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Soil Active Carbon - POXC

This week we have a guest author post from Fabricio Cano, Pablo Cremades and Nano Castro. They are members of reGOSH in Argentina where they have been developing open hardware tools and methods for measuring soil active carbon.

This week we are excited to share a guest author post. Fabricio Cano, Pablo Cremades and Nano Castro are members of reGOSH in Argentina where they have been developing open hardware tools and methods for measuring soil active carbon. This work is part of Open Agroecology Lab where they are developing open low-cost tools for organizatión farmers and researchers involved in the agroecological transition. We are extremely grateful to Fabricio, Pablo and Nano for sharing their work with the community and writing this week's guest post. To get in touch with the authors about their work, contact Nano Castro at ferhcastro@gmail.com. Also check out more ways to connect with them at the end of this post. We will hand it over to the authors from here - Jo & Will.

Soil Active Carbon - POXC

Fabricio Cano, Pablo Cremades and Nano Castro, reGOSH

Why is it important to measure active carbon POXC in soils?

Permanganate oxidizable carbon (POXC) also referred as active carbon, is the labile organic carbon fraction in soil and is considered to have a primary role in many soil functions related to productivity and environmental resilience. It is the organic carbon pool that is directly available for microbial activity and, hence, in considered the primary energy source for microorganisms. POXC is a good short term indicator of changes in soil quality so it is useful to evaluate management decision regarding soil related agricultural practices (Bongiorno et al., 2019).

POXC measurement protocol

We used the Culman protocol to determine active carbon in soil samples. It is an adaptation of the original method proposed by Weil (Weil et al., 2003).

PDF of protocol

Potassium permanganate (KMnO₄) Stock Solution

Different dilutions were obtained from a 0.2M KMnO₄ solution in order to build a calibration curve. To prepare the dilutions for a four points calibration curve you can use the volumes of the following table:

Concentration	0.02M KMnO4	Distilled Water
0.005 M	2.5 mL	7.5 mL
0.010 M	5.0 mL	5.0 mL
0.015 M	7.5 mL	2.5 mL
0.020 M	--	--

Open source colorimeters

To measure the absorbance of the calibration curve dilutions and the soil samples we used the IO Rodeo colorimeter and two self built colorimeters based on the previous old version of IO Rodeo. The measurement protocol suggest a 550nm light source, however we did not have LEDs emitting in that wavelength. We used the 520nm led for the IO Rodeo colorimeter and the green light of an RGB LED for the other two colorimeters. To know the emission spectrum and peak emission of those green LEDs we tested them on a commercial UV1800 Shimadzu UV-VIS spectrophotometer. You can find more details of the LED emission test here.

Colorimeter	LED Wavelength
IO Rodeo	520 nm
FCA	527 nm
FCEN	519 nm

Calibration curves

We measured the absorbance of the calibration dilutions with the three colorimeters.

Concentration	FCA	IO Rodeo	FCEN
0.005 M	0.098	0.09	0.117
0.010 M	0.18	0.17	0.205
0.015 M	0.272	0.27	0.308
0.020 M	0.363	0.36	0.411

With these results we built a linear regression and calculated the error of the regression parameters with a Python script for each colorimeter.

Colorimeter	Slope	Intercept	R2
IO Rodeo	18 ± 2	-0.01 ± 0.03	0.99
FCA	18 ± 1	0.01 ± 0.02	0.99
FCEN	20 ± 2	0.01 ± 0.03	0.99

IO Rodeo regression curve (left) and 95% interval (right)

Active carbon in soil

We measured the absorbance from four 2.5g air dried soil samples after its reaction with KMnO₄. The amount of oxidized carbon is a function of the amount of KMnO₄. This means that for greater amounts of POXC the less the color intensity and absorbance. Samples were taken from two different farms in the Valle de Uco region.

Soil Samples	FCA	IO Rodeo	FCEN
M1	1222	1132	1301
M2	954	873	1024
MC1	877	836	954
MC2	954	873	1024

For the POXC calculations we used the equation proposed by Weil. Results are given in mg of carbon for kg of soil

\[ \mathrm{POXC} (\mathrm{mg}/\mathrm{kg \, soil}) = [0.02 \, \mathrm{mol}/\mathrm{L} - (a + b \times \mathrm{Abs})] \times (9000 \, \mathrm{mg} \, \mathrm{C}/\mathrm{mol}) \times (0.02 \mathrm{L} / \mathrm{Wt}) \]

Where:

0.02 mol/L = initial concentration of the KMnO4 solution
a = intercept of the calibration curve
b = slope of the calibration curve
Abs = measured absorbance
9000 = [mgC/mol] of oxidized carbon for every mol of MnO4 that reacts from Mn+7 to Mn+4
0.02 L = stock solution volume used
Wt = [mg] weight of dry soil

Results

A 10-15% error was estimated for POXC in soil samples. You can check the details of this estimation here. To have a better understanding of what these values mean you can check scoring table from the Soil Health Evaluation Manual (Vanek et al., 2018).