inplamint

Modern agriculture with its high use of fertilisers and intensive soil cultivation leads to the loss of carbon and nutrients from the soil. This leads to excessive pollution of the atmosphere with greenhouse gases and of the groundwater with nitrate. Particularly in winter, nitrate is regularly leached from the upper soil layers into the groundwater. Typical crop rotations with winter cereals after nitrogenous preceding crops such as rapeseed do not allow the cultivation of fast-growing catch crops that could bind the excess nutrients.

The aim of the INPLAMINT joint project was therefore to find a solution for these frequent crop rotations to bind the excess nutrients and store them in the soil in the long term. The central approach of the project was to specifically stimulate the microorganisms in the soil, which are intensively involved in the soil's carbon and nutrient cycles, by adding carbon-containing substrates. This should enable the microorganisms to absorb the nutrients, convert them into a more stable form and make them suitable for long-term storage in the soil. To this end, numerous long-term trials were carried out in the field, in the greenhouse and in the laboratory at various locations in Germany with different soils and climatic conditions.

The decomposition of soil organic matter in autumn without corresponding plant nutrient uptake is the main cause of nutrient losses from arable land. However, spring drought can also lead to reduced uptake of fertiliser nutrients during the growing season and thus increase the need for nutrient fixation in the post-harvest period. The addition of carbonaceous substrates leads to an increase in microbial biomass and activity and thus to the stabilisation of nutrients in the soil if sufficient nutrients are available. Additions should therefore be made during the autumn mineralisation period if possible in order to bind excess nutrients directly, but not too late and in very moist soils so as not to stimulate denitrification and the associated nitrous oxide emissions. In particular, cellulosic substrates, such as wheat straw, should be used as they have been shown to be particularly effective. It has also been shown that straw incorporation does not lead to an increase in pest pressure for the subsequent crop.

The proposed measure can also be used to build up humus, as the added carbon is also firmly bound in the soil by fixing the nutrients. A medium humus content (at least 1% soil organic carbon, SOC) should be aimed for, as soils that are particularly poor in humus are characterised by a very low carbon efficiency of the microorganisms. This leads to a rapid conversion of the supplied carbon substrates to CO₂ and makes carbon and nutrient storage in the soil more difficult. Therefore, arable soils with <1% SOC should be brought above the threshold value of 1% SOC as quickly as possible through a combination of organic and mineral fertilisation. As a result, the microbial fixation of carbon and nutrients in the soil is particularly efficient without leading to increased leaching of nutrients and increased greenhouse gas emissions, as we were able to show in a three-year field trial.

Excessively high humus contents, on the other hand, can lead to increased nutrient release due to increased autumn mineralisation and thus have a counterproductive effect. However, our trials also showed that the fixed nitrogen is not available for the subsequent crop, or only to a very limited extent. Instead, it promotes the longer-term storage of carbon and nutrients in the soil, which contributes to a positive climate effect and increases the climate resilience of the soil. The use of biochar and sawdust to bind nutrients turned out to be economically unfeasible, as the production of biochar is still too expensive and the use of sawdust to bind nutrients in the soil would compete with industrial utilisation (pellets, plywood).

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Soil fungal community across a 52-year chronosequence of soil recultivation after open-mining in Inden, Germany