Greenhouse gas emissions from Boreal Lakes: Highlighting the impact of salinity and freezing period on emission dynamics.

Lakes and ponds in boreal regions are considerable natural sources of greenhouse gases (GHGs), including carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O). Although the seasonal variability of GHG emissions from boreal lakes is crucial for improving global emission models, emissions during the freezing period have not received sufficient attention. Focusing on two representative boreal lakes in China-Ulansuhai and Daihai-this study investigated variations in GHG emissions during both the non-freezing and freezing periods. The concentrations of CO₂ and CH₄ in lake porewater during the non-freezing period were observed to be 50 to 74 times higher than those during the freezing period. In both lakes, CO₂ and CH₄ emissions predominantly occurred at the water-air interface, with N₂O absorption. The Global Warming Potential (GWP) of GHGs in Ulansuhai was 234.35×10⁴ kg/yr, with CO₂, CH₄, and N₂O contributing 12.0 %, 87.4 %, and 0.6 %, respectively. In Daihai, the GWP was 40.47×10³ kg/yr, with CO₂ CH₄, and N₂O contributing 40.4 %, 24.5 %, and 35.1 %, respectively. Notably, the GHG 'storage' capacities of Ulansuhai and Daihai were 227.51 × 10⁵ kg/yr and 9.23 × 10² kg/yr, respectively. In both lakes, dissolved organic carbon and total nitrogen in the porewater exhibited a negative relationship with GHG concentrations. Compared to lake Ulansuhai, salinity exhibited a stronger correlation with GHGs in lake Daihai, which has high salinity. Our research reveals that the freezing period and the salinity (in high salinity lakes) have distinct impacts on GHG emissions in boreal lakes. The findings are crucial for understanding the contributions of boreal lakes to GHG emissions and their potential impact on climate change, and provide vital information for developing conservation and management strategies regarding these ecosystems.