Modelling water table depth effects on net ecosystem CO2 exchange of two contrasting forested peatlands – a tropical bog and a boreal fen

• Author / Creator

  Mezbahuddin, Mohammad

• Peatlands have been accumulating carbon (C) in wet soils under shallow water table (WT) over millennia. Increased frequency and intensity of droughts and artificial drainage for promoting agriculture have recently been causing peatland WT depth (WTD) drawdown. This could alter peatland C balance and shift peatlands from net sinks to sources of C. To conserve the resilience of these C stocks, improved predictive capacity is required to forecast how these C stocks would be affected by potentially deeper WT under future drier and warmer climates. Process-based peatland eco-hydrology modelling could provide such capacity. However, such modelling is thus far limited due to lack of prognostic WTD dynamics and poor representation of WTD feedbacks to peatland biogeochemistry. We aimed at using basic processes for water and O2 transport and their effects on ecosystem water, C and nutrient (nitrogen, phosphorus) cycling to model the effects of seasonal and interannual variations of WTD on surface energy exchange, water stress and net ecosystem CO2 exchange across contrasting peatlands under variable weather conditions. For this purpose we tested a process based ecosystem model ecosys under contrasting precipitation in a tropical drained Indonesian bog from a drier El-Niño year 2002 to a wetter year 2005 and in a boreal pristine Western Canadian fen from a wetter year 2004 to a drier year 2009. WTD was modelled from hydraulically-driven water transfers controlled vertically by precipitation (P) vs. evapotranspiration (ET), and laterally by discharge vs. recharge to or from an external reference WTD (WTDx). These transfers caused WTD drawdown and soil drying to be modelled during drier vs. wetter seasons and years in the tropical peatland, which reduced ET and caused plant water stress. WTD drawdown initially increased net ecosystem productivity (NEP) in the tropical peatland by increasing gross primary productivity (GPP) facilitated by improved plant nutrient (phosphorus) availability and uptake due to rapid mineralization in better aerated peats. This better aeration also enhanced microbial O2 availability and energy yields that increased ecosystem respiration (Re). When WT fell below a threshold of ~1.0 m below the hollow surface, increased Re along with reduced GPP from plant water stress reduced NEP. Negative NEP modelled and measured in this drained tropical peatland indicated that it was a large C source. Our undrained model projection showed that this peatland would have been a much smaller source of C had it not been drained. Gradually declining P to ET ratio in the boreal fen peatland caused WTD drawdown and peat drying from 2004 to 2009. Reduction in lateral recharge and increase in lateral discharge from the wettest to the driest year modelled from increasing WTDx also contributed to this WTD drawdown simulating watershed-scale drying effects on fen hydrology. When WT fell below a threshold of ~0.35 m below the hollow surface, intense drying of mosses caused reduction in late growing season ecosystem ET. However, rapid mineralization in better aerated peat improved plant nutrient (nitrogen) availability and uptake that increased vascular and hence ecosystem GPP. Improved microbial O2 availability and energy yields also increased Re in better aerated peats. Similar increases in GPP and Re with WTD drawdown, therefore, caused no net WTD drawdown effects on NEP. Our drainage projection showed that this peatland NEP would decline should WT fell below a threshold of ~0.45 m below the hollow surface due to reduced GPP from both vascular plant water stress and moss drying along with continued increase in Re. This study showed that non-linear interactions between peatland hydrology and ecology across contrasting peatlands can be modelled by adequately simulating dynamic WTD and its effects on peatland biogeochemistry and physiological ecology. The insights gained from this study would aid peatland C monitoring, assessing and restoration initiatives predicting how these C stocks would behave under future drier and warmer climates. This modelling can also provide a platform for scaling up peatland C modelling to regional, continental and/or global scale which is the major challenge current peatland C modelling community is facing.

• Subjects / Keywords

  • Water table depth
  • Net ecosystem productivity
  • Ecosys
  • Bogs and fens
  • Plant water relations
  • Gross primary productivity
  • Process based modelling
  • Ecosystem respiration
  • CO2 exchange
  • Peatland C balance
  • Eco-hydrology

• Graduation date

  Fall 2015

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R32805948

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.