[BabbyAnn]

Interestingly (to me, at least) when we kill off the top growth of vegetation with weedkiller, what happens to the nutrients, especially nitrogen, contained therein?  I'm guessing that the nitrogen escapes into the atmosphere, and I'm also guessing that JayG, for one, could come up with the definitive answer!

Plants take up nitrogen for a number of reasons - mainly as a means of synthesizing biomolecules such as RNA and DNA and amino acids.  When plants collapse and die for whatever reason, soil bacteria and fungus take advantage of the available nutrients and grow/increase their numbers (that's why nitrogen levels seem to go down during composting because these microorganisms are absorbing it)  When the food source runs out, they die and then nitrogen levels are returned to the compost/soil