The labile carbon pool is the fraction of Total SOC with the most rapid turnover rates. The labile pool consists of living microbes and their products besides soil organic matter. At the same time, this pool is important from the point of view of crop production. It fuels the soil food web and therefore greatly influences nutrient cycling for maintaining soil quality and its productivity.

Organic carbon oxidisable under 12 N H2SO4 was designated as the very labile pool (C frac1) and the difference in organic carbon oxidised between 18 and 12 N H2SO4 was termed the labile pool (C frac2). These two together were designated as the active pool of organic carbon because of their easy oxidisability (by weak 12 and 18 N H2SO4).

The passive pool represented the less labile pools (C frac3) and non-labile (C frac4) pools, the C frac3 is estimated by the difference in organic carbon extracted between 18 N and 24 N H2SO4  and C frac4 is the residual organic carbon after reaction with 24 N H2SO4 when compared with TSOC.

Look the Links.

http://www.soilquality.org.au/factsheets/labile-carbon

http://soilquality.org.au/factsheets/organic-carbon-pools

http://www.scielo.br/pdf/rbcs/v34n3/32.pdf

https://www.sciencedaily.com/releases/2013/02/130211102312.htm

http://www.fao.org/docrep/009/a0100e/a0100e06.htm

onlinelibrary.wiley.com/doi/10.1111/ejss.12209/pdf

Dear Zenon Hamkalo,

Please take a look at the attached article. it may helpful to understand Which is soil organic matter should be called “labile”.

Best of luck!

Asif

http://www.fao.org/3/a-a0100e.pdf

http://www.scielo.br/pdf/rbcs/v34n3/32.pdf

http://www.southwestnrm.org.au/sites/default/files/uploads/ihub/dalal-rc-etal-2005total-soil-organic-matter-and-its-labile.pdf

http://www.agriculturejournals.cz/publicFiles/08421.pdf

Labile SOM are usually small or simple molecules that can be readily decomposed by microorganisms. 

Soil organic matter is made up of four major pools viz.,  plant residues, particulate organic carbon, humus carbon and recalcitrant organic carbon.Labile carbon( kind of bio-available carbon, hence influences biological soil fertility  ) makes up a fraction of the total carbon pool;breaks down relatively quickly; s an active source of nutrition; an indicator of change in the soil;a major food source for soil microbes and ; size and quality of the labile carbon pool influences the decomposition rate.

To my mind , labile carbon is that fraction of organic matter that possess greatest sensitivity to any management input ..Some PDFs enclosed for further reading  in this context.

Dear Zenon,

Soil organic matter (SOM) is made up of different pools which vary in their turn over time or rate of decomposition. The  labile pool which turn over relatively rapidly (in less than 5 years) results from the addition of fresh residues such as plant roots and living organisms, while resistant residues which are physically or chemically protected are slower to turn over (in 20 to 40 years). The protected humus and charcoal components make up the stable SOM pool which can take hundred to thousand of years to turn over.

Residues of plants or animals, which decomposes easily and becomes in the nutrient release pool or the carbon of the microbial biomass, which takes active in the process of decomposition and synthesis of organic matter, results into labile organic matter. Also, the soft tissues of green manures are rich in labile organic matter. Amongst, humic substances, fulvic acid is quite labile compared to further resistant humic acid and humin in the humus component. 

How do we see ,  soil microbial biomass carbon contributing towards soil  labile C fraction..?

Let me add another response , how labile and stable fraction of  soil  organic carbon are important for both better plant nutrition and ecosystem service . Labile fraction of SOC is very important for maintenance of soil fertility, while enriching soil with stabilized fraction of SOC has positive impact on the environmental quality.Basing these fractions , we can develop carbon management index for different crops .

Carbon management index (CMI) was computed according to the formula given by

Blair et al. (1995) :

CMI = CPI × LI × 100

CPI =Total C of soil sample (mg g-1)/Total C of reference soil (mg g-1)

L =Carbon fraction oxidized by KMnO4/Carbon remaining unoxidized by KMnO4

LI =Lability of C in sample soil/Lability of C in reference soil

Where, CMI = Carbon management index; CPI = Carbon

pool index; LI = Lability index and L = Lability of carbon

PDF enclosed for further reading ...

