Why is the current atmosphere made of nitrogen (75.5%)? As I read a book on meteorology, 4 billion years ago, atmosphere was made by steam, carbon dioxide, carbon dioxide and a few nitrogen. Then photosynthesis came, run by underwater organisms, which led to a rise in the oxygen concentration in the atmosphere. But, where does the nitrogen comes from in the current atmosphere? Is there a reaction involved here?
 A: From what we know, the Earth's original atmosphere was not made of nitrogen (or oxygen, or carbon dioxide), but of gases such as hydrogen and helium. These would have been lost to space early on, because the Earth's gravity was not strong enough to hold on to them. The present-day oxygen came from organic sources, as you said. Here's what Wikipedia has to say about this newer atmosphere (emphasis mine):

The next atmosphere, consisting largely of nitrogen plus carbon dioxide and inert gases, was produced by outgassing from volcanism, supplemented by gases produced during the late heavy bombardment of Earth by huge asteroids.

The page attributes that passage to this, which concludes that volcanoes are one potential source of Earth's atmospheric nitrogen (see "Volcanic Atmospheres").
Also, I found another page that supports the outgassing theory:

The original atmosphere may have been similar to the composition of the solar nebula and close to the present composition of the Gas Giant planets, though this depends on the details of how the planets condensed from the solar nebula. That atmosphere was lost to space, and replaced by compounds outgassed from the crust or (in some more recent theories) much of the atmosphere may have come instead from the impacts of comets and other planetesimals rich in volatile materials. 

So it appears that the primary sources were volcanoes and extraterrestrial bodies.
A: 
The air around us is made of several different gases. Mostly nitrogen (78%) and some oxygen (21%) and traces other gases like argon, water, carbon dioxide, ozone, and more.
These gases weren't always in our atmosphere, and they came from all sorts of different places. Scientists believe that most of the nitrogen in the air was carried out from deep inside the earth by volcanoes.
The nitrogen molecule is heavier than most other molecules in the atmosphere, so it tends to settle towards the bottom. Other lighter gases, like hydrogen or helium, would tend to the top of the atmosphere, where it is more likely to be lost to space. So the heavier gases like nitrogen and oxygen tend to stick around, since they are at the bottom of the atmosphere.

Source: math dan's answer to why is there so much nitrogen in the air
A: it likely comes from nitrogen dissolved in the molten iron and nickel in the earth. A large comet would be roughly capable of delivering 1/10000 of the earths atmosphere if it where all nitrogen. They are not all nitrogen, but they have at least a fair amount. The interested reader will note that Mars has an atmosphere very rich in N2. Most likely originating in a similar way to that of earths. Things like magnesium, titanium, and lithium will all burn in nitrogen. It might have came with the water that was delivered to earth.   
