Leaf Color Chart-Based Nitrogen Management Affects Rice Enzyme Activities and Maintains Soil Nitrogen Balance

Real-time nitrogen (N) management based on the leaf color chart (LCC) is considered a potential alternative to traditional farmer practices. However, its physiological mechanisms for enhancing rice N utilization and its effects on paddy field N balance remain unclear. We aimed to elucidate the potential enzymatic mechanisms underlying LCC’s influence on rice N use and quantify the impact of LCC on paddy field N balance. In 2022 and 2023, a single-factor randomized block design experiment was conducted during the rice planting season. Four N treatments: no N (ONF), farmers’ conventional practices + urea [FNR] as the control, LCC + urea [SSNM1], LCC + controlled-release urea [SSNM2] were administered. Rice yield and N uptake were positive correlations with nitrate reductase (NR), glutamine synthetase (GS), glutamate-pyruvate transaminase (GPT), glutamate-oxaloacetate transaminase (GOT) and glutamate dehydrogenase (GDH) activities, which were higher under SSNM1 and SSNM2 compared with FNR, but were negative correlation with proteinase activity. Moreover, SSNM1 and SSNM2 increased rice yield by 9.2% and 9.4%, N uptake by 15.4% and 15.3%, and N use efficiency by 46.9% and 65.0%, and reduced reactive N losses by 46.2% and 66.7%, respectively. The annual net soil N inputs under FNR, SSNM1, and SSNM2 were 12.6, 8.9, and 4.2 kg N ha−1, respectively. LCC-based N management increased N uptake and rice yield by enhancing the activities of NR, GS, GPT, GOT, and GDH while reducing protease activity. Moreover, LCC maintained soil N supply capacity even with reduced nitrogen fertilizer application.