Labile C would likely be closer to fulvic acid than humic acid but this is not necessarily true in all cases.  As mentioned in the discussions above, it is that fraction that is easily metabolizeable by soil microbes and generally is closer in nature to fresh residues or organic materials added to the soil that is readily accessible to soil biota as a food and energy source.  And, some forms are more readily accessible to bacteria while other forms are more accessible by fungi.  So, it is not a clear definition of whether it is like fulvic or humic acid.  Probably by looking at it from the standpoint of : does the organic material stimulate biological activity would be a better definition.  But, again, the distinctions are not always clear.

Interesting response Larry . Why not debate , which fraction of SOM  trigger the biological activity of soil , labile fraction or non-labile fraction , i feel,  its labile fraction more active than non-labile fraction of soil SOM..But , interesting would  be to see , at what stage , non-labile having much stable carbon comes into picture ..

Organic matter which stimulate biological activity and easily metabolizeable by soil microbes is termed as active or labile organic matter. Non-humic organic molecules are released directly from cells of fresh residues such as proteins, amino acids, sugars and starches. This part of soil organic matter is the active or easily decomposed fraction. Active or labile soil organic matter is called detritus and stable organic matter is humus. In general, plant residues and living microbial biomass is active organic matter. But there are many advantages of high stable carbon, they are:

- It enhances aggregate stability and improving water infiltration

- It improves WHC

- It reduces surface crusting

- Increases soil CEC

- Improves ability of soil to resist pH

- Accelerates decomposition of soil microbes

- Produces food for the living organisms

- Enhances soil microbial biodiversity

- Enhances pore space through the action of microorganisms

The global climate change has increased the CO2 levels in the atmosphere and has increased high rates of photosynthesis in most plant species. This increase in CO2 has lead to various changes in chemical compositions of plant tissues and is therefore likely to have complex effects on the growth and composition of natural plant communities. Decreased whole plant water use of 5–20% under elevated CO2 will have consequences for the hydrological cycle of entire ecosystems, with soil moisture levels and runoff both increasing under elevated CO2. The “lability” of soil organic matter depends on the rate and type of biomass added into the soil. Recycling of biomass is essential through the action of soil-microorganisms to create soil organic matter which is so called labile.

interesting , what could be the simplest way estimate labile C of the soil..soil respiration or mineralization rate..?

Both are soil quality indicators. Soil respiration followed by mineralization rate.

The labile carbon pool is the fraction of Total SOC with the most rapid turnover rates. The labile pool consists of living microbes and their products besides soil organic matter. At the same time, this pool is important from the point of view of crop production. It fuels the soil food web and therefore greatly influences nutrient cycling for maintaining soil quality and its productivity.

Organic carbon oxidisable under 12 N H2SO4 was designated as the very labile pool (C frac1) and the difference in organic carbon oxidised between 18 and 12 N H2SO4 was termed the labile pool (C frac2). These two together were designated as the active pool of organic carbon because of their easy oxidisability (by weak 12 and 18 N H2SO4).

The passive pool represented the less labile pools (C frac3) and non-labile (C frac4) pools, the C frac3 is estimated by the difference in organic carbon extracted between 18 N and 24 N H2SO4  and C frac4 is the residual organic carbon after reaction with 24 N H2SO4 when compared with TSOC.

Sushanta Kumar Naik 

A follow up question please. On the literature i discovered that total organic carbon content decrease with depth and according to the trial i conducted, active or labile carbon pool does the opposite which is increasing with depth. I'm currently busy with the project report that need to be submitted any time soon. My only problem is that i cannot find the literature to support my finding . The only thing i could do is to speculate! Simply because the most previous and the current research is more based on determination of Total organic carbon with out paying attention on the active carbon

The soil organic carbon stock in terrestrial ecosystems is about thrice as large as the carbon storage in the plant biomass of such environments and almost twice as large as carbon stored in the atmosphere.

