Greenhouse gas emissions from peat soil thawing in spring : a comparison between fertilized and unfertilized soil

Sammanfattning: Peat soils drained for agriculture and forestry contribute to a significant amount of greenhouse gas emissions. Nitrous oxide is a greenhouse gas with high warming potential and agriculture in general accounts for about 70 % of nitrous oxide emissions. Much of the nitrous oxide emissions from soils occur during short periods of high emission peaks when conditions are favourable. For some soils, one of the most important events regarding nitrous oxide is spring thaw emissions, which is when most of the gas is released. Winter events, such as the spring thaw, are likely to change if the winter becomes warmer in the future, which in turn could affect the emissions. The fertilization rate might also affect the emission rate as substrate availability can affect the denitrifying organisms. Carbon dioxide contributes to the largest climate-warming potential from cultivated organic soils, so its emission trends are also interesting to study in freezing and thawing events. This study aimed to investigate the impact of autumn fertilization on carbon dioxide and nitrous oxide emissions in the spring, and further to see if certain soil properties could be connected to higher emissions. Measurements of the two treatments (fertilized and unfertilized) were performed in a field experiment outside of Uppsala, combined with a laboratory study with soil samples from the same site. Several freezing and thawing cycles were performed on soil samples in the laboratory. The results of the field measurement showed a large increase in emissions of both gases in late spring. There was no significant difference between the treatments for any of the gases, albeit the unfertilized samples showed a tendency of a higher average nitrous oxide emission level compared to the fertilized ones. In the laboratory experiment, the fertilized samples had significantly higher nitrous oxide emissions compared to the unfertilized samples in the first freezing and thawing cycle. There was no significant difference in the other cycles or between treatments of carbon dioxide emissions. Soil temperature in both field and laboratory experiments had a positive correlation with gas emissions, indicating that increasing soil temperature leads to higher greenhouse gas emissions. Soil water content did not correlate with the emissions, but volumetric water content was high in the soil samples, which indicates that the water content was within optimal range for denitrification. This study does not provide an unequivocal result on whether fertilization increases or decreases nitrous oxide emissions. There is uncertainty about the conflicting results between treatments from the field and laboratory measurement, but nitrate leaching may be a reason for the lower nitrous oxide emissions from the field, due to late autumn fertilization. A warmer climate may result in more freezing and thawing cycles in the future, several cycles were not visible in the field measurement, but by using a higher frequency measurement method could it be studied more closely in the future.