I've been asked a really good question and I pass along to you. If you cut plants, like say grass, they're green, so they're nitrogen for the compost. But if you leave them around a bit and they dry, they're yellow, so they're carbon for the compost. What happened to the N? Alchemy?
I like to think about grass clippings in a few ways:
1. Grass that's been cut when it's brown. Dead grass will have a very high C:N ratio since most of the Nitrogen has been moved into the root system while the grass was going dormant.
2. Grass that's been cut and left to dry out over a long period of time. This will probably have a very high C:N ratio since most of the Nitrogen has been broken down by bacteria & fungi and released back into the ground (probably via rain) as available nutrients.
3. Grass that's been cut and dried fairly quickly. This will have a fairly low C:N ratio, but not as low as its non-dried counterpart. When plants dry out, the bacteria living inside/on them tend to die off and release their Nitrogen into the atmosphere.
Sadly, no alchemy. It just gets washed away, blown away, and evaporated into the air.
The first thing to know about Nitrogen both in living plants and the soil is that it is Nitrate or nitrite that we are concerned with.
Free nitrogen (the gas, which is what we are talking about when we say nitrogen) is pulled in by bacterium and converted to nitrates, this is then further processed into nitrites which is the usual form plants can use.
Dormant or dead grasses will contain 3/4 less nitrite than green, living grasses, this is, as has been mentioned, because the plant moved as many nutrients as it could to the roots in preparation for dormancy.
Green, living grasses that are cut and allowed to dry will contain about 1/2 the nitrate and nitrite of the freshly cut, still green and moisturized grass.
For composting you can use grass in any stage, the fresh cut will provide the best conditions for heating up a compost heap since it contains the most nitrite and the most moisture.
Green cut and then dried grass will provide nitrite but will not help in the heating of a heap.
Dead cut (cut dormant or dead grass) will act more like a "Brown" component in a heap.