As it has been mentioned by most researchers the labile carbon is the carbon that is not recalcitrant or stable for decomposition. The fluxes of soil organic carbon vary in response to environmental and anthropogenic factors. Studies show that addition of purified labile C substrates such as glucose, fructose or cellulose generally stimulate mineralization of soil organic matter.

For a detailed account on this issue you may refer to this paper 'Manna MC, Swarup A, Wanjari RH, Singh YV, Ghosh PK, Singh KN, Tripathi AK, Saha MN (2006) Soil organic matter in a West Bengal inceptisol after 30 years of multiple cropping and fertilization. Soil Sci Soc Am J 70:121–129'

Interesting discussion on labile organic carbon.From the discussion it seems all the soil organic carbon, except for the stable(humified) and inert or recalcitrant (like charcoal) fractions, is labile.Some colleagues like Drs Kundu.Tarafdar and Agegnehu more or less conveyed this idea/view.It is the biological activty that decides the lability of carbon source.The labile carbon or the active carbon provides the carbon and energy to soil organisms.In the absence of the labile carbon the stable or humified carbon to a little extent provides the carbon and enery to microbes.

Yes I fully agreed with Annangi Subba Rao, Drs Kundu.Tarafdar and Agegnehu. The labile pool of organic C is the source of energy for soil microbes. The stable C or humified C is the part and parcel of soil and microbes can not use this C.

Soil labile organic matter fractions include easily oxidizable carbon (EOC), Particulate organic carbon (POC), Dissolved organic carbon (DOC), microbial biomass carbon (MBC). These fractions can turn over quickly after the land use and management practices. The short-term mangement can play important results for these labile fractions management. However, some site-specific problems and regional factors may influence distribution in soil layers.  furthermore our senior scientist can explain well. Thanks

Dear Zenon,

Please see the below given link

http://www.agriculturejournals.cz/publicFiles/08421.pdf

http://www.scielo.br/pdf/rbcs/v34n3/32.pdf

Regards

Krishna

Probably , this is the fraction of soil SOC which responds most readily to any  management input ...since it is the sensitive soil SOC fraction...

Rightly Anoop sir mentioned that the labile fraction of OM is most sensitive to any soil or crop management. I like to add that this fraction is also readily available to soil microbes and soil microbes mostly depended on this C fraction.

I agree with you Dr Islam. Most interesting would be to monitor changes in this fraction of SOC over time , scale, crops and soil types vis-a-via a variety of management practices , since residence time of this fraction of soil SOC is another important issue to deal with...

Dr. Zenon,

Answer of your question has already been given by the several scientist and many of them very clear and useful. Let me answer your question "The fraction of soil organic carbon which is utilized/available by/to soil microbes as food material called as labile carbon." 

You can also see the reference mentioned here to clarify the concept of labile fraction of carbon.

Calvelo Pereira, R., Kaal, J., Camps Arbestain, M., Pardo Lorenzo, R., Aitkenhead, W.,

Hedley, M., Macías, F., Hindmarsh, J., Maciá-Agulló, J.A., 2011. Contribution to

characterisation of biochar to estimate the labile fraction of carbon. Organic

Geochemistry 42, 1331–1342.

regards  

The portion of soil organic matter that can be readily decomposed by soil organisms is called labile fraction of organic matter (carbon). Labile organic carbon you can measure by a physical fractionation method (particulate organic carbon) or a chemical oxidation method (using potassium permanganate).

For detail fractionation, please go through following research article:

Six J., Conant, R.T., Paul E.A. and Paustian K. (2002) Stabilization mechanism of soil organic matter: Implications for C-saturation of soils. plant and Soil 241:155-176

Any spectroscopic protocol available for Labile carbon assessment

The SOC part which is most succeptible or easily decomposable by microbes called labile. It can be estimated by using chen method, the difference in organic carbon oxidised between 18 and 12 N H2SO4 was termed as labile SOC