Agricultural drainage impacts on the photosynthetic capacity and water use efficiency of boreal peatlands

Gyimah, Asare (2018) Agricultural drainage impacts on the photosynthetic capacity and water use efficiency of boreal peatlands. Masters thesis, Memorial University of Newfoundland.

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Abstract

Even though boreal peatlands cover a relatively small area of the Earth’s surface compared to forested areas, they are known to contain approximately a quarter of the total organic carbon in the soil. As a globally important store of carbon and terrestrial surface water, natural peatland ecosystems are known as a sink for atmospheric carbon dioxide (CO₂) since the rate of carbon accumulation through photosynthesis exceeds the rate of decomposition. The slow decomposition rate and the subsequent accumulation of carbon under the anoxic condition are mainly due to the high water table depth (WTD) in peatland ecosystems. However, peatlands have been drained for agricultural purposes. Peatland drainage for agricultural purposes helps to improve soil aeration condition by deepening the water table depth. This keeps the soil dry and improves plant growth and development. Since plants control their stomata in order to optimize the trade-off between the amount of carbon absorbed and the amount of water loss, I anticipated that the improved plant growth and development through drainage could also impact on the evapotranspiration rate (ET) (a critical component in the water balance of terrestrial ecosystems) and hence, the water use efficiency (WUE) of peatland ecosystems. Therefore, this study was aimed to investigate the effects of peatland drainage for agriculture on the photosynthetic capacity (represented as gross primary productivity (GPP)) and the WUE of boreal peatlands, on a plot and site scale. Here, a comparison was made between agriculturally drained and a natural peatland during the 2016 growing season. GPP was found to be significantly higher among the subplots affected by drainage compared to the hummock and hollow subplots at the natural site. Site average showed that, during the 2016 growing season, GPP (mgCO₂/m²/s) was significantly higher at the drained site (0.468 ± 0.04) compared to the natural site (0.093 ± 0.01) (p < 0.001). The seasonal pattern of GPP was linked to WTD. WTD variation and its effect on the physical and hydrothermal properties of the peat (e.g., electrical conductivity (EC) and peat temperature at 5cm depth (T₅)), was seen to account for most of the variation in GPP at the drained subplots and site relative to the natural site. Respectively, a decrease in soil moisture (SM), EC, and increase in T₅, significantly explained ~ 44%, 49% and 27% of variation in GPP at the drained site. The combined effect of EC and T₅ accounted for approximately 61% of the variation in GPP at the drained site. Growth in the hollow subplot was also seen to be at its peak when the WTD was at its deepest point. However, the deepest WTD seen at the hummock subplot did not lead to any production increase. This was attributed in part to the extremely lower water table depth at the hummock and its effect on water availability for plant growth and development, and the inability of the peat makeup to hold on to water during the summer when the water level dropped. In spite of statistically similar ET rate between the hummock and hollow subplots, the results showed a higher WUE (i.e., the ratio of GPP to ET) when GPP is high. However, when comparing the drained and natural sites, higher ET rate at the drained site still resulted in a higher WUE due to its significantly higher GPP rate, making GPP the primary controller of WUE. WUE (mgCO₂/mgH₂O) was significantly higher (0.047 ± 0.01) at the drained site relative to the natural site (0.005 ± 0.00). Between the hummock and hollow subplots, WUE (mgCO₂/mgH₂O) was significantly higher at the hollow subplot (0.010 ± 0.001) compared to the hummock subplot (0.005 ± 0.001). No significant difference in WUE was found among the plant functional types (PFTs) as GPP was statistically the same for the sedge, shrub and the grass subplots at the drained site. This study suggests that land use changes, microforms and PFTs would be significant in regulating the carbon and hydrological cycle in peatland ecosystems, and thus land use practices, microforms and PFTs need to be explicitly parameterized in modeling the carbon and hydrological cycle in peatland ecosystems.

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