Humus: Capture, conserve and promote

Farmers know the positive effects of humus-rich, revitalised soil with a favourable soil structure. At the latest since the World Climate Conference in Paris in 2015, the importance of humus has also been intensively discussed in connection with climate protection. Two goals complement each other perfectly, don't they?

Humus consists of half carbon, is stable in parts for centuries and is fed exclusively by biomass whose carbon comes from carbon dioxide in the air. Humus build-up therefore leads to a reduction in the CO2 concentration in the atmosphere until a new humus equilibrium is established. On the other hand, humus losses accelerate climate change through increased CO2 emissions from soils.

With the Germany-wide Soil Condition Survey Agriculture, consistent information on the humus reserves in agriculturally used soils and their influencing factors of more than 3,000 points is available for the first time. In Germany's agriculturally used soils, a total of 2.5 billion tonnes of carbon are stored in the upper metre as soil organic matter (humus and peat). This is more than twice as much as in the biomass of all German forests or about ten times the total annual emissions of CO₂ carbon in Germany.

However, a closer look shows that soil carbon stocks in Germany are distributed very heterogeneously. This is primarily due to the diversity of sites: a shallow vineyard soil in Rhineland-Palatinate with only 9 t/ha of carbon in the upper metre and a northern German lowland moor soil with 1137 t/ha of carbon form the extremes here. On average, 528 t of carbon per hectare are stored in the upper metre of agriculturally used peat soils. These soils, which account for only about 6 % of the total agricultural area, concentrate 24 % of the total organic soil carbon in Germany's agriculturally used soils. They are mainly located in the north and north-west and in the foothills of the Alps. This peatland carbon is not stable, as rapid peat mineralisation takes place after drainage, making agricultural peatland soils a major source of greenhouse gases in Germany.

In the case of mineral soils, it is mainly grassland areas that have the highest carbon stocks with an average of 135 t/ha and store almost 40 t/ha more than those under arable land use. This also fits in with the fact that the humus stocks in fields with a grassland past or grassland rotation lie between those of permanent grassland and purely arable sites. The difference in humus reserves between arable and grassland soils is not only explained by the year-round vegetation cover and the very intensive rooting, but also by the fact that grassland is mainly found where arable farming becomes difficult: for example on heavy, wet soils.

In the long term, humus reserves result from a balance between the decomposition of humus by soil organisms and the build-up of humus fed by crop residues such as straw and roots as well as organic fertilisers. If this balance shifts, the humus reserves change. Humus maintenance and build-up are therefore linked to the input of organic material into the soil, crop residues, green manure, composts, farm manure from livestock farming and biogas production. These materials differ in their composition, degradability and effectiveness for the reproduction of humus. In order to be able to evaluate the different carbon inputs comparatively with regard to humus maintenance and humus formation, a distinction is therefore made between the total input of organic carbon and the humus-effective fraction. The latter increases with the stability of the carbon input.

In Germany, an average of 3.7 t/ha of organic carbon and 1.2 t/ha of humus-active carbon are added to arable soils per year. By far the largest part of this (80 %) is added to the soil in the form of crop residues such as roots and straw. Permanent grassland receives slightly more humus-active carbon (1.3 t/ha, 82 % from crop residues).

On average, organic fertilisation contributes only 12 % to the humus-active C input across all agriculturally used soils. The lion's share of humus-active organic fertilisation, 68 %, comes from cattle farming in the form of liquid manure and stable manure. Farm manure from pig farming, dry chicken manure and fermentation residues are mainly applied to arable land. At 1 %, compost accounts for only a small part of the total humus-active carbon input into agricultural soils.

The available potential for humus reproduction and build-up in Germany is thus determined to a large extent by crop residues. Therefore, humus conservation and humus formation must primarily start with the crop residues. Sustainably high yields are important for humus conservation, because the amount of crop residues for humus reproduction increases with yields. However, looking at yields alone is clearly not enough. What is ultimately decisive is how much carbon is available in the crop rotations for humus formation. Humus-rich crop rotations can achieve higher humus contents even with lower yields, see organic farming. Ultimately, humus reproduction also requires the use of resources and inputs. Therefore, humus formation should not be evaluated in isolation from the expenditure involved. More plant material for humus formation can grow in the form of catch crops. This achieves the most permanent green cover possible. Their main aim is to keep nitrogen in the system and protect it from leaching. In Germany, winter catch crops are grown on only 12 % of arable land, while the share of summer crops is 32 %. There is therefore considerable potential. The biomass that grows in late summer and autumn contributes to the build-up of humus when incorporated. Drought in summer can prevent the successful establishment of catch crops. New cultivation methods are then necessary to establish catch crops as undersown crops.

Liquid manure, dung and fermentation residues are valuable sources of nutrients and at the same time influence humus reproduction. The use of these fertilisers is primarily about efficient and low-loss nutrient recycling and only secondarily about a contribution to humus preservation. Their use is limited by their nutrient content in order to avoid the negative effects of overfertilisation. They unfold their climate protection potential primarily by saving mineral fertiliser as efficiently as possible.

The various organic fertilisers differ in terms of their humus effectiveness. The more a substrate has already been microbially converted, the more stable it is and the higher its specific humus effectiveness. However, this comparison does not take into account how much organic carbon was invested in the formation of the fertilisers (e.g. as animal feed). If one takes into account the initial carbon investments and compares, for example, the humus reproduction performance of fresh biomass (maize, alfalfa) with that of manure and digestate from the same substrate, all three variants show very similar humus reproduction performances.