Just adding a little to this. Nitrogen is often the limiting factor in an ecosystem. Certain plants, mostly Legumes, evolved to form a symbiosis with rhyzoba bacteria that are able to convert gaseous nitrogen into usable NH3. These plants are generally "pioneer species"; popping up in disturbed or nitrogen deficient regions. Once these plants finish their life cycle and break down they increase the overall usable nitrogen in the ecosystem. More fascinating are the incredible traits that many taxa have evolved in adaptation to nitrogen deficient soils that don't use rhyzoba. One example being the several convergent carnivorous plants. Or plants that trick animals into defecating on them.