Greenhouse gas exchange and nitrogen cycling in Saskatchewan boreal forest soils
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Despite the spatial significance of Canada’s boreal forest, there is very little known about greenhouse gas emissions within it. The primary objective of this project was to study the atmosphere-soil exchange of CH4 and N2O in the boreal forest of central Saskatchewan. In the summers of 2006 and 2007, greenhouse gas emissions were measured along transects in three different mature forest stands (trembling aspen, black spruce and jack pine) using a sealed chamber method. In addition, the gross rates of mineralization and nitrification, and the relative contribution of nitrification and denitrification to N2O emissions, were measured at the trembling aspen site using a stable isotope technique in which 15N-enriched nitrate and ammonium were injected into intact soil cores. The amount of 14N found in the labeled pools was used to measure the gross rates, and the amount of 15N found in the emitted N2O was used to determine the relative contribution of the different N pathways to total N2O emissions. Results indicated that the jack pine and black spruce sites were slight sinks of CH4 (-1.23 kg CH4-C ha-1 yr-1and -0.17 kg CH4-C ha-1 yr-1 respectively in 2006 and -0.95 kg CH4-C ha-1 yr-1and 0.45 kg CH4-C ha-1 yr-1 respectively in 2007), whereas the trembling aspen site was a net source (46.7 kg CH4-C ha-1 yr-1 in 2006 and 196.0 kg CH4-C ha-1 yr-1 in 2007). All three sites had very low cumulative N2O emissions, ranging from -0.02 to 0.14 kg N2O-N ha-1 yr-1 in both years. Of the environmental controls examined for CH4, consumption at the jack pine site was correlated positively with organic carbon and negatively with water-filled pore space. Black spruce CH4 emissions were correlated negatively with both organic carbon and clay content, and emissions at the trembling aspen site were positively correlated with soil temperature and organic carbon, while also related to the presence of standing water (2006 and 2007 had very high precipitation, causing a high water table and ponding in depressions). The N2O emissions were not correlated with any of the environmental parameters measured at the jack pine or black spruce sites, but clay content was positively related to emissions at the trembling aspen site. The 15N results indicated that N cycling at the trembling aspen site was very rapid, allowing little N to escape the system as N2O; the majority of emissions that did occur were due to a nitrification-related process.
DegreeMaster of Science (M.Sc.)
SupervisorPennock, Dan J.