1 % during the wheat growing seasons, especially under NB6 treatment. The average yield-scaled N₂O and NO emissions were lowest in the NB6 treatment during the maize and wheat season. Overall, these findings suggest that 6 t ha⁻¹ is the optimal biochar application rate for reducing N₂O/NO emissions and yield-scaled N₂O/NO emission under four-year maize-wheat rotation in the NCP.