This is being examined by the Forest Research Institute Baden-Württemberg (FVA) in the SaMS project: Soils as Methane Sinks (in German). It takes a close look at this important ecological function of the forest soil for the breakdown of methane.

Methane is the leading greenhouse gas after CO2 and about 25 times more potent. As a result, even small amounts contribute significantly to global warming. Forest soils provide bacteria with an ideal habitat in which they can actively absorb and metabolise this atmospheric methane. This is called ‘methanotrophy’ because the bacteria literally feed on methane. On Earth, forest soils are therefore the most important ecosystem that can reduce the large amounts of methane released into the atmosphere by us humans.

Forest soils are intact methane sinks – but long-term data are still lacking

As soils are turned into agricultural land, metanotropic bacteria are increasingly losing their ability to break down methane due to tillage and nitrogen fertilisers. As a result, the breakdown of methane in agricultural soils is significantly lower than in forests. The methane sink represented by the forest soil has been considered intact. However, this assumption is mainly based on a lack of data, in particular over a long measurement period.

Under the hood

Greenhouse gas flows from soils are in most cases determined by what are known as chamber measurements: A hood is placed on the forest soil and the amount of methane metabolised by bacteria in the forest soil then leads to a concentration drop in the measuring hood during the measurement. The breakdown of the methane can thus be observed in real time. A lot of methane is broken down in active, uncompacted forest soils.

An elaborate and error-prone measuring method

This measuring method is expensive to operate over a long period of time: Due to the high energy, personnel and time requirements, most measurement projects for methane degradation in soils are limited to a few years, when they are conducted at all. Another problem: Long-term chamber measurements have a pronounced impact on the soil. The question then arises at the end whether the forest soil being examined is still the same as at the beginning of the measurement, and whether all observed changes are really caused only by external environmental influences – or by the chamber measurement itself. 

FVA has developed a new process

At the FVA in the early 1990s, a new measurement method was developed to determine the breakdown of methane in the forest soil. Plastic tubes in different soil depths, known as gas collectors, are installed on the environmental monitoring intensive measurement sites – a total of 13 measurement sites at six locations in Baden-Württemberg. In them, the surrounding soil air can be collected in a special vial. Once installed, these gas samples only need to be exchanged at regular intervals – every four weeks by us – and analysed in the laboratory for the methane content. This method does not require any power supply in the forest and is ideally suited for long-term and regular monitoring due to the low number of personnel required.

Dank dieses Verfahrens war es der FVA möglich, Zeitreihen von 10 bis 20 Jahren, in Einzelfällen auch bis zu 30 Jahren, zu erhalten – eine Messreihe seit Sturm Lothar 1999, über die Trockenjahre 2003, 2015 und 2018. Das macht die Untersuchung einer Vielzahl an Einflussfaktoren möglich.

SaMS evaluates long-term measurements

The SaMS project is now evaluating all of these long-term series for the first time. Additional chamber measurements of the last year and a half on all measurement sites under different climatic conditions validate the long-term measurement series. The FVA is also fortunate that Alexander Schengel, who examined the first gas samples in the laboratory at the beginning of the 1990s and co-developed and optimised the measurement methodology, is still working in this field today, which is a decisive factor for the continuity and quality of long-term measurements.

What do the evaluation results suggest so far?

Short-term influencing factors on methanotropic bacteria and their activity are already known. Methane breakdown is much faster in summer than in winter. But the crucial question is this: Did the breakdown of methane in the forest soil take place in the summer 20 years ago in a way that is similar to today? And if not, why is that?

Whether and how much methane can be broken down in the soil depends in particular on soil moisture and the temperature of the topsoil, the area where most bacteria and microorganisms can be found. Soil temperature and humidity are also the parameters that will change greatly as climate change progresses. The breakdown of methane in the forest soil clearly shows how methanotropic bacteria influence the climate by reducing the greenhouse effect through the depletion of methane. The question remains, however, whether or when the climate also has a decisive influence on the bacteria.

What influence do other factors have on the breakdown of methane?

Irrespective of climatic influencing factors, differences between the measurement sites can also be observed in the breakdown of methane. This is because the intensity of management, soil type, nutrient input with precipitation or the prevailing tree species and above all (as yet) unknown factors influence the bacterial composition in the soil and thus also the amount of methane that is metabolised. For example, it is assumed that considerably more methane can be broken down in deciduous and mixed forests than in coniferous forests, since the essential oils contained in the needle litter have a negative effect on the breakdown. Therefore, a conversion of forests to mixed forests is also desirable with a view to preserving the methane sink.

A look at the status quo and the future

The SaMS project aims to identify changes that have already taken place, but also how incremental climate change – especially the increase in heavy rainfall or drought events – will affect the methane-reducing function of the forest soil.

Studies from the USA have shown a marked decrease in this sink capacity, but with a much smaller data basis. Whether this trend also occurs in our forest soils will only be clearly evident from the project results at the beginning of 2024 – but a general decline is not yet apparent. Nevertheless, there is a concern that this may not continue with ongoing climate change. In recent years in particular, forests have been struggling with great drought and heat, which has caused the death of a lot of forests. The aim is therefore to preserve forests in such a way that they can bind a lot of CO2 in soil and wood in the long term and continue to provide microorganisms with a healthy habitat.