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Imagine an alien species arrived on Earth, to find that the nitrogen in the air was toxic to them. Imagine they wanted to terraform our planet, and decided to fix this problem by nitrifying a lot of our atmosphere.

They wouldn't want to completely halt the nitrogen cycle; they'd just want to convert enough nitrogen into nitrates so that they wouldn't die the minute they left their spaceship. So my question is, if they took this course of action, and, say, the nitrogen concentration in the atmosphere was halved, would that create some kind of ecological crisis that would only make their living here more problematic?

And if it is so that converting so much nitrogen into nitrates would be pointless because denitrifying bacteria will only return the nitrogen to the atmosphere, could they convert the nitrogen into another unreactive compound without effecting all the plants that need nitrates for growth and protein production?

Note: I know that there are actually two steps to convert nitrogen into nitrates, but I've just used 'nitrification' as a shortcut.

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    $\begingroup$ I wouldn't recommend trying to establish a colony on a planet at the same time you start terraforming it. Just wait for the end of the process. Welcome to the site! $\endgroup$ – Frostfyre May 16 '15 at 18:32
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It's fine-ish.

It happens already, the aliens have friends here on Earth. Rhizobia and other bacteria convert atmospheric nitrogen for us higher order species on a regular basis, since we can't use atmospheric nitrogen anyway.

Atmospheric Nitrogen constitutes a large portion of the atmosphere, so you will have a few years of climactic effects, but on the ground, not a huge difference (other than effects of climate change), because the vast majority of atmosphere is in the 5km-16km location, not the 1% or so that we live in.

So brace yourselves for weather issues and very fertile growth. I hope the Aliens come hungry.

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  • $\begingroup$ Would a severe drop in atmospheric nitrogen kill off the Rhizobia? If we lost the Rhizobia (and other such), would we be in trouble? $\endgroup$ – PotatoEngineer May 16 '15 at 22:39
  • $\begingroup$ It appears that the mass majority of nitrogen consumed by Rhizobia (etc) is in the lower 1% of the atmosphere, so if the dramatic reduction were in the upper atmosphere, it shouldn't have a huge effect, but as the aliens send it downward, it only benefits them. Maybe we'll have an overgrowth of bacteria, and higher-order animals that benefit from them. This answer is (must be) full of a lot of speculation, since we don't even know fully our atmosphere and climate as it is. $\endgroup$ – Mikey May 16 '15 at 22:59
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I'm going to have to argue against your selected answer.

The problem is, just how are yon aliens going to get rid of the nitrogen? You referred to "converting so much nitrogen into nitrates" which suggests that this is what you had in mind. So the N2 molecules get converted to NO3 ions, right? Mmm, no. In fixing the nitrogen you deplete the atmosphere of much more oxygen than you do nitrogen, specifically 3 times as much. So, since the earth's atmosphere is (roughly) 80% nitrogen and 20% oxygen, when about 8.3% of the 80% has been converted to NO3, ALL of the oxygen will be used up.

The other alternative, ammonia, is much worse. Ammonia, NH4, takes 4 times as much hydrogen as nitrogen, and since the total free hydrogen in the atmosphere is only 55 parts per million (by volume, not weight) there's just not a lot of leverage you can get.

Of course, if the aliens are examples of Clarke's Law ("Any sufficiently advanced technology is indistinguishable from magic), They can get around this by transmuting about 3/8 of the nitrogen to oxygen, binding it to 1/8 of the nitrogen, and thereby nitrify 1/2 of the total nitrogen.

Is this a good idea? I'd suggest not. Let's ignore the energy release caused by converting oxygen to nitrogen - those aliens are really good at the "indistinguishable from magic" part. If we're not willing to overlook this, the surface of the earth turns into crispy critters, and this is generally considered to be A Bad Thing.

NO3 is the standard measure of nitrogen in fertilizers, and dumping this much nitrate into the soil would constitute (essentially) massive overfertilization and would quickly kill most plants. The NO3 produced accounts for about half of the total atmosphere, and atmospheric pressure is 15 lb/sq in, so the total NO3 will amount to about 7 pounds per square inch, which is far beyond anything we have experience with. For a somewhat specialized perspective, see http://www.growweedeasy.com/nitrogen-toxicity-cannabis. About 70% of this nitrate would be directly deposited in the oceans, and water runoff would contain extraordinarily high levels of nitrogen, and this would quickly produce algal blooms in the world's oceans which would deplete the oxygen levels and kill off all surface fish, followed more slowly by the deeper species as the oxygen depletion worked its way downwards. The local, surface version of this phenomenon is already seen in places like the Gulf of Mexico, where fertizer runoff from farms along the Mississippi River cause "dead zones". Of course, this assumes that the nitrate levels would not directly kill the existing algae (not a good assumption, IMO, but I'm not an expert in the field).

With all due respect, this is a good example of the principle that you should wait at least 24 hours before selecting an answer.

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  • $\begingroup$ I wonder how thick a later of nitrates it would cover the Earth with. $\endgroup$ – RonJohn Mar 11 '18 at 7:06

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