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Breakdown of protein/amino acids. We do not absorb nitrogen in the air. If we did, that would be a problem because it would probably boil out in our blood. We also can't fix the nitrogen from atmospheric nitrogen in order to use it. Bacteria in soil have to do that, then it's upcycled from there via plants and herbivorous animals to usable amino acids that we eat.

In our bodies? No, it's not from the air, it comes from food you eat. Pure Nitrogen is too strongly bonded together to be useful in plants and animals. Microorganisms break down the N2 into nitrites and nitrates so that plants can use it, then animals eat the plants (or animals eat the animals that eat the plants.) Nitrogen compounds are also manufactured industrially for fertilizers.

See: The Nitrogen Cycle

Ultimately the main Nitrogen compound in your urine (urea) is produced in the liver from the breakdown of proteins in the blood. This would normally create ammonia (like what aquatic animals do) but Ammonia is a powerful oxydizer and highly toxic, so our livers bond two Ammonia molecules to a Carbon Monoxide molecule to make it safer for our urinary track.

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And urea requires less water for excretion, something important when you're living on land. Although urea requires more energy, it's excretion is safer and conserves water.

Also, around 10% of kidney nitrogen excretion is in the form of ammonia (in normal conditions)

Further read: Urea excretion in humans

Nitrogen in urine comes from the breakdown of proteins in the body. When proteins are metabolized, they are broken down into amino acids, and then converted into ammonia through a process called deamination. Ammonia is then further converted into urea, which is excreted in urine. So, essentially, the nitrogen in urine comes from the nitrogen-containing amino acids in the proteins we eat.

You are absolutely right. I wanted to touch on that but didn't want to get too wordy. Plus other land animals (like birds) have a different solutions for the toxic Ammonia problem.

This is one of those things that can get super complicated the further down you get into the minutia. OP's question was fairly simple so it's basically food+metabolism+liver=Nitrogen in urine.

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I remember reading about nitrogen, urine and Lovelock's Gia Theory. There was something about the way mammals process nitrogen which is inefficient, but provides plant - available nitrogen in their urine. What would this inefficient process be?

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Answers here are focused on proteins and that is correct, but nucleotides (DNA and more abundant RNA) are also an important source of nitrogen in urine. In bacteria, there is about half as much nucleic acid by dry weight as protein. Nucleotides are less nitrogen-dense than proteins but still a major contributor to nitrogen in urine.