Grassland cultivation is bad for the climate
When grassland is ploughed in, this can lead to the emission of laughing gas, which is a powerful greenhouse gas. Scientists at Aarhus University will in a new project study the formation of laughing gas in soil and management strategies to reduce emissions.
This may sound strange, but grassland is a source of laughing gas (nitrous oxide). Nitrous oxide is a greenhouse gas because it contributes to global warming. The global warming potential of nitrous oxide is about 300 times higher than that of CO2. The emission of nitrous oxide is particularly high when grassland is converted: in other words, when it is ploughed in.
- The optimisation of management when cultivating grasslands has for many years focused on reducing nitrate leaching. Very little is, on the other hand, known about the effect on nitrous oxide emissions, says the leader of the project, senior scientist Jørgen Eriksen from Aarhus University.
Plants and ploughs
Grassland under conversion makes up about eight per cent of the farmed area in Denmark. Grasslands are perceived as being good for the environment and the climate because they are good at retaining nitrogen and fixing CO2. These fields are nevertheless regularly ploughed in to make way for other crops and this practice also helps to control weeds, pests and diseases. But it turns out that this turning over of the grassland can lead to significant emissions of nitrous oxide.
Several factors affect the formation of nitrous oxide in the soil and the risk of its emission to the atmosphere. The factors that the scientists will be focusing on are:
- Grassland botanical composition
- Tillage method
- Nitrogen turnover when using nitrification inhibitors
Plants, ploughs, earthworms and bacteria
Grassland is typically a mixture of different grasses and nitrogen-fixing plants such as clover and lucerne. The plants have different chemical compositions and are degraded at different rates. Under a conversion, the composition of plants in the grass turf can therefore influence how much nitrous oxide is produced and released. This is investigated in a comparative study in plots with grass, clover and a grass-clover mixture.
New studies indicate that also the soil tillage method used is of importance. If the turf is broken up using a cultivator so that decomposition takes place closer to the soil surface, it may be possible to reduce the emission of nitrous oxide.
Ploughs and plants are cultivation factors that affect the risk of nitrous oxide emission. But the generation of nitrous oxide in the soil is biological and the work of bacteria that transform nitrogen in soil via nitrification and denitrification processes (see fact box). It is important to determine which species are active and the conditions under which they generate nitrous oxide.
Larger soil organisms such as earthworms also play an, as yet, unknown part. Earthworms and their gut bacteria break down and redistribute plant material in the soil and in this way they can affect the decomposition of the grass turf and the emission of nitrous oxide. The project will therefore be looking at how earthworms and their gut bacteria are affected by the grassland conversion method. Will a location close to the soil surface affect their contribution to nitrous oxide emission, for example?
Lab and field
The experiments will take place both in the laboratory and in the field.
- In the laboratory we fill glass containers with soil and plant material and expose them to treatments that mimic the conditions in the field, including the distribution of grass and clover residues following ploughing and cultivation, but under more controlled conditions. This means we can study the decomposition in both time and space, explains Jørgen Eriksen.
The earthworms are also put under the microscope in the laboratory. They are collected from experimental plots in the field to study how much nitrous oxide is emitted by the worm, or rather – by its gut. The turnover of plant residues in the field is clarified via measurements of nitrous oxide emissions and carbon and nitrogen emissions in soil. The most promising strategies will be tested under field conditions.
The three-year project is a joint effort between the Department of Agroecology and the Department of Bioscience at Aarhus University, University of Kiel and other international partners. The project has received 5 million Danish kroner from The Danish Council for Independent Research | Technology and Production.
Facts about nitrogen cycling
Nitrification: Aerobic conversion of ammonia via nitrite to nitrate. Nitrification under anaerobic conditions can lead to the generation of nitrous oxide.
Denitrification. Converts nitrate to nitrous oxide and nitrogen gas under anaerobic conditions.
Nitrification inhibitors: A group of chemical compounds that can suppress the conversion of ammonia to nitrite.
Senior scientist Jørgen Eriksen, Department of Agroecology, e-mail: firstname.lastname@example.org, telephone: +45 8715 7672, mobile: +45 5168 0554 or
Senior scientist Søren O. Petersen, Department of Agroecology, e-mail: email@example.com, telephone: +45 8715 7756, mobile: +45 4026 0627