There is a thing called C:N ratio. That is Carbon to Nitrogen. The microbes in the soil prefer I believe about 20 or 30 to 1. Things like grass have lower C:N ratios than things like wood chips. When the microbes come to things like grass clippings, they are happy because they don't have to work very hard to get the N that they want out of it. However when they come to things like wood chips, they still want that N, but have to go through the C to get it. It takes a bit longer for them to get the N out of the wood chips, so while they are doing that, they aren't processing N from more viable sources.