Nitrous oxide (N₂O) is a greenhouse gas. It is not as well-known as CO₂, but it has a global warming potential nearly 310 times higher than CO₂. Nitrous oxide also has a long lifespan in the atmosphere, which makes it important to minimize emissions. Nitrous oxide is naturally produced during the microbial turnover of nitrogen in the soil, but it is also affected by agricultural management. Thus, the addition of both synthetic and organic fertilisers will affect the production of nitrous oxide. But what role does soil structure play in this process? A team of researchers from Aarhus University and University of Padua has studied what happens when soil is compacted under heavy agricultural machinery. It is called soil compaction when the weight of heavy machines presses the soil.

“Our study shows that soil compaction, at different depths, reduces the amount and size of the soil pores, which are voids in the soil where air and water move. It also affects how these pores are connected. When they become smaller and the connections fewer, it reduces the ability of air to enter the soil, and under wet conditions with low oxygen this causes anaerobic conditions, which favours the formation of nitrous oxide, but at the same time  affects the movement and thus emissions of nitrous oxide,” explains Mansonia Pulido-Moncada from the Department of Agroecology at Aarhus University.

In other words, compaction acts like a double player in agricultural soils. On one hand, it promotes the production of nitrous oxide, but on the other hand, it controls the movement of the gas towards the atmosphere, some it is trapped, and some it is released. Although it may seem positive at first glance that nitrous oxide cannot move out into the atmosphere, the study shows that compacted subsoil layers can act as a trap for nitrous oxide, increasing the risk of emissions when the topsoil is drained of water and there are again connecting pores with the compacted subsoil layers.

Small changes can have a big impact

Using advanced X-ray imaging called microtomography, the researchers analysed the soil’s pore structure. They found that pore connectivity is the best indicator of how nitrous oxide moves and ultimately escapes into the atmosphere.

“When you are able to visualise the pore structure of a non-compacted soil sample, you can see a large network of connected pores. This pore structure facilitates desirable gas and water transport, particularly after a rain event. But for compacted soils, you can see fewer, smaller, and disconnected pores, and you can also quantify the remaining connected pores that can be used as pathways for gases such as nitrous oxide to move upwards within the soil layers, or for oxygen to move downward into deeper layers. However, the fate of nitrous oxide produced in compacted layers is complex, as it is influenced by the interaction between pore architecture and the soil's chemical and biological properties among others,” says Mansonia Pulido-Moncada, emphasising the importance of maintaining a healthy soil structure and adapting agricultural practices accordingly.

For farmers, this means it is crucial to avoid soil compaction. Practical measures such as reduced tillage, the use of cover crops, and proper handling of machinery can help preserve the soil’s structure and functionality.

“There are many good reasons to avoid soil compaction. It is not only to reduce nitrous oxide emissions but also to improve soil health and thereby crop yields and human health,” says Mansonia Pulido-Moncada, stressing that even small changes in agricultural practices can have a significant impact on the environment.

“The study gives us a better understanding of how nitrous oxide moves from deeper soil layers to the surface. It also shows how important it is to take good care of the soil to reduce greenhouse gas emissions and keep the soil healthy,” concludes Mansonia Pulido-Moncada.