Soils gain nitrogen from ammonia released by agricultural sources and from nitrogen dioxide (NO2) produced during energy generation. In regions with intensive agriculture, some soils receive as much as 60kg of nitrogen per hectare per year; the average for Europe is between 10 and 20kg per hectare per year. Once in the soil this nitrogen can be converted into gaseous nitrogen oxide by soil-dwelling microbes. Microbes are less active when temperatures are cooler so it had seemed likely that nitrogen oxide emissions from soil are negligible during the winter months.

In order to test this assumption, Klaus Butterbach-Bahl from the Karlsruhe Institute of Technology, Germany, and his colleagues obtained measurements of soil nitrogen oxide emissions from the Högwald Forest in Germany for the years 1994 to 2010. Focusing on the winter months, where daily average air temperature was less than 3°C, they calculated the nitrogen oxide emissions and compared them with emissions during the summer.

The team found that on average over one fifth of nitrogen oxide emissions occurred during the cold season, with variation from 13% to 41% between individual cold seasons. Temperature was the main influence on emissions.

"Soil nitrogen oxide emissions were all correlated with soil respiration rates, with warmer winters – where there are significant periods with temperatures above freezing – producing more emissions," said Butterbach-Bahl. Snow cover didn't appear to have any impact on emissions.

Högwald is a representative temperate forest, with similar levels of nitrogen deposition from agriculture to most of central Europe. "Similar forests can be found in the eastern US and across large parts of central Europe, including Germany, Poland, Czech Republic, France and The Netherlands," said Butterbach-Bahl, who published his findings in Environmental Research Letters (ERL).

This means that winter nitrogen oxide emissions from soil are likely to be far more significant than previously thought, in particular during mild winter years. "We expect that the importance of winter-time soil nitrogen oxide emissions will increase with climate warming," said the researcher.

So in addition to reducing greenhouse gases such as carbon dioxide and methane, the new findings indicate that we need to focus on mitigating ammonia emissions from agricultural sources, and reducing nitrogen dioxide emissions from combustion processes too.

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