Nitrous Oxide

According to researchers, there is a need for changing the way of accounting greenhouse gases from agriculture. The current inventory of nitrous oxide from plant residues relies solely on the amount of nitrogen in the residues, while crucial factors such as the degradability of plant residues are not included. According to the researchers, this leads to misleading inventories, which also misrepresents possible mitigation measures. According to researchers from -- among others -- Aarhus University, there is a need for changing the way of accounting greenhouse gases from agriculture. The current inventory of nitrous oxide from plant residues relies solely on the amount of nitrogen in the residues, while crucial factors such as the degradability of plant residues are not included. According to the 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 very 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. "All in all, there is large diversity in the quality of the crop residues that are added to the soil as part of the general cultivation practice," says Professor and Department head Jørgen E. Olesen from the Department of Agroecology at Aarhus University. He has headed a new scientific study highlighting the differences in crop residues and how they affect nitrous oxide emissions from agricultural fields in different ways. "Crop residues are essential for maintaining soil fertility, and in addition, they play a major role in maintaining or increasing the carbon content in the soil. Unfortunately, they also contribute to the emission of nitrous oxide," says Jørgen E. Olesen. Clash with current emission inventories IPCC -- The Intergovernmental Panel on Climate Change -- prepares guidelines for how to make national inventories of greenhouse gas emissions, including nitrous oxide emissions, when crop residues are returned to the fields. Fact box: IPCC -- The Intergovernmental Panel on Climate Change (FN´s Klimapanel) The IPCC is an international organisation established in 1988 by the World Meteorology Organization (WMO) and the United Nations Environment Programme (UNEP). The purpose of the IPCC is to assess scientific knowledge about climate change, its causes, impacts and possible adaptation and mitigation strategies. The IPCC 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. IPCC reports are used as the basis for international climate negotiations and policy development. The organisation is known for publishing comprehensive assessments of climate science, including reports on greenhouse gases, sea level rises, temperature changes and other aspects of climate change. The work of the IPCC therefore plays a crucial role in understanding and managing climate change at global level. 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," explains Jørgen E. 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," says Jørgen E. Olesen. Difference between ripe and unripe crop residues According to the researchers, there may be large differences in how much nitrous oxide the crop residues emit. "Many different factors come into play when you look at the extent of nitrous oxide emissions from crop residues. 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," says Jørgen E. Olesen. You often see a high concentration of both easily degradable carbon and nitrogen in immature crop residues, such as cover crops, grass, legumes and vegetables, 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 Jørgen E. Olesen. Targeted mitigation of nitrous oxide emissions "There is a critical need for new and targeted strategies when it comes to reducing nitrous oxide from crop residues. Globally, approx. 9% of agricultural emissions of nitrous oxide stem from the input of crop residues to the fields. However, such strategies require that the accounting of emissions from crop residues becomes more accurate," says Jørgen E. Olesen. A distinction between mature and immature crop residues may be an approach that, according to the researchers, could improve the accuracy of the inventories. It will also improve the possibilities of targeting and 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 Jørgen E. Olesen, who points out that there is a need for more research into:

Nitric oxide (NO) is a central molecule in the global cycling of nitrogen, and also toxic. Little is known about if and how microbes can use NO as a substrate for growth. Scientists have now managed to grow a microbial community dominated by two, so-far unknown species on NO for more than four years (and counting) and study their metabolism in great detail. Their research provides insight into the physiology of NO-respiring microorganisms, which have pivotal roles in the control of climate active gases, waste removal, and the evolution of nitrate and oxygen respiration. Nitric oxide (NO) is a fascinating and versatile molecule, important for all living things as well as the environment. It is highly reactive and toxic, organisms use it as a signaling molecule, it depletes the ozone layer in our planet's atmosphere, and it is the precursor of the greenhouse gas nitrous oxide (N2O). Moreover, NO might have played a fundamental role in the emergence and evolution of life on Earth, as it was available as a high-energy oxidant long before there was oxygen. Thus, despite its toxicity, it makes perfect sense that microbes use NO to grow. However, research on the topic is scarce and, to date, microbes growing on it have not been cultivated. That has now changed, as reported by scientists around Paloma Garrido Amador and Boran Kartal from the Max Planck Institute for Marine Microbiology in Bremen, Germany, in the journal Nature Microbiology. They have managed to enrich two yet unknown species of microorganisms growing on NO in bioreactors and reveal exciting aspects of their lifestyle. From the wastewater tank to the bioreactor The study started off with a trip to Bremen's wastewater treatment plant. "We collected sludge from their denitrifying tank," Garrido Amador tells. "Back in our lab, we added the sludge to one of our bioreactors and we started the incubation by feeding it with NO." Bioreactors are designed and optimized to grow microorganisms under controlled conditions, which closely mimic their natural environment. This bioreactor setup was very challenging, though, Garrido Amador reports, "Because NO is toxic, we needed special equipment and had to take great care when handling them for our own safety. Nevertheless, we managed to keep the cultures growing for more than four years now -- and they are still happy and healthy!" Two new microorganisms The living conditions in the bioreactor thus favored microorganisms that could survive and grow anaerobically with NO. "Eventually, two previously unknown species turned out to dominate the culture," says Boran Kartal, group leader of the Microbial Physiology Research Group the Max Planck Institute in Bremen. "We named them Nitricoxidivorans perserverans and Nitricoxidireducens bremensis." Garrido Amador adds, "From just two microorganisms growing on NO, we gained valuable insight into how non-model microorganisms, in particular NO-reducers grow. Some of our observations showed us that these microbes did not conform to how model organisms -- organisms which easily cultivated and thus extensively studied -- behave, and showcased the limitations of metabolic predictions based solely on genome analyses." Importance in the environment and applications for waste removal "Currently we know little about the contribution of microorganisms growing on NO to nitrogen cycling in natural and engineered environments," explains Kartal. "Nevertheless, we can speculate that these microorganisms could potentially be feeding on NO and N2O released by other microorganisms while removing nitrosative stress and minimizing the emission of these climate active gases to the atmosphere." The enriched microorganisms converted NO to dinitrogen (N2) very efficiently. "There were virtually no emissions of the greenhouse gas nitrous oxide," Kartal adds. The latter -- the sole production of N2 -- is particularly relevant for application: Many other microorganisms convert NO to nitrous oxide, which is a potent greenhouse gas. N2, in contrast, is harmless. Thus, each molecule of NO that is transformed into N2 instead of nitrous oxide is one less molecule adding to climate change. In a next step, the Max Planck researchers are cultivating other NO-respiring microorganisms using samples from natural and engineered environments. "Cultivation and enrichment of further NO-respiring microorganisms will help to elucidate the evolution of N-oxide reduction pathways and the enzymes involved. It will also allow to decipher the role of NO in known and yet-unknown processes of the nitrogen cycle and its importance in the natural and engineered environments where these processes take place.," Garrido Amador concludes.