Nitrous oxide from cover crops

Disclaimer: This article has been machine translated from Danish to English, so please note that there may be errors in the translation.

Cover crops are a good and widespread ecological tool for achieving better production results and limiting losses of nitrogen and other nutrients to the environment. It is also considered a climate change mitigation tool, but we lack knowledge about the extent to which nitrous oxide is formed and emitted after crumbling. Nitrous oxide is a potent greenhouse gas, so even relatively small amounts emitted can have a significant climate impact.

A memo and two videos from the Innovation Centre for Organic Farming describe the main factors for nitrous oxide formation and emissions from cover crops and preliminary results from field trials where nitrous oxide was measured after mulching.

Uncertainty in climate calculations

Measuring the actual nitrous oxide emissions from a field is difficult and expensive. Instead, when making climate calculations, it is assumed that 1 per cent of the nitrogen applied is emitted as nitrous oxide. There is a great deal of uncertainty about this figure, which is not helped by the fact that the amount of nitrogen in crop residues is not measured either, but determined based on the crop yield. This approach is difficult to transfer to catch crops, which is the reason for the studies carried out by the Innovation Centre for Organic Farming in the project Climate catch crops - destruction without emissions.

Immature cover crops emit the most emissions

According to a meta-study from Aarhus University, there is a correlation between crop residue maturity and nitrous oxide emissions. Maturity is understood here as the content of easily convertible carbon and C/N ratio. The lower the C/N and woody matter content, the more immature the crop residue is and the greater the nitrous oxide emissions. Green cover crops, clover grass and vegetable residues are examples of immature plant material.

Weather and soil matter

Nitrous oxide (N2O) is formed when microbes decompose organic matter in the soil. During decomposition, organic nitrogen is converted to mineral nitrogen in a chain of chemical processes. Nitrous oxide is one of the intermediate products. The risk of emissions is highest in oxygen-free pockets in the soil.

Soil temperature also plays a role. The higher the temperature, the greater the microbial activity and thus the risk of nitrous oxide formation. The soil's ability to drain water is important, as a compact soil with poor soil structure increases the risk of an anoxic environment.

Crumbling clover grass trials

The Innovation Centre for Organic Farming is conducting trials measuring nitrous oxide emissions from mulched clover grass on JB 3 and 5. The clover grass is ploughed down with and without prior milling. The control is unploughed clover grass. It is clear from the test results that nitrous oxide emissions are higher when the soil is tilled. The trial shows no significant difference in nitrous oxide emissions between the different mulching methods tested.

Watch the video: Measuring nitrous oxide from mulched cover crops