Abstract
The specific mechanisms by which nitrogen affects the nodulation and nitrogen fixation of soybean plants are unclear. Investigating the relationship between nitrogen, nodulation and nitrogen fixation can provide new insights for the rational and proper use of nitrogen fertilizer in soybean plants. In this study, we grafted soybean roots to construct a dual-root system with a single nodulated side. Experiment I was performed at the third trifoliate leaf to initial seed filling (V3-R3) growth stages (for 30 days) for long-term nitrogen supply, and Experiment II was performed at the third trifoliate leaf to fourth trifoliate leaf (V3-V4) growth stages (for 5 days) for short-term nitrogen supply. For the two experiments, a nutrient solution providing 15NH415NO3 or NH4NO3 as the nitrogen source was added to the non-nodulated side, while a nitrogen-free nutrient solution was added to the nodulated side. The concentrations of nitrogen supplied were 0 mg/L, 25 mg/L, 50 mg/L, 75 mg/L, and 100 mg/L. The results showed the following: (1) Short-term nitrogen supply systematically regulated the specific nitrogenase activity (SNA), thereby inhibiting the acetylene reduction activity (ARA). Under long-term nitrogen supply, the recovery of SNA was generally consistent across treatments, and the concentration of nitrogen supplied systematically regulated the growth of root nodules, thereby inhibiting the ARA. (2) Using the 15N tracer method, the concentration of fertilizer nitrogen was positively correlated with the amount of nitrogen redistributed to other organs. Although the percentage of nitrogen derived from the atmosphere (Ndfa%) decreased significantly with increasing concentrations of nitrogen supplied, the effect on the accumulation of nitrogen fixed by nodules (Naccumulation of nodules) was not significant. By establishing the relationships between the ARA (measured by the acetylene reduction method), Ndfa% (based on 15N calculations), and Naccumulation nodules (based on 15N calculations), it was found that the ARA reliably reflected the Ndfa% but not the Naccumulation of nodules.
Highlights
The severe global climate changes have a serious impact on the development of global agriculture [1]
We proposed the following hypotheses: (1) Both long-term and short-term nitrogen treatments systematically inhibit the nitrogen fixation ability of root nodules of the non-nitrogen supply side, but different nitrogen supply times lead to different underlying mechanisms (short-term nitrogen supply systematically regulates the specific nitrogenase activity (SNA), thereby inhibiting the acetylene reduction activity (ARA); under long-term nitrogen supply, the concentration of nitrogen supplied systematically regulates the growth of root nodules, thereby inhibiting ARA). (2) Under an increasing nitrogen concentration, the ARA, Ndfa%, and Naccumulation of nodules did not change synchronously
In dual-root systems of soybean plants with a nodulated side, we revealed that short-term nitrogen supply systematically regulated SNA, thereby inhibiting nitrogen fixation capacity
Summary
The severe global climate changes have a serious impact on the development of global agriculture [1]. China occupies 9% of the world’s arable land, but it accounts for 30% of the global fertilizer use [2]. Excessive fertilizer application leads to serious negative impacts on the ecosystem [3]. To address these impacts, more attention has been given to research on crop biological nitrogen fixation in order to foster sustainable agriculture [4,5]. Soybean (Glycine max (L.) Merr.) is an important nitrogen-fixing crop. The amount of nitrogen fixed by soybean plants during their lifetime accounts for 25–85% of the total aboveground nitrogen [6,7,8].
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