Combined application of biochar with fertilizer promotes nitrogen uptake in maize by increasing nitrogen retention in soil

Combined application of biochar with fertilizers has been used to increase soil fertility and crop yield.However,the coupling mechanisms through which biochar improves crop yield at field scale and the time span over which biochar affects carbon and nitrogen transformation and crop yield are still little known.In this study,a long-term field trial(2013-2019)was performed in brown soil planting maize.Six treatments were designed:CK-control;NPK-application of chemical fertilizers;C1PK-low biochar without nitrogen fertilizer;C1NPK,C_(2)NPK and C_(3)NPK-biochar at 1.5,3 and 6 t ha^(−1),respectively,combined with chemical fertilizers.Results showed that theδ^(15)N value in the topsoil of 0-20 cm layer in the C_(3)NPK treat-ment reached a peak of 291‰at the third year(2018),and demonstrated a peak of 402‰in the NPK treatment in the initial isotope trial in 2016.Synchronously,SOC was not affected until the third to fourth year after biochar addition,and resulted in a significant increase in total N of 2.4 kg N ha^(−1) in 2019 in C_(3)NPK treatment.During the entire experiment,the ^(15)N recovery rates of 74-80%were observed highest in the C_(2)NPK and C_(3)NPK treatments,resulting in an annual increase in yields significantly.The lowest subsoilδ^(15)N values ranged from 66‰to 107‰,and the ^(15)N residual rate would take 70 years for a complete decay to 0.001%in the C_(3)NPK.Our findings suggest that biochar compound fertilizers can increase C stability and N retention in soil and improve N uptake by maize,while the loss of N was minimized.Biochars,therefore,may have an important potential for improving the agroecosystem and ecological balance.