Measured and modelled nitrous oxide and dinitrogen emissions from drained peatland soil in Finland

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Title: Measured and modelled nitrous oxide and dinitrogen emissions from drained peatland soil in Finland
Author: Rajan, Nandita
Contributor: University of Helsinki, Faculty of Science
Publisher: Helsingin yliopisto
Date: 2019
Language: eng
Thesis level: master's thesis
Discipline: Atmosphere-Biosphere Studies
Abstract: Nitrous oxide (N2O), a major greenhouse gas and long-lived trace gas has been increasing in concentration in the atmosphere since the 1900s. Natural sources including soils account for approximately 62% of the total N2O emitted annually into the atmosphere. Biological processes lead to the production and consumption of nitrous gases in soils. Denitrification, the anaerobic reduction of nitrate to dinitrogen (N2), produces N2O as one of the intermediate volatiles. This process is driven by soil environmental factors such as temperature, moisture and nutrient availability as well as various underlying factors. Anthropogenic activities greatly affect these processes. Boreal peat soils are an important source of nitrous gases due to high content of soil nutrients (carbon and nitrogen) and drained peat soils can become potential ‘hotspots’ for N2O emissions. The environmental factors regulating the emissions are complex and remain unclear. The aim of this study is to investigate the importance of these factors on the emissions of N2O and N2, from drained peatland soil under changing temperature and moisture using measured data and process modelling. Soil samples from drained peatland sites in Southern Finland were collected, the N2 and N2O direct fluxes were measured using the helium-gas-flow soil-core method. To further examine the importance of different factors on the nitrous gas emissions in drained organic soil, a dynamic process-based model, CoupModel was used to simulate similar conditions as those in the laboratory experiment setup for measurement of the fluxes. The model includes a number of parameters which aim to quantify the detailed nitrification-denitrification process. A simple parameterization method was used to define parameter values and estimate the emissions based on the experimental conditions and known soil properties. Parameters were carefully selected and tested for their impact on the N-emissions. Key parameters and factors determining the soil N2O and N2 emissions were thus identified. The results show that nutrient-rich Lettosuo peat produced relatively high N2O (6 x 10-4 g N m-2 day-1) and N2 emission (2.6 x 10-3 g N m-2 day-1) on average as compared to nutrient poor Kalevansuo peat, which showed uptake of N2O (-1.5 x 10-5 g N m-2 day-1) and low N2 emission (0.7 x 10-3 g N m-2 day-1) on average. A significant response to changes in soil environmental conditions, temperature and moisture, was observed in both sites. The model output was consistent with the measurements of abiotic (soil temperature and soil moisture) and biotic responses (soil respiration). The model explained 53% of the variability in measured N2O emissions of the nutrient-rich soil. The dynamics of N2O and N2 emissions were simulated successfully by the model but included overestimation (up to 88%) in the case of N2O. The optimum soil water content for N2O production was between 80-85% vol. Most of the N2O and N2 emissions occurred under anaerobic conditions and hence originated from denitrification. In conclusion, denitrification is the main source of N2O in drained peat soils and soil temperature and water content regulating soil oxygen along with nutrient availability are the major factors influencing this process. Management practices as well as climate change can potentially influence these factors thus leading to changes in the greenhouse gas emissions origination from soils.

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