Good point Birhanu , time taken in analysis of such a large number of samples is surely an important criterion towards the method to be used , but comparable values to be obtained also must be considered . Lets consider , not to compromise with the precision of the analysis for the want to lesser time invested in such analysis...

The simple procedure to determined the easily degradable organic C by using rapid titration method. In this method, known amount of soil is oxidized with a mixture of potassium dichromate and concentrated sulfuric acid, utilizing the heat of dilution of H2SO4. Unused potassium dichromate is back titrated with ferrous sulfate or ferrous ammonium sulfate (Walkley and Black,1934; FAO, , 1974).

 Dear Giuseppe Badagliacca, 

Labile and recalcitrant C pools can be estimated using modified Walkley and Black method following Chan et al., (2001). Total SOC is divided in four pools of decreasing oxidizability:

Pool 1 [(very labile C): Organic C oxidizable with 12.0 mol L−1 H2SO4], Pool 2 [(labile C): Difference in organic C oxidizable with 18.0 mol L−1 and that with 12.0 mol L−1 of H2SO4], Pool 3 [(less labile C): Difference in organic C oxidizable with 24.0 mol L−1 and that with 18.0 mol L−1 of H2SO4 (24.0 mol L−1 H2SO4 is equivalent to the standard Walkley and Black method)], Pool 4 [(non-labile C): Residual organic C after oxidation with 24.0 mol L−1 H2SO4 when compared with total SOC].

Pools 1 and 2 together constitute the labile C pool, while Pools 3 and 4 constitute together the recalcitrant C pool (Chan et al., 2001; Bhattacharyya et al., 2011).

 For this we need to add 5, 10 and 20 ml of H2SO4 during wet oxidation respectively. You may refer to "Chan, K.Y., Bowman, A. and Oates, A., 2001. Oxidizible organic carbon fractions and soil quality changes in an oxic paleustalf under different pasture leys. Soil Science, 166(1), pp.61-67" for details procedure.

Or you may find other methods of labile SOC measurements in "Strosser, E., 2010. Methods for determination of labile soil organic matter: an overview. Journal of Agrobiology, 27(2), pp.49-60."

All the best

The particulate carbon, light fraction,KMnO4-oxidizable carbon and microbial biomass carbon are relatively active or easily decomposable fractions in soil.In the recent past the C-KMnO4 fraction received considerable attention as labile C fraction.For details the following references may be consulted.

Relationships between labile soil organic carbon fractions under different soil management systems. de Souza,G.P. et al (2016) Sci.Agric. 73(6):535-542

www.scielo.br/pdf/sa/v73n6/0103-9016-sa-73-06-0535.pdf

Estimating active carbon for soil quality assessment:A simplified method for laboratory and field use.Weil,R.T et al American Journal of Alternative Agriculture 18(1) 2003.pdf availble on internet for both papers.

Labile carbon , as an indicator of change in soil and the major source of energy to micorbes, ,  makes up a fraction of the total carbon pool, which  breaks down relatively quickly, and is an active source of nutrition.however, the size and quality of the labile carbon pool influences the decomposition rate.

Hot-water-soluble C as a simple measure of labile soil organic matter: the relationship with microbial biomass C.(AGR:IND21969883)

Soil Biology & Biochemistry [01 Sep 1998, 30(10/11):1469-1472],

DOI: 10.1016/S0038-0717(98)00040-

Badagliacca sir,

Please do as follows:

Take 1 g of air-dried Soil (0.5 mm sieved) and add / mix 25 mL of 0.033 mol per L KMnO4. Leave 30 minutes. Pipette 1 mL of the supernatant into 250 mL volumetric flask and make up the volume with distilled water.  Absorbance at 565 nm is to be determined with a spectrophotometer.

The change in the concentration of KMnO4, estimated from an analytical curve with known C values (take glucose or ascorbic acid), is to be used to estimate the amount of oxidized C (C-KMnO4), considering that 1.0 mmol of Mn will be consumed in the oxidation of 0.75 mmol (9 mg) of C.

Ref.:  Blair et al. (1995), Soil carbon fractions based on their degree of oxidation, and the development of a carbon management index for agricultural systems. Australian Journal of Agricultural Research 46: 1459-1466.

some method, such as  Zimmerman et al. 2007 method, could clolse related with some Model(RothC). Please check.

In continuation of my earlier  suggested references,the following recent reference may also be consulted.

Total and permanganate oxidizable organic carbon in the rooting zone of US coastal plain soils as affected by forage radish cover crops and N fertilizers by Wang ,F et al. 2017  Soil and Tillage Research 165:247-257.

http://dx.doi.org/10.1016/j.still.2016.08.022

The modified walkley and Black method might be used as suggested by our colleagues. The problem is to think how time consuming is the 500 (large) mber of samples. To save your time, you can use Mid infrared spectroscopy. You may need about 20% of the total samples = only 100 samples to be analyzed following wet chemistry test for calibration purpose. Because PLSR analysis indicates that mid-infrared spectroscopy works best for measurement of soil organic carbon.

Good point Birhanu , time taken in analysis of such a large number of samples is surely an important criterion towards the method to be used , but comparable values to be obtained also must be considered . Lets consider , not to compromise with the precision of the analysis for the want to lesser time invested in such analysis...