# What is the residue after plant matter is completely decomposed?

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.

• If it is completely decomposed then nothing, it means anything not released as a gas has been consumed. – John Dec 21 '16 at 14:31

From Mary, et al., 1996, Figure 1 on the second page shows a nice breakdown of there the Carbon and Nitrogen go when a plant decomposes. Hadas, et al., 2002 has experimental data from plants with C:N ratios from 11:1 to 136:1.

In summary, most carbon ends up in microbial biomass or as CO$_2$. If the process is allowed to proceed to infinity, then presumably, the microbial biomass will itself be decomposed by other microbes until all of it in the form of CO$_2$.

The nitrogen is mineralized, first into NH$_4^+$, then NO$_2^-$ and eventually all into NO$_3^-$ by various decomposing microbes. Normally these ions are attached to something else that they got from the soil and thus enter the soil. In the regular nitrogen cycle, the ions that eventually end up as NO$_3^-$ will be processed by other bacteria into atmospheric nitrogen (N$_2$). So, if decomposition proceeds to infinity, all the nitrogen would be expected to return to the atmosphere eventually too.

That leaves us with the small amounts of other biologically common elements such as phosoprous, calcium, etc. This will be the only stuff left. From Latshaw, 1934 (side note, not the oldest paper reference I've seen on Stack Exchange, but close!), and the chemical composition of a corn plant, considering all parts of the plant together, there is on average 44.9% oxygen, 44.0% carbon, 6.3% hydrogen, and 1.5% nitrogen by mass; all of these would be expected to leave as as gasses.

Of the remainder, representing 3.3% of the mass, probably most of that would stay. Whatever elements are left are most likely to form molecules with oxygen, since it is freely available in the air and likes to react with just about anything. Silicon, potassium and calcium don't seem to have any likely gaseous forms they could take, while sulfur (SO$_2$) and phosphorous (possibly P$_2$H$_4$) might take some more volatile forms. In the end, little will be left of the former plant, just a scattered residue of oxidized minerals, around 3% of the original mass.

• Ammonia (NH3) should end as (NO3) due to the action of bacteries. – Santiago Dec 21 '16 at 20:06
• @Santiago Thank you, I was not answering the right question with the nitrogen section, edited it a little better now. – kingledion Dec 21 '16 at 20:14
• Thank you, that is a very clear answer. Do think this answer, whilst using Corn for the sake of example, also applies in principle at least to to trees such as those that I mentioned? – Puffin Jan 14 '17 at 12:40
• @Puffin I believe so. The big difference in a tree is all the cellulose in the wood, which would (pun!) take much longer to decompose; decade as opposed to years. But ultimately, cellulose is all carbon, oxygen, and hydrogen, so it will mostly end up as gasses too. – kingledion Jan 14 '17 at 17:01