Quantifying biophysical and biogeochemical influence on northern high latitudes greenhouse gases dynamics
In this dissertation, I examined the impacts of biogeophysical-biogeochemical interactions on northern high latitudes (NHLs) greenhouse gases (GHGs) emission in a terrestrial modeling framework. To achieve this objective, I extended and calibrated a land surface model, the Integrated Science Assessment Model (ISAM), to explore how the physical and chemical interactions within the atmosphere-land-soil continuum determined the production, transport and flux of GHGs from site to regional and global scales. I focused on the impacts of three key GHGs that can largely contribute to climate change in terrestrial processes: (1) Carbon Dioxide (CO2), (2) Methane (CH4) and (3) Nitrous Oxide (N2O). In Chapter 1, I provided a brief introduction of the overall objectives and content of this dissertation. The importance of representing (1) vertical heterogeneity of soil column, (2) CH4 transport pathways from soil to the atmosphere and (3) the soil oxygen availability and soil anoxia condition in the land surface model are highlighted. Measured data from multiple sources have been highlighted. In Chapters 2, I developed vertically-resolved soil biogeochemistry (carbon and nitrogen) module and implemented it into a land surface model, ISAM. The model captures the vertical heterogeneity of the Northern High Latitudes permafrost soil organic carbon (SOC). I also implemented Δ14C to estimate SOC turnover time, a critical determinant of SOC stocks, sequestration potential, and the carbon cycle feedback under changing atmospheric CO2 concentration [CO2] and climate. ISAM accounted for the vertical movement of SOC caused by cryoturbation and its linkage to the frost heaving process, oxygen availability, organo-mineral interaction and, depth-dependent environmental modifiers. After evaluating the model processes using the site and regional level heterotrophic respiration, SOC stocks and soil Δ14C profiles, the vertically-resolved soil biogeochemistry version of the model (ISAM-1D) estimated permafrost SOC turnover time of 1443 years, which is about three times more than the estimation based on the without the vertically-resolved version of ISAM (ISAM-0D). ISAM-1D simulated SOC stocks for permafrost regions was 319 PgC in the top 1 m soil depth by the 2000s, about 80% higher than the estimates based on ISAM-0D. ISAM-1D SOC stock and turnover time were compared well with the observations. However, the longer SOC turnover time preserves less SOC stocks due to the lower carbon use efficiency (CUE) for SOC than ISAM-0D, thus respires more SOC than being transferred downward by cryoturbation. ISAM-1D simulated reduced SOC sequestration (3.7 PgC) compared to ISAM-0D (4.8 PgC) and published ESMs over the 1860s-2000s, due to weaker [CO2]-carbon cycle and stronger climate-carbon cycle feedbacks, highlighting the importance of the vertically heterogeneous soil for understanding the permafrost SOC sinks. Chapter 3 estimated the distribution of CH4 emissions and sinks from wetlands (including freshwater and coastal wetlands) and non-wetland (including wet and dry soils) with a newly-developed vertically-resolved soil CH4 model, integrated into ISAM. I calibrated and tested this integrated model with CH4 observations at test sites in the Contiguous United States (CONUS). ISAM is applied across the CONUS to estimate CH4 emissions and sinks given both recent past observed climate and wetland extent and future climate and wetland extent driven by two scenarios, RCP4.5 and RCP8.5. Estimated net CH4 emissions for the 2000s are 13.8 TgCH4 yr-1, mostly from wetland soils. Estimated net emissions under RCP4.5 and RCP8.5 are 30% and 64% higher, respectively, in the 2090s than in the 2000s due to 1) higher temperature and seasonal wetland extent (driven by higher precipitation in the climate scenarios), which increase modeled methanogenic activity more than methanotrophic activity in soils, and 2) altered transport in the soil column and exchange with the atmosphere by modeled transport processes (diffusion, ebullition, and aerenchyma transport). Non-wetland soils emit CH4 (1.4 TgCH4 yr-1) in some areas and take up CH4 (-2.9 TgCH4 yr-1) in other areas, resulting in a net estimated sink for the 2000s; the net non-wetland soil sink increases by 15% and 46 % by the 2090s under RCP4.5 and RCP8.5, respectively, mainly due to drier soil conditions, which enhances the methanotrophic activity and oxidation of CH4 diffused into the soil from a future atmosphere with higher CH4 concentration. In Chapter 4, I investigated the improvement of estimation of N2O through applying soil oxygen availability as a factor instead of the water-filled pore space approach, which is a widely used empirical proxy of oxygen availability. The results showed the WFPS approach overestimated the soil N2O emission for almost all different plant biome types, which may cause an overestimation of N2O response to the change of environmental factors such as soil temperature. I also incorporated the harvest of animal feed and the manure production scheme into ISAM to estimate the N2O emission from the globe after applying N fertilizer and manure. The addition of N fertilizer and manure N increase the N2O emission from the soil by 24% over the decade 2001 – 2010. Finally, Chapter 5 provides an overall summary and potential direction for further works related to this thesis.
Northern High-Latitudes, Permafrost, Soil Organic Carbon Dynamics, Soil Methane, Wetland Methane Emissions, Non-wetland Methane Emissions, Methane Dynamic Model, Nitrous Oxide
University of Illinois - Urbana-Champaign
Networked Digital Library of Theses and Dissertations
University of Illinois at Urbana-Champaign