At first glance, it seems like a really good idea: farmers get paid "on the side" for the contribution they make to climate protection by building up humus. Companies certify the binding of CO2 carbon through humus growth and sell the certificates to other companies, which can use them to advertise their products and services as "climate-friendly" or even "climate-neutral". A wide variety of climate protection projects are already offered through voluntary CO2 compensation: Reforestation, peatland restoration or the promotion of low-emission technologies. There are internationally recognised clear criteria for the recognition of such projects on the voluntary market: Verification, permanence, additionality and no displacement effects.
If one evaluates the CO2 certificates for humus build-up (also called humus certificates) in agriculture against this background, then the "good idea" does get a bad rap.
This is a labour-intensive challenge: soils and their humus contents are highly variable. Measures to build up humus have different effects depending on the location and soil properties. It is therefore necessary to take and analyse many soil samples in order to prove both the initial state of the humus stock and the humus build-up achieved. Moreover, humus stocks change slowly. This can be illustrated by a few figures. On average, 61 t of carbon are stored in the topsoil (0 - 30 cm depth) of German fields. In good conditions, humus-amplifying crop rotations with perennial crops can store an additional 0.3 t of carbon annually. This would result in 3 t more soil carbon in the form of humus after ten years. With the usual detection uncertainty of humus changes (5 to 10 %), after ten years one would for the first time reach a range in which changes would be detectable.
To determine the humus reserves, it is also necessary to determine not only the humus content but also the stone content and the storage density of the soil. Often, at least the first humus inventory for a CO2 certificate must be paid for entirely by the farmer. CO2 certificates are therefore first profitable for the soil laboratories that carry out the measurements. The risk that there will be no verifiable humus build-up and thus no revenue from CO2 certificates lies with the farmer.
A more cost-effective option is to use soil carbon models. Such models must be adequately tested and validated, but they still lead to higher uncertainties in humus detection compared to direct measurements. The reasons for this are, firstly, uncertain information on the initial humus stock and the question of whether it is already in equilibrium and, secondly, uncertain assumptions on the input of organic carbon into the soil, especially via roots and root exudates.
Only humus that is permanently stored in the soil can be counted as a climate protection benefit. If a measure to build up humus is terminated, the built-up humus is also lost again and with it the complete climate protection effect. Does the farmer have to pay back the payments he received if the humus reserves decline at a later date? How can climate protection projects ensure that the climate protection service invoiced is really maintained permanently (even beyond ten years)? The reversibility of humus accumulation generally calls into question the suitability of humus accumulation for CO2 compensation. Long-term established measures such as the planting of new hedges or additional riparian strips are much better suited than arable farming to ensure the permanence of humus build-up.
Humus is broken down in the soil by microorganisms, and they are particularly active when it is warm. Only additional carbon storage in stable plant carbon behaves differently. Its degradability is very low even at elevated temperatures and also largely unaffected by soil management. However, plant charcoal is currently still much more expensive than the proceeds from humus certificates, and open questions about pollutant contents, suitable substrates, their overall ecological assessment and the legal framework for their application need to be clarified.
Sustainable humus management and the targeted promotion of humus reproduction are essential components of agricultural soil management and a prerequisite for maintaining soil fertility. Whether measures are taken within the framework of humus certification that lead to the build-up of humus beyond the usual humus management and thus fulfil the criterion of additionality must be examined on a case-by-case basis. The criterion further stipulates that the additional income from CO2 certificate trading must be a prerequisite for the humus build-up achieved. The measures must therefore also be set beyond existing humus-effective agricultural support programmes.
In organic farming, for example, higher humus contents are achieved on average than in conventional farming. These higher humus stocks contribute to climate protection, yet they do not fulfil the criterion for humus certificates. This is because they are considered to be "common to the system". Moreover, organic farming is already promoted and realised without additional income from certificate trading.
A clear distinction between "usual" and "additional" humus is difficult and severely limits the measures for the designation of humus certificates. However, it also reveals a general problem: some farmers have long taken up the task of humus building, strengthen humus contents and fertility of their soils through regular inputs of organic matter and have thus achieved considerable humus build-up over decades. It is precisely these farmers who will benefit least from humus certificates: Since they have already implemented measures to build up humus, it is difficult for them to achieve further increases in humus. It is easier to build up humus on land whose humus reserves have been neglected and have declined. Such areas could benefit most from humus certificates. This raises the question as to how fair the certification system is and whether it always sets target-oriented incentives.
The situation is different when heavily degraded and infertile sites, e.g. in developing countries, are made arable again through meliorative measures and humus build-up. In many cases, this requires start-up investments, which could be supported by humus certificates.
The easiest way to quickly build up humus stocks is to purchase organic fertiliser, preferably compost. This is done on most of the areas participating in humus certification projects so far. However, this leads to the fact that the organic carbon applied with the compost is no longer available elsewhere for humus maintenance. On the CO2-certified land, humus stocks increase with the application of compost; on the land from which organic carbon was exported for compost production, they decrease. Thus, only a redistribution of humus takes place. There is no net increase in humus and thus no climate protection.
This describes the shifting of greenhouse gas emissions when the implementation of a climate protection project increases emissions at another location or lowers the stocks of organic carbon at another location. In the case of humus build-up, this is usually the case if it is based on inputs of organic materials produced on other land. Therefore, the requirement for CO2 offset projects is clear: such displacement effects must be excluded as far as possible or deducted from the climate protection effects achieved. This also applies to displacement effects within a farm. In other words: Humus build-up for climate protection must be based on the productivity of the site and must not be achieved by importing organic materials from other land, which then loses humus.
Humus certificates for climate protection must fulfil the generally recognised quality criteria for CO2 compensation projects. All business models and project initiatives based on the sale of humus certificates must meet these requirements. The reversibility of humus accumulation, displacement of humus with organic fertilisers and the limited potential to achieve humus accumulation based on site productivity severely limit the possibilities to achieve real climate protection contributions via humus certificates. Not all humus certificates that advertise the label climate protection actually contribute to climate protection.