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Soil – the underestimated carbon store

Von Michael Welling, Folkhard Isermeyer, Heinz Flessa and Axel Don (Wissenschaft erleben 2018/2)


AK Institute of Climate-Smart Agriculture

In a large-scale project carried out over several years, soil science teams from the Thünen Institute for Climate-Smart Agriculture swarmed through all parts of Germany to collect soil samples and analyze the stocks of organic carbon bound there.

3,104 times 1x1x1. This formula represents more than seven years of work in the nationwide agricultural soil survey (so called Bodenzustandserhebung Landwirtschaft, BZE-LW). For this largest sampling since the founding of the Federal Republic of Germany, a grid of 8x8 kilometers was laid over the entire area of Germany. If there was an arable, grassland or special crop area at the intersections of this grid, this plot became a sampling point: After consultation with the farmers, employees of the Thünen Institute dug a pit with a base area of one meter by one meter and a depth of one meter, characterized („addressed“) the soil, took soil samples at various depths for analysis in the newly established soil laboratory oft he Thünen Institute in Braunschweig, and recorded the small-scale variability in the surrounding area using soil cores. In total, there were exactly 3,104 sample sites.
 

Goals of the soil survey

The first main objective was to obtain a representative record of soil carbon stocks. Why carbon? The organic carbon (humus) bound in the soil, which is constantly produced by the metabolism of plants, represents an important variable in climate processes. It is of great importance for climate protection that this store is managed responsibly and, if possible, continues to grow. Germany has committed to report regularly on this, and the BZE-LW represents an opening balance in this respect.

The second main objective was to learn more about the causes and effects of different humus contents. For this reason, both site factors and recent management history were recorded. The cause-effect relationships are complex, especially since the goal should not be humus maximization at any cost, because under certain conditions too much humus can also lead to undesirable nitrogen outputs. It will therefore be important to determine the optimal constellation in each case, taking into account soil type, nutrient dynamics, erosion risk and other aspects.
 

Results

As expected, the BZE-LW confirmed many known relationships, such as the overriding importance of peatland soil conservation for climate protection. Although peat soils and peatland-like soils account for only about 6% of Germany's agricultural land, they store about a quarter of the total soil carbon stocks of agriculturally used soils - based on soil depth 0 to 100 centimeters.

In mineral soils, the stocks are primarily determined by soil conditions and site factors: Soils rich in clay store twice as much organic carbon in the topsoil (0 to 30 cm) as soils poor in clay. Proximity to groundwater is also important for mineral soils: Soils with a groundwater level above 80 centimeters soil depth contain significantly more organic carbon than soils far from groundwater. The influence of land use is most evident in the topsoil: Use as permanent grassland allows intensive rooting throughout the year and does not disturb the soil with tillage operations. This leads to significantly higher carbon stocks than with arable land use. The importance of subsoils (30 to 100 cm soil depth) is also relevant for climate protection: On average, 35% of the total stocks of soil organic carbon contained in our mineral soils are found in these deeper soil layers.

Based on the individual findings, the total organic carbon stock was extrapolated. The result: with around 2.5 billion tons of carbon at soil depths of 0 to 100 centimeters, agricultural soils are by far the largest terrestrial store of organic fixed carbon in Germany. A great treasure for climate protection and soil fertility that must be preserved.

Where do we go from here?

Detailed evaluations, some of which are still ongoing, will provide numerous indications on the design of humus-promoting measures. In addition, model calculations used by the Thünen Institute to estimate the future development of greenhouse gas inventories in German agriculture are also important for climate protection policy.

The model calculations carried out to date indicate that losses of an average of 0.19 tons of carbon per hectare and year can be expected for arable soils in the future. These occur particularly where current inventories are relatively high and inputs of organic carbon via plant residues and manure and slurry are low. The higher the temperatures and the lower the precipitation during the growing season, the greater the losses of organic carbon can be. This is due to accelerated decomposition of soil organic carbon, but also to lower carbon inputs due to lower yields. Good adaptation of agriculture to climate change is therefore also an important contribution to climate protection.

How carbon stocks will develop under future conditions and whether measures to protect and build up humus will have the hoped-for effect, can only be answered with certainty by repeated inventories. These should be carried out at intervals of ten years, because changes in soil organic carbon stocks take place slowly over many years. The basis for such a time series analysis has been laid with the current BZE-LW.

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