Variation of CO2 reserves in land occupied by a photovoltaic plant

To analyze the evolution of the sinking capacity of land where it is planned to build a photovoltaic solar plant, we must first quantify the existing carbon reserves before its transformation. The CO2 sink in a photovoltaic plant could be done experimentally, but it would be very costly in time and money, so we are going to make estimates instead.

We must take into account two elements: organic carbon in soil (COS) and the carbon reserve in vegetation above and below the ground (CVEg).

Organic carbon in soil (COS)

Since we will not rely on measurements, to calculate the amount of organic carbon in reference soil in the humus layer from 0 to 30 centimeters (measured as a mass of carbon per hectare) we must take into account the climate, soil type, land occupation, land management and inputs. The calculation methodology proposed by the Decision 2010/335/EU of the European Commission provides us with reference values that we can use.

Alternatively, organic carbon in soil can be estimated using geographic information systems. Thus, for example, in the Region of Murcia there is geographical information available that allows us to know the value of the COS (or SOC for its acronym in English) at any point in the region.

Typical values would be between 13 and 36 tons of carbon per hectare, although the land where solar plants are usually projected is closer to 21 t of C/ha.

The carbon reserve in vegetation (CVEG)

Calculating the carbon reserve in vegetation above and below the ground (measured as the mass of carbon per hectare) is no easy task. For example, in annual cereal crops in which all waste is harvested and removed, we can establish that the value of the CVEG would be equivalent to that of asphalt soil, that is, it would be equal to zero. Meadows and pastures would have intermediate values of 3.1 t of C/ha; and in forest plantations and in woody crops such as fruit trees, maximum values (43.2 t of C/ha) would be observed.

In the case of agricultural vegetation, the CARM in the Region of Murcia calculates the CVEG based on the LessCO2 initiative. As for absorption, only the carbon retained in the roots, trunk and main branches is considered (the harvest and leaves are not taken into account because they are a very short carbon cycle) and the emissions necessary to maintain the crop are subtracted (emissions of N2O from nitrogen fertilizer and CO2 from the use of machinery). In this way, it establishes an average of 22 tons of C/ha of perennial crop.

For its part, the Generalitat Valenciana has prepared a territorial cartography of the carbon stock that quantifies the carbon fixed in vegetation covers for the entire territory of the Valencian Community. This cartography is subdivided into forest, agricultural land and artificial naturalized green spaces.

The before and the after

Changes in average carbon stocks can be monitored using a variety of methods including IPCC data sets and secondary calculations. We will choose the method of differentiating between the existing reserves at the beginning and the estimation of the balanced carbon reserve that the lands will reach with their new use, which will be the production of photovoltaic solar energy.

Taking into account that the solar plants being developed in our country are large in size, are relatively standardized and that they usually occupy rainfed agricultural land, we can establish that a percentage between 3 and 10% of the surface occupied by the fence will be sealed due to the construction of roads, ditches, drifting, etc. This situation will be assimilated to a total destruction of the existing carbon stock.

The variation in the rest of the surface where there is no permanent occupancy or excessive compaction depends, of course, on the starting conditions. If it is an abandoned almond field, there will be a significant loss of CVEG, but if instead there is a wheat field, we can say that adventitious vegetation would increase existing carbon reserves compared to the starting situation. In the case of a meadow without the presence of woody plants, we can assimilate both situations without significant losses or gains.

Remember that we can always calculate CO2 from COS and CVEG by applying the relationship between the atomic weights of C (12) and the CO2 molecule (44), which is a 44/12 or 3.67 ratio.

In addition to the experiences of the Valencia Community and the Region of Murcia, it is worth highlighting a tool for estimating CO2 emissions from the loss/gain of the carbon stock and sink capacity of the impact of urban planning on forest or cultivated areas developed by the Generalitat de Catalunya which has already had several updates since it appeared in 2012.

The compensation

The lack of transparency in the acquisition of tons of CO2 in many countries has generated controversy and doubts about these international compensation mechanisms. On the other hand, in recent months there has been a great deal of interest among the promoters of renewable projects on Fingerprint registration, compensation and CO2 absorption projects which establishes a clear methodology for calculating compensation for forest plantations. The species established for these plantations are sometimes not the most suitable for generating biodiversity spaces adapted to the biogeographic region and it is important not to fall into a reductionist vision when we want a measure to be sustainable as a whole.

The Autonomous Communities mentioned above are already requesting that the possible loss of reserves associated with the change of use be calculated, prevented and compensated for. The compensation proposals established to date are:

a) Planting vegetation in the project environment (Comunitat Valenciana and Region of Murcia).
b) Investing in projects to improve the management of nearby carbon sinks. These projects may promote forest management but also the improvement of other ecosystems of interest such as wetlands or even cultural agricultural landscapes (only Comunitat Valenciana).
c) Economic compensation, taking as a reference the price of a ton of carbon dioxide in the international market at the date of approval of the project (only Comunitat Valenciana).
d) Self-consumption projects of any form of renewable energy (including photovoltaic modules), without occupying land, of buildings for public use or non-profit entity of social interest, in the municipality where the project is located (Region of Murcia only).
e) Fleet electrification projects (unique project in the Region of Murcia).

Other proposals

A compensatory measure that we would like to see in the future is the transformation of the form of production of surrounding agricultural land towards regenerative agriculture. It is a more complex measure that requires greater support, but we consider it to be very powerful because we are not only talking about mitigating climate change, but also about the adaptation of our production. The basic principles by which it is governed are:

• zero tillage (or minimum tillage);
• permanent ground cover with vegetation (common crops and additional plants) or plant residues;
• direct seeding through permanent soil cover or crop residues;
• biomass production and land cover with plant materials using adapted species.

At Ideas MedioAmbiental we have been carrying out this type of calculation for years and we have been pioneers in incorporating them into our Environmental Impact Studies. During this time we have successfully submitted different reports and compensation projects that have helped to make different projects a reality. Do not hesitate to contact us if you are interested in more information.

Iago Vázquez, Director of the Sustainability Area