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Browsing by Subject "FTC"

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  • Hakkola, Saana (2023)
    The average global surface temperature has risen 1.1 degrees from pre-industrial times, mainly due to anthropogenic greenhouse gas (GHG) emissions. Nitrous oxide (N2O) is a strong and ozone-depleting greenhouse gas. It has 265 times stronger Global Warming Potential than carbon dioxide (CO2) and an atmospheric lifetime of 120 years. Over half of the global N2O emissions originate from agriculture, mainly from cultivated soils. In soils, facultative microbial-driven denitrification and chemoautotrophy- or heterotrophy-driven nitrification are the dominant N2O -producing processes. These processes are driven by soil properties and environmental variables. In northern agricultural soils, over half of the annual N2O emissions are estimated to be produced outside the growing season, mostly during soil freeze-thaw cycles (FTCs). However, due to lack of seasonal measurements the emissions outside growing season are still poorly understood. The aim of this study was to investigate how soil FTCs affect N2O emission dynamics from Finnish grassland soils, and whether different soil types (fine sand, peat soil, clay loam) affect N2O emissions and nitrogen dynamics during soil freezing and thawing. The study was conducted as a laboratory incubation experiment using a newly built automated measuring system. N2O emissions from undisturbed soil samples were measured continuously for 21 days, which included total of 7 freeze-thaw cycles from -5°c to +5°C. N2O production or consumption was calculated for each freeze-thaw cycle, and soil mineral nitrogen (min N), C:N ratio, C%, N%, and pH(H2O) were determined during each cycle from separate undisturbed soil samples. Also, net ammonification, net nitrification and total net mineralisation rates were calculated from the min N concentrations for each FTC. Each soil type emitted N2O during the thawing of the soil, while frozen soils appeared to act as a small N2O sink. Contradictory to other studies, steady N2O production was seen throughout the seven FTCs and there was no significant increase or decrease in N2O emissions as the FTCs proceeded. Peat soils produced 4-10 times more N2O than fine sand and clay loam, respectively. Peat soil was also the largest sink of N2O during freezing. N2O emissions did not correlate with soil chemical properties, with few exceptions during soil thawing: C:N ratio (positive correlation) and pH (negative correlation). This is probably due to the episodic nature of N2O, and the complex and overlapping processes driving N2O production and consumption. FTCs increased NH4+ concentration especially in fine sand and peat soil while NO3- concentration decreased in all soils except in clay soil. Based on these results, denitrification was suggested as the main N2O producing process in peat soil and in fine sand. High NH4+ and low NO3- concentration in peat soil and fine sand indicate fast mineralisation and rapid denitrification during thawing or low nitrification and dissimilatory nitrate reduction (DNRA) activities, both of which may also have limited N2O emissions. This study shows that organic grassland soils have a high potential to both produce and consume N2O compared to other soil types during FTCs. The capacity to produce N2O during consequent FTCs indicates that these soils have a persistent and long-lasting capacity to produce N2O in freeze-thaw conditions. The N2O emission dynamics most likely reflect rapid changes in soil nitrogen turnover processes, which calls for further studies and method development to link the gross N turnover rates to N2O production and consumption during soil freezing and thawing.