N-enriched biochar with an optimized C:N ratio enhances crop growth by regulating enzymatic activities, soil properties, and photosynthetic efficiency under varying N fertilization

Biochar is widely used to enhance soil fertility and productivity, but the role of its C:N ratio and its interaction with varying nitrogen (N) application rates remains poorly understood. This study evaluated the effects of N-enriched biochar (NB) with different carbon-to-nitrogen (C:N) ratios at various N levels on the N fractions, growth, and yield of maize and wheat. Three kinds of biochar [pristine biochar (PB) and two NB types (NB1 and NB2)], along with a control (without biochar; CK), were applied at 100% (N100), 70% (N70), and 40% (N40) of recommended N fertilizer. NB1 and NB2 significantly increased NO 3 ⁻-N, available N (AN), N uptake efficiency (NUPE), and N partial factor productivity (NPFP), irrespective of N level. At N70 level, NB1 significantly improved soil organic matter (SOM) by 20.5% in maize and 18.5% in wheat at maturity, compared to CK-N70. Under N70, NB1 increased N-acetyl-glucosaminidase (NAG) and leucine aminopeptidase (LAP) activities by 114.9% and 98.5% at tasseling, and 68.0% and 64.3% at maturity in maize, respectively, as compared to CK-N70. In wheat, NAG and LAP activities rose by 70.1% and 47.1% at heading, and 30.1% and 42.0% at maturity under NB1-N70. The greatest increase in grain yield and harvest index (HI) was observed with NB1 at N70. NB also enhanced photosynthetic efficiency and chlorophyll content even at N70. NB1 at 70% of the recommended N rate improved soil N dynamics, enzymatic activities, and photosynthetic efficiency, ultimately enhancing crop growth and yield. These findings highlight the potential of NB to reduce dependence on chemical N fertilizers while maintaining crop productivity. Long-term and multi-site studies are required to confirm its effectiveness under diverse environmental conditions.