<p>From <a href="http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.474.7358&amp;rep=rep1&amp;type=pdf" rel="nofollow noreferrer">Mary, et al., 1996</a>, Figure 1 on the second page shows a nice breakdown of there the Carbon and Nitrogen go when a plant decomposes. <a href="https://www.researchgate.net/profile/Emine_Kara/publication/222149241_Rates_of_decomposition_of_plant_residues_and_available_nitrogen_in_soil_related_to_residue_composition_through_simulation_of_carbon_and_nitrogen_turnover/links/54934ea30cf22d7925da2ecb.pdf" rel="nofollow noreferrer">Hadas, et al., 2002</a> has experimental data from plants with C:N ratios from 11:1 to 136:1. </p>

<p>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$_2$.</p>

<p>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. </p>

<p>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 <a href="https://naldc.nal.usda.gov/download/IND43966853/PDF" rel="nofollow noreferrer">Latshaw, 1934</a> (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. </p>

<p>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.</p>

CC BY-SA 3.0
2016-12-21T20:06:03.507Z
Ammonia (NH3) should end as (NO3) due to the action of bacteries.
Santiago
CC BY-SA 3.0
2016-12-21T20:14:07.250Z
@Santiago Thank you, I was not answering the right question with the nitrogen section, edited it a little better now.
kingledion
CC BY-SA 3.0
2017-01-14T12:40:00.490Z
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
CC BY-SA 3.0
2017-01-14T17:01:40.007Z
@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](https://en.wikipedia.org/wiki/Cellulose) is all carbon, oxygen, and hydrogen, so it will mostly end up as gasses too.
kingledion
CC BY-SA 3.0
2016-12-21T16:21:48.917Z
2016-12-21T20:13:36.380Z
2016-12-21T20:13:36.380Z
kingledion
<p>What typical fraction of decomposing plant matter, left on the surface rather than buried, is not released in gaseous form?</p>

<p>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.</p>

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

<p>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.</p>

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

CC BY-SA 3.0
2016-12-21T14:31:58.190Z
If it is completely decomposed then nothing, it means anything not released as a gas has been consumed.
John
<atmosphere><soil><co2>
What is the residue after plant matter is completely decomposed?
Puffin
# What is the residue after plant matter is completely decomposed?

(Asked by: username_0 on 2016-12-20)

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.

**Tags:** atmosphere, soil, co2

> **Comments:**
> **username_1**: If it is completely decomposed then nothing, it means anything not released as a gas has been consumed.  

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## 1 answers

### Answer by username_2 on 2016-12-21. Score: 6 | Accepted answer

From [Mary, et al., 1996](http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.474.7358&rep=rep1&type=pdf), 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](https://www.researchgate.net/profile/Emine_Kara/publication/222149241_Rates_of_decomposition_of_plant_residues_and_available_nitrogen_in_soil_related_to_residue_composition_through_simulation_of_carbon_and_nitrogen_turnover/links/54934ea30cf22d7925da2ecb.pdf) 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](https://naldc.nal.usda.gov/download/IND43966853/PDF) (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.

> **Comments:**
> **username_3**: Ammonia (NH3) should end as (NO3) due to the action of bacteries.  
> **username_2**: @username_3 Thank you, I was not answering the right question with the nitrogen section, edited it a little better now.  
> **username_0**: 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?  
> **username_2**: @username_0 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](https://en.wikipedia.org/wiki/Cellulose) is all carbon, oxygen, and hydrogen, so it will mostly end up as gasses too.  

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