The circumference is the distance of the earth that is measured around the poles. Eratosthenes method of finding the circumference of a planet with the sun's rays cast a shadow on the equator a vertical stick of 2,500 km making an angle of 40° with the planet shows a distance of 40,075.017 km.
He also estimates that the meridian has a length of 252, 000 stadia. And described these techniques in his book measure of the earth.
Nitrogen fixation is mostly done by bacteria living in the soil. Plants need nitrogen to grow, but they cannot use it straight from the atmosphere or as ammonia from the soil.