Nick correctly said that nitrogen is only a small component of the Martian atmosphere which is much thinner than the Earth's atmosphere. Nick quoted a value of ~1.6% N2. I normally use a molecular nitrogen mole concentration value of 2.7%. I'm too lazy to check whether or not Nick's value is actually a mass percentage. However I will show that this concentration is not an issue for a Mars colony through mass considerations.

Let us assume that 1.6% is the nitrogen mass concentration of the Martian atmosphere. The mass of the Martian atmosphere is 25 x 10^12 metric tons. Therefore the mass of nitrogen in the Martian atmosphere is 4 x 10^11 metric tons. What is the mass of nitrogen contained by the entire human population currently on Earth? The world's population is 6446131400 people. The average person is 3.2% nitrogen by mass. Assume for easy math that the average person is an over weight 100 kg. The mass of nitrogen contained by the human race is 2 x 10^7 metric tons. Of course, a Mars colony must have other living things in it other than people, e.g. plants, livestock, pets, etc. However even if the nitrogen contained by the human biological component was only 1/10000 of the total nitrogen mass of the colony, there would still be sufficient nitrogen in the martian atmosphere.

I should emphasize that IMHO the Earth is currently over populated with 6446131400 people. I believe the Earth's sustainable carrying capacity is under 2 billion people. A Mars colony by definition must be extremely energy intensive. Therefore the population carrying capacity of Mars must be orders-of-magnitude less than Earth (Mars has no energy resources except for solar and nuclear). My gut feeling is the carrying capacity of Mars is probably in the ball park of 20 million. The bottom line is the nitrogen in the Martian atmosphere does not set the planet's biological carrying capacity. Energy is probably the main limiter but that's another story.

Gary

p.s. More fun with numbers:

Assume 500 colonists are transported to Mars by spacecraft (a number sufficient to avoid in-breeding). Assume the population doubles every 20 years. How long does it take for a Martian colony to reach its carrying capacity of 20 million? Answer: 306 years.