Soil organic carbon
Soil organic carbon (SOC) is part of soil organic matter and is a measurement of the total amount of SOC per unit area (t C/ha) at a given soil depth, irrespective of its origin or decomposition.
This indicator is being monitored at a soil depth of 0-30 cm in the LIFE IP URBAN KLIMA 2050 pilot projects, where practices such as converting from conventional to organic farming systems, changing to less intensively managed forest species, naturalising a dam and restoring peripheral areas are expected to increase SOC stocks. Sampling was carried out at the start of the activity and is expected to be repeated during the final phase of the project, as changes to SOC are slow.
- Soil organic carbon is a universal indicator of soil quality and changes to it can have implications for many environmental processes such as soil fertility, erosion and greenhouse gas fluxes.
- The average soil organic carbon in the Basque Country is estimated at 72 t C/ha.
- There are currently several absorption projects underway in the Basque Country that are expected to increase organic carbon in the soil
Relationship of the indicator to climate change
Soils can act as a sink or source of greenhouse gases (CO₂, N₂O, CH₄). On the one hand, they have the capacity to store or sequester carbon for a long time if appropriate practices are followed. On the other, soil organic matter can decompose, releasing carbon dioxide and other greenhouse gases (CO₂ y N₂O) into the atmosphere. Similarly, anoxic phenomena can lead to methane (CH₄) emissions in soils.
Just as the dynamics of soil organic matter can fix or emit greenhouse gases (GHGs) and therefore affect climate change, climate change can also act on the dynamics of organic matter. In fact, the decomposition process is affected by the temperature and humidity of the soil, among other factors.
SOC is the largest carbon pool in terrestrial ecosystems (Lal, 2008), and contains approximately 2344 Gt of organic C globally (Stockmann et al., 2013). The estimated SOC pool in agricultural soils in the EU at a depth of 0-30 cm is 17,63 Gt. In the Basque Country this pool is estimated at 0.046 Gt, with average stocks of 72 t C/ha
Trends over time and indication of climate change
The trend and results in terms of carbon sequestration cannot be analysed for the time being. Several years need to elapse (for temperate climates, soil organic carbon balance is generally considered to be reached in about 20 years) before the effectiveness of the practices can be tested and their evolution assessed.
In the absence of second samples to check the evolution of this indicator, the information presented for it is as follows:
Map of organic carbon stocks in the first 30 cm of soil in the Basque Country (t C/ha)
Figure 2. Map of SOC at a depth of 0-30 cm for the Basque Country (t C/ha). Source: KLIMATEK. Mapa de existencias de carbono y mapa de textura para los suelos de la CAPV (Lur-Carbon-Text) - Suelo - Euskadi.eus.
Location and results of the first SOC sampling in the pilot projects carried out as part of the LIFE IP URBAN KLIMA 2050 project (these samplings are scheduled to be repeated at the end of the project)
Sixteen plots (39.5 ha in total) were sampled between 2020 and 2021 in six different environments (Artikutza dam, Jundiz industrial estate, Mt. Oberan, Bolivar, Aberasturi and Bermeo). The actions carried out are expected to increase organic carbon stocks (t C/ha).
Figure 3. Map showing the location of SOC sampling at the LIFE IP URBAN KLIMA 2050 pilot projects.
Figure 4. Results of SOC sampling (t C/ha) at the LIFE IP URBAN KLIMA 2050 pilot projects.
Sampling is carried out following the methodology outlined by Stolbovoy et al.(2007). This methodology is based on the AFRSS-Area-Frame randomised soil sampling method, which uses a random sampling template represented in a grid of 100 cells (10x10) that makes it possible to obtain random samples over a threshold distance. The numbering of the cells is selected randomly to prevent the cells to be sampled from being too close together (Figure 1).
Figure 1. Area-frame randomised template and its parameterisation. Source: Stolbovoy et al., 2007.
A number of sub-plots are sampled on each plot, between 3 and 6 depending on the plot size. 25 samples are taken from each sub-plot (ISO recommendation) to make up the composite sample. Undisturbed soil samples are taken at the central point of the grid using cylinders of a known volume to estimate the apparent density.
The sampled points are georeferenced so that the same points can be re-sampled in subsequent surveys.
Lal, R. 2008. Carbon sequestration. Philosophical Transactions of the Royal Society B 363, 815-830.
Stockmann, U., Adams, M. A., Crawford, J. W., Field, D. J., Henakaarchchi, N., Jenkins, M., Minasny, B.,McBratney, A. B., Courcelles, V. R., Singh, K., Wheeler, I., Abbott, L., Angers, D.A., Baldock, J., Bird, M.,Brookes, P.C., Chenu, C., Jastrow, J. D., Lal, R., Lehmann, J. O’., Donnell, A. G., Parton, W. J., Whitehead, D.and Zimmermann, M. 2013. The knowns, known unknowns and unknowns of sequestration of soil organic carbon. Agriculture, Ecosystems and Environment 164: 80 – 99.
Stolbovoy, V., Montanarella, L., Filippi, N., Jones, A., Gallego, J., Giacomo, G. 2007. Soil Sampling protocol to certify the changes of organic carbon stock in mineral soil of the European Union. EUR 21576 EN / 2, 48 pp Oficina de Publicaciones Oficiales de las Comunidades Europeas, Luxemburgo.
The Basque Country
72 t/ha
Average value at present