Organic C stocks in topsoil mineral layers
Garcia-Pausas, J. 1; Casals P. 1
1 Forest Science and Technology Centre of Catalonia
Last updated: 28/09/2026
Recommended citation:
Garcia-Pausas, J.; Casals P. 2026. Organic C stocks in topsoil mineral layers. In: Pere Casals, Davide Ascoli, Lena Vilà-Vilardell, Imma Oliveras, Madoka Krick (eds.) 2026. Methods handbook for characterizing fire use behavior, assessing fire effects, and monitoring soil and vegetation responses at the stand level. FIRE-ADAPT project (number 101086416), deliverable D1.1
Indicator
Ecosystem responses: Soil responses
Brief introduction
Organic C content is an essential soil property, as it determines its fertility, structure and water holding capacity. Also, soil C accumulation contributes to climate change mitigation. The impact of fire on mineral soil C mostly depends on the intensity of fire. The direct effect (driven by high temperatures) is usually restricted to the first few centimetres of soil or could even be negligible in low-intensity fires that affect only the organic layers. Due to the high variability of soil organic C and the inertia of its dynamics, this method is generally not recommended for detecting short-term variations (< 5 years).
Long-term indirect effect is driven by changes in the leaf- and root-litter C inputs and changes in environmental conditions that cause shifts in mineralisation rates of organic matter. The accumulation of char or partially charred materials, which are recalcitrant to decomposition, could cause an increase in C stability.
Method specifications
Method type
Soft
Hard
Method timing
Before
fire
During
fire
After fire
(short-term)
After fire
(mid-term)
After fire
(long-term)
Field skills
Low
Medium
Hard
Lab skills
Low
Medium
Hard
Material cost
Low
Medium
Hard
Time cost
Low
Medium
Hard
Calculation expertise
Low
Medium
Hard
Equipment
For field sampling:
- Prismatic soil core sampler (e.g., 5 cm x 5 cm cm x 30-50 cm depth)
- Drime hammer
- Metric measuring tape (~30 – 50 cm)
- Hydraulic jack
- Knife
For laboratory analysis:
For soils without carbonates 1:
- Elemental analyser
For soils with carbonates 1:
- basic glass laboratory material (tubes, pipettes),
- heating block and
- chemicals (potassium dichromate, sulfuric acid, Mohr salt (ammonium iron(II) sulphate) and ortophenanthroline monohydrate)
1 As elemental analyser analyses total C, it should be avoided in soils containing carbonates. In this case, organic C should be analysed by dichromate oxidation (Mebius, 1960).
Experimental design
To assess the organic C stocks at plot scale, at least 15 sampling points in a regular grid should be sampled. Each sampling point should be separated by at least 5 m. Any microsite exhibiting visible surface disturbances (e.g. animal bioturbation, trail paths) should be avoided. When a sampling point falls, offset the location to the nearest adjacent undisturbed area. Each microsite should be marked adequately (using durable field stake), or coordinates recorded with a sub-metric GPS for future samplings.
Methodology
- Locate the sampling points, avoiding non-representative locations
- If organic layers are not sampled, push them aside to expose the underlying mineral soil by hand in a small area (about 10 x 10 cm) to allow the insertion of the core sampler
- Insert the core by hammering to the desired depth. Core samplers that can be opened longitudinally are recommended. This will allow us to easily separate the samples into depths. If any rock or stones are found in depth, it is recommended to change the sampling point location, repeating the process described above.
- A hydraulic jack can be useful to extract the core sampler.
- Cut the sample in the core using a knife to separate it into depth intervals. Usual depth intervals are: 0 – 5 cm, 5 – 15 cm, and 15 – 30 cm. For assessing the short-term direct effects of fire, we may not sample more than 5 cm depth. The depth intervals of each layer should be measured as carefully as possible to assess the soil bulk density of each layer.
- Keep the samples in separate labelled bags and bring them to the lab.
- Before C analyses, samples should be dried (air-dried or oven-dried at 60ºC) and sieved with a 2 mm mesh.
- Weigh the dry fine earth (< 2 mm) and coarse particles (i.e. gravels and stones) (> 2 mm)
- Analyse organic C of the fine earth with an elemental analyser or by dichromate oxidation (Mebius, 1960) 1,2.
1 As elemental analyser analyses total C, it should be avoided in soils containing carbonates. In this case, organic C should be analysed by dichromate oxidation (Mebius, 1960).
2 The presence of charcoal in the soil can cause a subestimation of soil organic C when it is analysed by dichromate oxidation. To avoid this, non-oxidized residue can be then analysed with an elemental analyser and the C content of the residue can be added to the C measured in the previous analysis.
Calculations
Soil organic C stocks (SOCS, kg m-2) are calculated as the sum of the C content at each layer, considering the organic C concentration (OC, %), the bulk density (BD, kg m-3), the coarse particles content (CP, kg m-3), and the thickness (th, m) of each layer i, as follows:
\mathrm{SOCS}=\sum_{i=1}^{n}OC_i\times(BD_i-CP_i)\times\frac{th_i}{100}
Relation with other methods
If organic layers are also sampled, it is recommended to take the samples from the same sampling points, which will be able to relate both pools of C.
Illustrations
Data templates
NA
Further information
Mebius LJ. 1960. A rapid method for the determination of organic carbon in soil. Analytica Chimica Acta 22: 120-124