What typical fraction of decomposing plant matter, left on the surface rather than buried, is not released in gaseous form?

My simple world view is that the plant structure has all come from the air, water content in the ground and a few nutrients and so I am wondering if the process of decomposition leaves just a tiny percentage of nutrients or if some larger fraction is left as solid material.

I had thought of posting this on sustainability.stackexchange though I am looking for a rational explanation rather than a practical experience.

Lastly, I appreciate the answer is likely to vary according to type of plant matter. If it helps I'm thinking of the UK, and a mixture of deciduous (beech, hazel, and oak), yew, spruce, and other grasses and shrubs.

Clearly I understand that the timescales applicable to woody stems and branches will be much longer than for green shoots and leaves.