Assume that the soya bean plant could use free nitrogen from the air. Explain how the nitrogen content of the air would then be kept constant?
Are we in the middle of our A-level science exams?
The process is this: The bacteria that live in symbiosis with the Soya plant allow it to use free nitrogen from the air as you ask us to assume. The Soya bean is used as a foodstuff for animals which in turn produce urine which contains urease. The urease is food for micro organisms in soil which convert the urea to ammonia, which is then absorbed by plants and eaten by animals.
So how does the nitrogen get back into the air to keep it constant? Well the actual volume of nitrogen in the air is not a constant nor is the percentage of nitrogen as a volume - it is about 78%. Perhaps you mean 'How does the nitrogen get back into the air for the next cycle?'.
It doesn't, is the short answer, as 78% of the air is nitrogen, the amount used in agriculture is a very small percentage of this so there is always more.
If you'd like a deeper understanding try this link, it's quite helpful. www.ext.colostate.edu/pubs/crops/00550.html