Engineered biochar improves nitrogen use efficiency via stabilizing soil water-stable macroaggregates and enhancing nitrogen transformation

The use of inorganic nitrogen (N) fertilizers has increased drastically to meet the food requirements of the world's growing population. However, the excessive use of chemical nitrogen fertilizer has caused a series of soil and environmental problems, such as soil hardening, lower nitrogen use efficiency (NUE), nitrate pollution of water sources, nitrous oxide emissions, etc. In this review, we aimed to elaborate and discuss the role of engineered biochar in inducing the stability of water-stable macroaggregates, improving inorganic N transformation, and utilization efficiency to address the current uncertainties of nitrogen loss and maintaining soil and water quality. Firstly, we elucidated the characteristics of engineered biochar in improving biochar quality to work as a multifunctional player in the ecosystem and promote resource utilization, soil conservation, and ecosystem preservation. Secondly, we discussed how the engineered biochar modulates the stability of water-stable macroaggregates and soil inorganic nitrogen transformation to enhance plant response under various toxic or deficient nitrogen conditions in the soil. Thirdly, the role of engineered biochar in biological nitrogen fixation, mediating nirK, nirS, and nosZ genes to promote the conversion of N2O to N2, and decreasing denitrification and N2O emission was reviewed. Altogether, we suggest that engineered biochar amendment to soil can regulate soil water-stable macroaggregates, reduce N input, improve nitrogen metabolism, and finally, NUE and crop growth. To the best of our knowledge, this is the first time to evaluate the combined interactions of "engineered biochar x soil x NUE x crop growth," providing advantages over the increasing N and water utilization and crop productivity separately with the aim of enhancing the stability of water-stable macroaggregates and NUE together on a sustainable basis. Excessive application of nitrogen (N) chemical fertilizers causes N loss and lower nitrogen use efficiency.Engineered biochar can build micro and macro soil structures and induce the stability of water-stable macroaggregates.Engineered biochar can improve biological nitrogen fixation, promote the conversion of N2O to N2, and decrease denitrification and N2O emissions.Engineered biochar compensates for high and low N stress by regulating N metabolism, transformation, and remobilization in plants.Engineered biochar improves NUE by reducing N input and denitrification and enhancing soil water-stable macroaggregates and N transformation.