Carbon farming is a form of farming that increases the amount of carbon stored in soils and plant biomass, thereby removing CO2 from the atmosphere. In most cases, the term carbon farming also includes an income component: farmers produce not only food and plant raw materials, but also monetary climate protection services. These range from the rewetting of organic soils (former moorland soils) to protect the immense carbon stocks stored therein, to carbon enrichment in mineral soils (all other soils) through the cultivation of catch crops, legumes or perennial plants, the use of plant charcoal and green manure, to increasing the carbon fixed in woody plants over the long term by planting hedges, establishing agroforestry systems or afforestation. The benefits that carbon farming measures yield are correspondingly diverse. In general, an increase in soil carbon levels in mineral soils is associated with an increase in biological activity, better water infiltration and water-holding capacity, and higher soil fertility. All this also helps to better withstand consequences of climate change such as drought or heavy rainfall events. Many measures also promote the diversity of agricultural landscapes and create habitats for beneficial insects, which play an important role in natural pest control.

Building up organic matter in arable soils has always been a central concern of agriculture, as this can improve soil fertility. Although humus buildup in conventional agriculture lost importance for a time with the availability of synthetic nitrogen fertilizers, a change in thinking has since taken place. In organic farming, where synthetic fertilizers are not used, humus buildup continues to play a central role. However, targeted measures for this are associated with costs and are therefore not yet implemented on the majority of areas. The best established measures are those that are comparatively easy to integrate into farm operations, such as the cultivation of catch crops or the cultivation of legumes. In contrast, measures that require massive changes, like rewetting or agroforestry systems, are rather rare. However, these measures in particular have great climate protection potential.

The greatest challenge is to achieve a long-term effect with the measures, since carbon is not simply stored in the soil, but is part of a dynamic equilibrium. On the one hand, there is a regular supply of carbon through plants and possibly organic fertilization. On the other hand, the carbon is constantly consumed and respired by microorganisms, creating CO2 again. Carbon farming measures usually shift this balance by increasing the amount of carbon inputs. However, when the measures are stopped, the system returns to its initial state and the additional stored carbon is released again. This is comparable to a diet: if you fall back into your old eating habits, any successes achieved are soon lost. Another challenge is the cost associated with carbon farming. These can be considerable, for example for the rewetting of organic soils. In addition, carbon farming measures must be implemented on a permanent basis in order to have a climate impact. There is still a lack of incentive systems that take this permanence into account.

Most commonly, carbon increment is determined via laboratory analysis from the difference in carbon content of two soil samples, one taken before and the other taken a few years after the introduction of carbon farming. This can produce acceptable accuracy with good sampling practices. Finally, there are providers who combine soil measurements to determine the baseline situation with model calculations of future developments to determine the carbon increment. Here, reliability depends on the quality of the model used, for example, whether releases of carbon generated by climate change are also correctly represented. Purely remote sensing methods, such as using aerial or satellite imagery to record soil carbon, are not yet in use. Theoretically, this would greatly reduce the cost of measurements, but the accuracy of such approaches is still insufficient.

In Germany, there is currently no regulation of the so-called humus certificates. Anyone can issue certificates and each certificate provider sets its own methods. Humus certificates suggest that they can be used to offset emissions from other sectors. However, their climatic effect is probably much smaller than the certified amounts of CO2, so that money for climate protection is not used efficiently. Private certificate providers cannot guarantee the permanence of carbon storage. Even if policies continue, climate change may cause carbon to be removed. For example, a warming climate promotes humus degradation, and modeling predicts that carbon farming measures alone will not be sufficient to preserve current carbon levels. Since carbon measurements end after 10 years at the latest for certificates, subsequent carbon losses are not even noticed.

Another problem is that the certificates based on measurements do not check how the carbon increase comes about - for example, through the application of fertilizer or compost, which leads to a rapid increase in carbon in the soil. For the climate, this is a zero-sum game. It is also questionable whether some carbon farming measures would not have been implemented even without the certificates.

A final problem concerns all types of emissions offsets based on carbon sequestration: Currently, it is assumed that one ton of stored carbon offsets the climate impact of one ton of released carbon. However, some research suggests that the climate impact of released carbon may be greater than that of stored carbon due to climate buffering systems. More research is urgently needed here.

Interview: Beatrix Boldt