PhD Thesis
Ecosystem-atmosphere exchange of carbon dioxide and water vapour in typical East-Asian croplands
Peng Zhao (07/2014-07/2014)
Support: Johannes Lüers
Agricultural areas comprise a large portion of the land surface of the earth. Under the current climate and environmental changes, the role of cropland ecosystems in the surfaceatmosphere energy and matter exchange has drawn a lot of attention. This thesis investigated the potential change on ecosystem-atmosphere exchange of heat, water vapour and carbon dioxide if crop species planted or irrigation management is changed in croplands. Improvement of the Penman-Monteith model to simulate evapotranspiration and light-response model to simulate carbon dioxide ux for croplands is studied. Field campaigns were carried out in an irrigated rice eld and a non-irrigated potato eld in an intensively managed agricultural area in Korea Peninsula in 2010 and 2011. The eddy-covariance technique was used to observe net ecosystem carbon dioxide exchange and energy uxes of sensible and latent heat with additional measurements of meteorological variables and biomass change.
Results show that the conventional Penman-Monteith approach which estimates the stomatal resistance as a function of single leaf stomatal resistance and leaf area index performs well for the estimation of evapotranspiration when the vegetation is fully developed in the potato eld. In the case of evaporation-dominated croplands, i.e. the rice eld with standing water or the potato eld with small leaf area index in this study, it is shown that the stomatal resistance should be estimated as a function of meteorological variables rather than leaf area index.
The study on the light response function indicates that the primary cause of seasonal change in gross primary productivity was the change in leaf area index during the whole growing season under the summer monsoonal condition. Thus, a site-speci c time window approach could signi cantly improve the model performance. In its standard form, however, the light response function does not account for leaf area index changes. In order to simulate longer time series, which is needed for lling large gaps in the observations, a new leaf-light response function is proposed.
A signi cant change in ecosystem-atmosphere exchange of heat, water vapour and carbon dioxide is found if irrigation management is changed. The non-irrigated potato eld had 140% more sensible heat and 30% less latent heat than the nearby irrigated rice eld. The dierence in evapotranspiration between these two elds was mostly attributed to less evaporation (rather than transpiration) in the potato eld than in the rice eld. The seasonal sum of carbon dioxide ux was 12% less for gross primary productivity, 7% less for ecosystem respiration, and 20% less for net ecosystem exchange in the potato eld than in the rice eld. The rice eld acted as a sink of carbon dioxide through the whole season, while the potato eld turned from a sink to a slight source at the late growing stage when the above-ground green biomass disappeared. Besides the decline in solar radiation and the warm conditions in summer monsoon, the enhancement of ecosystem respiration caused by the large amount and the rapid growth rate of the biomass is suggested to be a cause of the mid-season depression in net ecosystem exchange.