Accurate accounting of nitrous oxide needed: researchers

According to researchers from Aarhus University, among others, there is a need to change the way greenhouse gases from agriculture are accounted.

Read Also

The prospect of farming in the Great Clay Belt

It’s an area larger than the total acreage now farmed in Ontario, yet the prospect of clearing, tiling and bringing…

The current inventory of nitrous oxide from plant residues relies solely on the amount of nitrogen in the residues, while crucial factors such as degradability are not included. According to researchers, this leads to misleading inventories, which also misrepresents possible mitigation measures.

Crop residues are an important resource in agriculture. They contribute carbon to the soil, increase soil fertility and play an important role in the agricultural ecosystem, but they also play a role in relation to energy supply and recycling of nutrients.

Crop residues can be diverse and have widely different composition and properties. They can be cover crops, grass, grass-clover, vegetables, straw, etc. They may consist of residues from roots or from aboveground crop parts.

Professor Jørgen E. Olesen from the department of Agroecology at Aarhus University is heading a new scientific study highlighting the differences in crop residues and how they affect nitrous oxide emissions from agricultural fields in different ways.

The Intergovernmental Panel on Climate Change prepares guidelines on how to make national inventories of greenhouse gas emissions, including nitrous oxide emissions, when crop residues are returned to the fields.

The IPCC is an international organization established in 1988 by the World Meteorology Organization and the United Nations Environment Programme. Its purpose is to assess scientific knowledge about climate change, its causes, impacts and possible adaptation and mitigation strategies.

The panel plays a key role in gathering and assessing the latest scientific literature on climate change and preparing reports that inform politicians and decision-makers worldwide. These reports are used as the basis for international climate negotiations and policy development.

The IPCC’s inventory method includes nitrous oxide from crop residues such as annual cereal and seed crops, root crops, vegetables, fodder crops and grassland renewal.

”Non-harvestable crops such as cover crops are not taken into account,” says Olesen, who adds that the biochemical properties of crop residues and their degradability of carbon and nitrogen are also not included in the accounting of emissions.

”The current method only considers the nitrogen content in the plant residues, but our studies show that the degradability of carbon in plant residues actually is more important. There is therefore a need for an improved accounting method so that inventories are accurate and mitigation measures can be effective.”

According to researchers, there may be large differences in how much nitrous oxide the crop residues emit. Olesen says many factors come into play.

“A very important factor is the concentration of degradable carbon and nitrogen. When the concentration is high, the potential for producing nitrous oxide also increases.”

A high concentration of both easily degradable carbon and nitrogen in immature crop residues, such as cover crops, grass, legumes and vegetables is often seen, while mature crop residues such as straw do not have such high concentrations.

“A distinction between mature and immature crop residues could help to ensure a more accurate estimation of the short-term effects of crop residues on nitrous oxide emissions. For the more long-term effects, i.e., years and decades, we should account for the residual effects on soil quality and nitrogen content. They are affected by local climatic conditions, just as the soil conditions are of importance,” says Olesen.

Nine per cent of agricultural emissions of nitrous oxide stem from the input of crop residues to the fields. A distinction between mature and immature crop residues may be an approach that, according to researchers, could improve the accuracy of the inventories.

It will also improve the possibilities of finding appropriate mitigation strategies.

“Reviewing how we calculate emissions from specific crop residues and determine the right time and place to use them requires more research. There are also important questions in the research into emissions from crop residues that we still need to answer,” says Olesen.