Here is some info about citric acid, and other ornic acids exudates by roots and microbes that solublize organic matter. For info on citric acid see the references below, esp. the first few, hope this is helpful. (thanks MM for the email)
I hope I didn't come across as claiming their are saprotrophic plants; AFAIK there is no such thing, plants that where termed saprophytes** are really parasitic upon mycorrhizal fungi. Plants that were termed saprophytes are now termed "myco-heterotrophs" (see references below). I was referring to normal plants the can break down organic matter essentially feeding themselves, not myco-heterotrophs like non-photosynthetic orchards. Dicots tend to emit more organic acids than other monocots, esp. leugems, to my understanding.
** The term saprotrophic or saprophyte is now detritivores or saprophage. So saprophyte fungi are now called saprophage fungi.
I believe P is a major element that is freed (solubized via mineralization) from organic matter when roots emit or create weak acids in the rhizosphere and soil solution. Other elements like Ca, Fe, and I think N, etc, can be solublized too. The amount of ions the plant solublizes via acids isn't high, but can make a difference for P anion which has poor soil solution mobility and often gets tied up.
I'm sure you know most of this already, so please ignore any info that seems to assume you don't know something (like info about the soil solution).
Below I use "solublize" and "solublization" interchangeably with "mineralization"; to mean ions are freed from organic matter.
Here are some acids emitted or made by roots:
- Carboxylic acid: emitted by roots, lowers rhizosphere pH which effects ion availability (esp. P) and also solublizes some organic matter releasing ions the plant cant take up (ex. soft rock phosphate especially when the bulk soil pH is weakly acidic like 6 to < 7).
- Carbonic acid: similar to carboxylic acid but carboinic acid dissociates back into Co2 and water in the presence of water. Carbonic acid is formed when the roots emit Co2 and the Co2 mixes with water. Carbonic acids are pretty stable, however, in the presence of water it can dissociate into Co2 and water. Carbonic acid are similar to carboxylic acid in terms of rhizsphpere pH, and pH of soil solution and mineralization of some types of organic matter. Carbonic acid can be dissociated into Co2 and water via. exudates by bacteria (i.e. "carbonic anhydrase") that prefer neutral to basic pH levels.
- H+ protons: emitted by roots when roots take up ammonicial nitrogen such as ammonium (NH4+). This is why ammonium is said to be acidic, because of the reaction by the roots after taking in the cation. When H+ protons are emitted by roots it lowers rhizosphere pH and also can solublize some organic matter freeing ions the roots can use. When roots absorb nitrate N they emit bicarbontes, which are basic, and is why nitrate is said to be a basic (alkaline) form of nitrogen (roots also emit anions when nitrate is absorbed).
- Citrate (ex. citric acid): emitted by roots and lowers rhizosphere pH and can solubilize some organic matter. One of the acidic exudates with the highest level release by roots
- Oxalate: can provide nearly the same level of mineralization of soil P (from organic matter) as citrates. Oxalate is an acidic exudate of some mycorrhizal fungi (ex. AM fungi) used to solublize soil P.
The acids listed above is not a complete list, just an example. By the process of mineralization of organic matter the exudates (ex. citric acid) releases ions into rhizosphere and the soil solution (thin film of water surrounding media particles; where microbes and ions can be found). The roots can then use the ions in the soil solution or rhizosphere depending upon the pH of the liquid and solubility of the ions. The soil solution has an equilibrium of cations to those held by media cation exchange sites. Citrate has a large affect upon P anion in terms of increasing their bio-availability to roots, this is in regards to solubility of P anions already in the soil solution and/or rhizosphere.
Below are only a couple of quick references I have handy, I will put together other references for you. I think the weak acids I listed above are the same weak acids the Luebkes (e.g. Controlled Microbial Composting) refer to in terms of roots exudates that can solublize some organic matter.
Here is a good quote from the paper: Bioavailability of soil inorganic P in the rhizosphere as affected by root-induced chemical changes: a review
1. Bioavailability of soil inorganic P in the rhizosphere as affected by root-induced chemical changes: a review
Plant and Soil 237: 173–195, 2001.
(full text) https://www.plantstress.com/Articles/...ailability.pdf
2. Effects of root exudates on nutrient availability in the rhizosphere
Plant Nutrition Developments in Plant and Soil Sciences, 2002, Volume 92, Symposium 9, 626-627,
(abstract only for now) https://www.springerlink.com/content/n216w5006k27754p/
3. The release of root exudates as affected by the plant physiological status
R. Pinton, Z.Varanini, Z. Nannipieri (eds.)
The Rhizosphere: Biochemistry and organic substances at the soil-plant interface. Marcel Dekker 2000.
4. Root exudates as mediators of mineral acquisition in low-nutrient environments
Felix D. Dakora1,3 & Donald A. Phillips
Plant and Soil 245: 35–47, 2002
(full text) https://www.fsl.orst.edu/~bond/fs561/references/Dakora%20and%20Phillips%202002 %20Root%20exudates.pdf
5. Carbonic Acid Decomposition
Richard E. Barrans Jr., Ph.D.
PG Research Foundation, Darien, Illinois
(full text) https://www.newton.dep.anl.gov/askasc.../chem99661.htm
6. "Ecosystem Physiology: The Plant-Microbe Dance"
(I do not agree with everything written in this reference, but the info about carbonic acid is sound)
by Leslie H. Kirkegarrd
(full text) https://grow-orchid-grow.com/science_...obe_Dance.html
7. "A Touch of Chemistry"
8. "Troubles in the Rhizosphere"
9. Plants parasitic on fungi: unearthing the fungi in myco-heterotrophs and debunking the ‘saprophytic’ plant myth
JONATHAN R. LEAKE
Mycologist, Volume 19, Part 3 August 2005.
(full text) https://www-mykopat.slu.se/Groningen/Leake.pdf
10. Myco-Heterotrophs: Hacking the Mycorrhizal Network
Mycena News, March 2006
(fell text) https://www.mykoweb.com/articles/Myco-Heterotrophs.html