Objective Carbon-nitrogen metabolism imbalance resulted from returning spent rice straws on tobacco-rice rotation cultivation fields that ill-affected the leaf quality was investigated.

Methods Xiangyan No. 7 was used in a two-factor split-plot experimentation that returned spent rice straws from crop of previous year into the soil at a rate of 0 t·hm⁻² (A1), 2.8 t·hm⁻² (A2), or 5.6 t·hm⁻² (A3) along with nitrogen fertilization at a rate of 150 kg·hm⁻² (B1), 180 kg·hm⁻² (B2), or 210 kg·hm⁻² (B3) to determine the effects on carbon-nitrogen metabolism and post-curing leaf quality of the tobacco plants.

Results Increasing nitrogen in soil with a constant amount of returned straws significantly promoted the accumulation of total sugars, total nitrogen, and nicotine as well as the activities of invertase (INV), amylase (AL), and nitrate reductase (NR) in the leaves of tobacco plants grown on it. The tobacco leaves after flue-curing showed rising contents of total nitrogen and nicotine, declining total sugar, reducing sugar, and chloride, and an escalating followed by a falling content of potassium as nitrogen was increased. Primarily resulting from the increased nitrogen in soil that activated INV and AL and stimulated carbon fixation and conversion of the plants. On the other hand, increasing the quantity of returned straws under same nitrogen fertilization encouraged formation of sugars, starches, nitrogen, and nicotine in the leaves. As a result, the INV, AL, and NR activities in the leaves increased, and so were the total nitrogen, nicotine, total sugar, and reducing sugar, coupled with declined chlorine and firstly rising then lowering potassium as straw-returning increased, in the cured leaves. Synergized with nitrogen fertilization, the moderate straw returning of A2 optimized the activity of the key enzymes, INV, AL, and NR, to maximize the overall carbon-nitrogen metabolism and accumulation of metabolic products in the tobacco plant. It appeared that the INV, AL, and NR activities positively correlated to total nitrogen, nicotine, and potassium but negatively to chloride content in the leaves. And through the enhanced activities, the total nitrogen, nicotine, and potassium of the flue-cured tobacco leaves were heightened with a reduced content of chloride.

Conclusion The A2B2 combination not only intensified the carbon-nitrogen metabolism of tobacco plants during the early and mid-stages but also ensured timely transition to carbon in the late stage. On the other hand, B1 instigated a decline in enzyme activity, which led to early starch accumulation and premature leaf senescence, while B3 caused excessive vegetative growth and delayed maturity that resulted in carbon-nitrogen metabolism imbalance. Therefore, by combining A2 and B2, high-quality flue-cured tobacco with desirable chemical composition could be obtained.