Plants need nitrogen to grow, a nutrient that most crops solely get from fertilizer. Only a few plants, such as peas, clover, and beans, can manage without it. They live in symbiosis with special bacteria that convert nitrogen from the air into a form that the plant can use.

Researchers around the world are striving to understand the genetic and molecular mechanisms behind this special ability, so that it one day may be transferred to crops such as wheat, barley, and maize.

This would make the plants self-sufficient in nitrogen and thus reduce the need for artificial fertilizer, which currently accounts for about two per cent of the world’s total energy consumption and emits large amounts of CO2.

Kasper Røjkjær Andersen and Simona Radutoiu, both professors of molecular biology at Aarhus, have identified the small changes in the plants’ receptors that cause them to switch off the immune system and allow a symbiosis with nitrogen-fixing bacteria.

Plants use receptors on the surface of their cells to pick up signals from microorganisms in the soil.

Some bacteria emit a chemicals which signal that they are “enemies” and that the plants need to defend themselve. Others are “friends” that help provide nutrition.

Legumes, such as peas, beans, and clover, invite special bacteria into their roots. Here, the bacteria convert nitrogen from the air and pass it on to the plant. This cooperation is called symbiosis, and it is the reason why legumes can grow without artificial fertilizer.

The Aarhus University researchers discovered that this ability is largely controlled by two amino acids – two small “building blocks” of a protein in the plants’ roots.

The protein in the roots functions as a “receptor” that receives signals from the bacteria. It decides whether the plant should sound the alarm (immune system) or welcome the bacteria (symbiosis).

The researchers found a small area in the protein, which they have called Symbiosis Determinant 1. The area acts as a kind of switch that determines which message is sent inside the plant cell.

By changing just two amino acids in this switch, the researchers could get a receptor that normally triggers an immune response to instead start symbiosis with nitrogen-fixing bacteria.

In the laboratory the researchers successfully modified the plant Lotus japonicus. But the same principle proved to apply in barley.

If the modification can be transferred to other crops, eventually it may be possible to breed cereal plants such as wheat, maize, or rice with the ability to fix nitrogen themselves – just as legumes do today.

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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.

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