Landscape-scale N mineralization and greenhouse gas emissions in Canadian Cryosols

Abstract

Arctic soils store great amounts of soil organic matter (SOM) that are likely to be affected by future climate changes. Knowledge of the ability of the soil to mineralize nitrogen (N) and release greenhouse gases (GHG) at the landscape scale is critical to predict and model future effects of climate change on Arctic SOM. The objective of this study was to investigate how soil gross N mineralization and GHG emissions vary across landscapes and Arctic ecosystems. This study was conducted in three Arctic ecosystems: Sub-Arctic (Churchill, MB), Low-Arctic (Daring Lake, NWT), and High-Arctic (Truelove Lowlands, NU). The topography was divided into five landform units: 1) upper (Up), 2) back (Back), and 3) lower (Low) slopes for catena sites and 4) hummock and 5) interhummock for hummocky sites (i.e., hummock in Churchill and ice-wedge polygons in Truelove). All sites were sampled near the end of their growing seasons (i.e., from two to three weeks before plant senescence). Soil gross N mineralization was measured in situ using a 15N dilution technique, whereas soil GHG emissions (N2O, CH4, and CO2) were measured in situ using a multicomponent Fourier transform infrared gas analyzer combined with an automated dark chamber. For all ecosystems, topography significantly influenced soil gross N mineralization and CO2 emissions. Topography had no significant impact on N2O and CH4 fluxes most likely because net fluxes were extremely low throughout landscapes. Soil gross N mineralization and CO2 emissions increased from Up, Back, to Low and from hummock to interhummock landform units. For example, at Churchill, soil gross mineralization rates averaged 4mg N-NH4+ kg−1d−1 in upper slopes and progressively increased to about 25mg N-NH4+ kg−1d−1 in the lower slopes. Similarly, CO2 emission rates at Daring Lake averaged 0.5μmol CO2 m−2s−1 in upper slopes and progressively increased to about 2.3μmol CO2m−2s−1 in the lower slopes. Comparisons among ecosystems showed that Churchill (Sub-Arctic) had the highest gross N mineralization rates followed by Truelove (High-Arctic) and Daring Lake (Low-Arctic). Furthermore, Daring Lake had significantly higher CO2 emissions than Churchill and no difference in CH4 and N2O emissions between both ecosystems was found. These findings suggest that all factors influencing C and N cycling processes such as climate and human induced changes may not have similar effects across landscapes or across Arctic ecosystems.