PhD defence by Line Vinther Hansen
Nitrous oxide and methane fluxes from agricultural soils: Understanding the mechanisms and their controls
Soil ecosystems actively regulate methane (CH4) and nitrous oxide (N2O) concentrations in the atmosphere. In Denmark, where farmland covers 60% of the country's area, agricultural practices significantly contribute to greenhouse gas emissions, and in the face of climate change, mitigating emissions from agricultural systems is paramount.
Upland soils are recognised as CH4 sinks, but the contribution is poorly quantified. This PhD explores CH4 uptake rates in Danish upland agricultural soils, revealing that under conventional management across dominant soil types, crop rotations, and regional weather differences, soils are small net sinks, with regional variations and significant influence of soil moisture and temperature on CH4 uptake.
Nitrous oxide emissions from soils constitute a major concern in Danish agriculture, with soils as large emitters.
This PhD investigates three key drivers of N2O emissions: N fertilisation, rainfall, and freeze-thaw events, which all have the potential to promote N2O production due to microbial stimulation, increased nutrient availability, and lowered oxygen conditions in the soil. Results show that N2O emissions are higher from organic liquid fertilisers than synthetic ones, with regional differences and dependence on soil type and weather conditions. The timing and magnitude of rainfall in relation to spring fertilisation are crucial for N2O emissions. Freeze-thaw events also showed potential for high N2O emissions with intense frost. This research highlights the importance of country-specific estimates of N2O emissions and underline the need for tailored mitigation strategies based on fertilizer types, and the potential impact of rainfall distribution and freeze-thaw events.
Assessment Committee
Professor David Chadwick, Bangor University, United Kingdom (online)
Senior Scientist Arezoo Taghizadeh Toosi, Teknologisk Institut, Denmark
Associate Professor Klaus Steenberg Larsen (Chair), IGN, University of Copenhagen, DK
Supervisors
Professor Sander Bruun, PLEN, University of Copenhagen, Denmark
Professor Lars Stoumann Jensen, PLEN, University of Copenhagen, Denmark
Assistant Professor Azeem Tariq, PLEN, University of Copenhagen, Denmark
The defence is followed by a reception in meeting room R322/23 on 3rd floor