Fluxes of carbon dioxide and water vapour at a Waikato peat bog

Abstract

New Zealand wetlands are threatened entities and the remaining 15% of pre-European freshwater wetlands are important refuges for plant and animal species, while also providing historical information on past environmental changes. Hydrological regimes ultimately determine wetland ecosystem structure and function, with peat accumulation believed to be promoted by a high water table that minimises aerobic decomposition of peat. The carbon budget is relevant to the successional development of wetlands, as feedback relationships occur between vegetation, soils, hydrology and bog formation. Research on carbon transfer provides knowledge of bog development, along with information for managing wetland resources.

Moanatuatua Scientific Reserve is a 114 ha remnant of a raised peat bog located 18 km southeast of Hamilton. Drainage of surrounding agricultural land is believed to have lowered the water table at the remnant bog, with peat degradation occurring as a consequence. The aim of this research was to investigate fluxes of carbon dioxide and water vapour at Moanatuatua Scientific Reserve to identify controls of carbon sequestration and determine whether a lowered water table has led to net carbon loss. This knowledge will provide more accurate information for long-term management of raised peat bogs in the Waikato.

A closed-path eddy covariance system was used to measure half-hourly fluxes of CO₂ and water vapour at Moanatuatua between 15 December 1998 and 15 December 2000. Meteorological data and water table elevation were also monitored at half-hourly intervals. A portable chamber system was used to measure fluxes of CO₂ from the peat substrate during field visits.

The present precipitation regime is dominated by low magnitude, high frequency events and this ensures that moisture is generally available in the plant canopy so that 64% of precipitation was removed as evaporation. While the dense plant canopy restricts evaporation from the moist peat surface, plant physiological regulation of transpiration is relatively less important and λЕ is controlled more by availability of radiation than by stomatal controls observed over short periods during summer by previous researchers.

At the diurnal scale, carbon uptake is greatest during summer mornings. The bog was a sink of carbon for almost all of 1999 and 2000, apart from 2-3 months during winter when the wetland was a source of CO₂. Annual sequestration of carbon was 1.85 and 2.10 tCha⁻¹ for 1999 and 2000 respectively: greater than observations at other wetlands around the world. Ecosystem respiration is driven by temperature and GEP was modelled from inputs of PPFD. Reduced magnitude of solar radiation under increasingly cloudy conditions caused by climate change scenarios will have negligible effect on carbon sequestration due to improved radiation use efficiency under these conditions. Compared with annual sequestration for 1999, the magnitude of annual NEP was not reduced in 2000 despite a deeper water table during the latter year. Photosynthetic production may be stimulated by a deeper water table, which would offset increased respiration losses under these conditions. This is encouraging for the sustainability of the wetland habitat, however further research is required to test the hypothesis that sequestered carbon is being stored as peat and not as aboveground biomass.