The role of the grassland biome in the global carbon cycle

This thesis starts from the observation that the terrestrial biosphere exhibits a large annual CO2 sink which equates 50% of anthropogenic CO2 emissions. The study explores how grassland ecosystems could have a role in this net CO2 sink. It analyses how a CO2 balance emerges from its constituent processes. Its chapters develop how diurnal cycles of assimilatory and respiratory activity aggregate to an annual cycle of CO2 exchange between atmosphere and grassland ecosystem. It establishes why the annual net CO2 exchange or CO2 balance can differ among years. Particular attention is given to drained peat grassland ecosystems. Their aerobic soil profiles are characterised by the accelerated decomposition of peat, which results in a release of CO2. In the succession of chapters the temporal and spatial scales of the CO2 exchange processes increase. It navigates from instant grass sward processes to instant ecosystem processes, which then aggregate to annual patterns of CO2 exchange.The lowest level of process aggregation introduces an experiment in which the photosynthetic activity in an in vivo grown grass sward was measured under laboratory conditions. Photosynthetic rate was measured as a function of irradiance, temperature and ambient CO2 concentration. Photosynthetic dynamics are analysed by applying a process description where photosynthetic rate is determined by both electron transport rate in the thylakoid membrane and carboxylation at low irradiance. It shows that it is difficult to apply biochemical processes to in vivo leaf area. Aggregated hyperbolic response functions instead show that the leaf photosynthetic rate at zero irradiance consistently decreased with temperature; the initial rate consistently increased with CO2 concentration. Photosynthetic rate decreased with temperature in summer and showed a temperature optimum in autumn.It reports on aerodynamic gradient measurements of CO2 flux and energy done in a grazed drained peat grassland during a period of two years. The measured instantaneous net CO2 flux was separated into a respiratory and a gross assimilatory CO2 flux. The respiratory CO2 flux responded to temperature in a Q10 type of relationship, whereas the assimilatory CO2 flux responded to irradiance hyperbolically. Low temperature strongly limited the initial response of the assimilatory CO2 flux to irradiance during much of the growing season. Effects of vapour pressure deficit and calculated surface conductance on the assimilatory CO2 flux were not detected, possibly as a result of the maritime weather conditions. Moderate temperatures suppressed respiratory activity more than assimilatory activity. It is analysed how the downward net irradiance dissipates into an upward latent heat flux and sensible heat flux. Their interdependence is reflected in the Bowen ratio. Several months during the growing season were identified with an increased Bowen ratio, thus pointing at a limiting ecosystem surface conductance and thereby suppressing the assimilatory CO2 flux more than the respiratory CO2 flux.It reports on eddy covariance measurements of CO2 flux done during a growing season in a grazed peat grassland at two different drainage levels. The experiment aims to measure the difference in the respiratory CO2 flux between both drainage levels, as a measure for the difference in decomposition of peat in both aerobic soil profiles. Direct comparison of the respiratory CO2 fluxes was not possible because the greatest part of these fluxes are associated with living biomass, with differences too small to be detectable. An approach was chosen in which the instantaneous net CO2 flux itself responded to the gross assimilatory CO2 flux and temperature. The ecosystem net CO2 flux tended towards sequestration at an increasing assimilatory CO2 flux and towards release at increasing temperature. Comparison of the net CO2 fluxes at both drainage levels indicated that the net CO2 release at deep drainage was higher than at a more shallow drainage.The instantaneous CO2 fluxes and their environmental parameters from preceding chapters are subsequently aggregated to monthly values. This results in the annual cycle of CO2 exchange at a time resolution where effects of differences in weather conditions on the CO2 flux components and the annual net CO2 exchange can be assessed. It shows that the annual cycle of the gross assimilatory CO2 flux responded strongly and linearly to irradiance. The annual baseline response was mediated by limiting effects of low temperature and high vapour pressure deficit. Low temperature had relatively little impact on the annual assimilatory CO2 flux, generally occurring at low irradiance. However, high vapour pressure deficit appeared to be a major limiting factor in the annual assimilatory CO2 flux. The annual cycle of the respiratory CO2 flux responded to temperature following a Q10 type of relationship. The hysteresis in the response of the respiratory CO2 flux to the assimilatory CO2 flux was related both to higher temperatures and to an increase in amount of decomposing litter as the season progresses. For the grazed drained peat grassland ecosystem annual net CO2 releases of 623 and 920 g CO2 m-2 y-1 were calculated. An export of C in dairy produce equivalent to approximately 200 g CO2 m-2 y-1 should be partly or entirely added to the ecosystem respiratory CO2 flux.The general discussion reflects on the research methodology and weaves together the processes at the successive levels of aggregation. It demonstrates how patterns of photosynthetic activity in the grass sward re-appear in instant ecosystem CO2 exchange and how patterns of instant ecosystem CO2 exchange translate into an annual cycle of CO2 exchange. In the annual cycle of CO2 exchange higher levels of assimilatory activity lead to a higher net CO2 sequestration whereas higher temperature through its dominating effect on respiratory activity leads to a lower net CO2 sequestration. This emphasises the significance of conditions where a moderately warm but productive growing period and low temperatures outside the growing period enhance net CO2 sequestration. Such conditions particularly apply to ecosystems in the temperate and boreal climate zones. It discusses how grasslands could have a substantial role in this biospheric net CO2 sequestration. It is argued that reinforcing the natural role of the biosphere by restoration of degraded ecosystems such as peatlands is an effective approach to mitigating anthropogenic CO2 emissions.