Examining the role of climate, carbon and nitrogen interactions in the terrestrial biosphere

There is compelling evidence showing that terrestrial biosphere has acted as a net carbon (C) sink in recent decades. However there is large uncertainty regarding the magnitude and location of the C sink predominantly due to large uncertainties associated with C emissions from land use change and C sinks due to other processes such as the CO2 fertilization effect, the effect of climate change and the effect of nitrogen (N) deposition. This thesis focuses on understanding and assessing the interactions between the C cycle, N cycle and climate and how they might impact terrestrial C sources and sinks in the context of increasing atmospheric CO2 concentrations ([CO 2]), climate change, N deposition and land use change by using a global terrestrial C-N cycle model in the modeling framework of the Integrated Science Assessment Model (ISAM).;In the first part of this thesis, the geographically explicit terrestrial C cycle component of ISAM is first developed and used to study the C emissions from land use change and the impacts of changes in [CO2] on C sequestration through no-till cropland management. It is shown that C emissions during the 1980's due to land cover changes for cropland and pastureland varied between 1.6 and 2.06 GtC yr-1 globally. The divergence in the estimated land use emissions from two sets of land use change data was primarily due to the differences in the rates of changes in the amount of land area for croplands, and not a result of pastureland emissions or global land use practices. It is also shown that changes in climate and [CO2] were responsible for about 11 Tg or 3% of the soil C sequestered under no-till land management in U.S. and Canada, with the impacts in cold regions are relatively small compared to that in warm and tropical regions.;In the second part of this thesis, a comprehensive model of terrestrial nitrogen (N) dynamics is developed and coupled with the geographically explicit terrestrial C cycle model of ISAM. Observations from the Long-term Intersite Decomposition Experiment (LIDET) dataset were compiled for the calibration and validation of the decomposition submodel. The terrestrial C-N cycle model was then used to evaluate how the introduction of N dynamics and interactions between C, N and climate influences terrestrial C sources and sinks in response to changes over the 20th century in global environmental factors including atmospheric CO2 concentrations, N deposition, climate and land use. This study shows (i) The terrestrial C sink from CO2 fertilization effect is reduced due to the limitation of N (by 0.53GtC/yr in the 1990s), (ii) the positive feedback between climate warming and terrestrial C cycle is attenuated due to the interactions between C, N and climate (by 0.34 GtC/yr in the 1990s), (iii) an enhanced terrestrial sink associated with N deposition (of 0.26 GtC/yr in the 1990s) and (iv) an enhanced source associated with changes in land use due to N limitation (of 0.08 GtC/yr in the 1990s). This study also suggests that the C sink associated with increasing atmospheric CO2 in subtropics is overestimated and the C source associated with changes in temperature and precipitation in higher latitude regions is underestimated when terrestrial N dynamics are not considered. This study highlights the importance of including the N dynamics when assessing terrestrial C sources and sinks with coupled C-climate system models.