The OP's original question: "So, if you leave them to dry and turn brown (before you apply them as mulch) what becomes of the nutrients? Do they somehow wither away?" Your question has already been answered, but I wanted to present it in a little different way. Dried grass is hay. the purpose of hay is as a way to preserve the nutrients in the grass so that those nutrients remain somewhat stable and palatable for animals. Properly dried (cured) and stored hay keeps the majority of its nutritional value for a least a couple of years. The control factor is moisture, and the art of producing quality hay is all about moisture management. Once moisture enters the hay/grass it is the catalyst that begins the process of breaking down/decomposition/composting. The answer to "what happens to the nutrients" is linked the moisture levels.

When grass clippings are piled up and emit that ammonia smell that is Nitrogen escaping to the atmosphere. Grass clippings put into a compost pile and covered with adequate amounts of carbonaceous material will "trap" that Nitrogen and the bacteria will use it to digest that carbon. Depending on how the grass clippings have been dried could affect the amount of Nitrogen they have so the brown color may not really reflect the C:N ratio of those clippings. Just as Alfalfa hay that has been sitting around awhile may turn brown and that does not indicate a loss of protein that can be as true of aged grass clippings.. If, while the clippings were laying about and emitted a strong ammonia odor they most likely do have a high C:N ratio and are now carbons rather than nitrogen's. Keep in mind that some "browns", manure, feather meal, blood meal, are high Nitrogen sources, while most all greens are also. kimmq is kimmsr

First impression using the Chapman 20v is that it was really great. Performed as expected. Not too heavy when full. Straps seemed strong. It was comfortable on the back. Quiet. One disappointing thing that I experienced and that I have read somewhere else before is that the cap is really difficult to deal with. Cross threading with a bad seal is easy and even if you think you have a good seal nice and tight, well, product splashes and leaks from the top. You need to be VERY careful. I used blue dye and when I took the backpack off, the back of my shirt was wet with blue dye. Luckily the Prodiamine didn't make it through to my undershirt, but I rushed off to the shower regardless. Be careful of that cap!

If there's iron in the fertilizer it can make the grass dark green. But it's also possible the mix has changed and the cultivars are genetically darker.