Emission of nitrous oxide (N₂O) and leaching of nitrogen from agriculture create problems for both climate and the environment. And despite the fact that researchers have previously found that the environmental and climate impact of organic farmers is lower than that of conventional ones when measured from an average per area, the impact according to researchers from the Department of Agroecology at Aarhus University seen the same when measuring from the amount of finished product, because the yield from organic plant production is lower. 

The researchers have now reviewed the literature on emissions and leaching from organic and conventional plant production, respectively, to see how the two differ from each other and whether it can be used to develop strategies to reduce N₂O emissions and nitrogen leaching. 

“The idea was to investigate how the two production systems differ and how it affects nitrogen loss. We have chosen to focus on greenhouse gas emission and nitrogen leaching,” explains Professor and Head of Department Jørgen E. Olesen of the Department of Agroecology.

Organic matter also affects nitrous oxide

One of the major differences in organic and conventional plant production is that in the organic systems, nitrogen is typically added in organic form when fertilising. For example, it may be plant residue or green manure that is plowed into the soil prior to the sowing of the next crop.

"When organic farmers use organic nitrogen as fertiliser in their fields, it means that the mineral nitrogen in the soil does not exhibit quite as large fluctuations as it does for example in fertilisation in conventional farming," says Jørgen E. Olesen.

Traditionally, the emission of nitrous oxide from the field is expected to depend on how much nitrogen is supplied to the soil. According to the IPCC (Intergovernmental Panel on Climate Change), it is estimated that 1% of the nitrogen added to the soil in the form of fertiliser, manure or green manure will be discharged as N₂O. And with a lower nitrogen input into organic systems than conventional ones, the IPCC's calculation will also result in lower emissions of nitrous oxide.

“But that doesn't seem to be the case. We can see that there can also be large amounts of nitrous oxide emitted from organic fields. And it can be linked not only to the amount of nitrogen added, but rather to the processes in the soil that condense the emission of nitrous oxide. The conditions depend on how much microorganism activity there is in the soil, as well as the amount of oxygen consumption in the soil from the decomposition of the organic material,” Jørgen E. Olesen explains.

Nitrous oxide is formed under oxygen-free conditions in the soil, and the oxygen-free conditions can occur partly when the soil is very wet or when the microorganisms consume oxygen. The latter happens more easily when adding a lot of organic matter, as is often the case in organic plant production. The more organic nitrogen added, the more the microorganisms have to react, and the more they react the more oxygen they use, which creates a higher emission of nitrous oxide, which the researchers also found in their review of the literature. 

"The advantage we could expect in the organic systems, because not so much nitrogen is added, is eaten up to the large amount of organic matter and carbon that is returned to the soil," Jørgen E. Olesen explains.

Leaching dependent on other than nitrogen

The review of the literature showed that it is sparse what is found by measurements of nitrogen leaching in organic and conventional production systems. However, the data available do not show systematic differences in nitrogen leaching between the two production systems.

"In fact, this is probably because nitrogen leaching is more dependent on how the crops and catch crops in particular are handled in the systems than on the amount of nitrogen fertiliser added," explains Jørgen E. Olesen.

The leaching of nitrogen is not concerned whether it is an organic plant production or a conventional one, because the conditions for the formation of nitrate in autumn are not different for the two systems, as long as the soil is not overfertilised in the conventional system. It is about growing crops and catch crops in the right way, and to cultivate the soil correctly and at the right time.

“In fact, other factors play a role in the extent of the leaching. Catch crops reduce the leaching of nitrogen, but we also see that they can help increase nitrous oxide emissions in some situations. Therefore, it is important that all crops are handled and grown correctly,” explains Jørgen E. Olesen. 

The researchers' review of the literature does not show major differences in the environmental and climate impacts of organic and conventional plant production, but it also shows that they will both be able to reduce their impact through targeted cultivation strategies.

Behind the research