Partitioning evapotranspiration and carbon flux in a tallgrass prairie: effects of rainfall variability and grazing

• Ano de defesa: 2022
• Autor(a) principal: Rocha, Adolpho Emanuel Quintela da
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: eng
• Instituição de defesa: Universidade Federal de Viçosa
• Palavras-chave em Português: Evapotranspiração; Análise de covariância; Pastagens; Plantas - Transpiração; Uso eficiente da água; Agrometeorologia
• Link de acesso: https://locus.ufv.br//handle/123456789/29009; https://doi.org/10.47328/ufvbbt.2022.168

Partitioning evapotranspiration (ET) and carbon flux is key to understanding ecosystem responses to rainfall variability and management practices. In this study, we evaluated the effects of rainfall variability and grazing on carbon and water vapor fluxes in tallgrass prairies. In addition, a footprint model was used to assess the spatial representativeness of eddy covariance (EC) measurement and the influence of surface heterogeneity on energy balance closure (EBC). The research was conducted in two tallgrass prairie sites located in Kansas, U.S. The effects of rainfall variability on fluxes were evaluated during the 2017, 2018 and 2019 growing seasons. The impacts of grazing on fluxes were assessed during the 2003, 2004 and 2005 growing seasons. Carbon and water vapor fluxes were measured using the EC technique. Net ecosystem CO 2 exchange (NEE) partitioning into gross primary productivity (GPP) and ecosystem respiration (R eco ) was performed using the relationship between the air temperature and nighttime NEE data. ET partitioning into transpiration (T) and evaporation (E) was obtained using the concept of underlying water use efficiency (uWUE). To evaluate the uWUE approach, we compared daily E estimates obtained from the uWUE with E observations provided by microlysimeters (ML) during the 2018 growing season. EBC was poor during nighttime and stable atmospheric conditions, but was not affected by surface heterogeneity. The correlations between vertical wind velocity and scalars indicated that low-frequency processes affected EBC. The contribution of T to ET (T/ET) was lowest during the dry growing season (0.50) and higher during wet growing seasons (0.63 and 0.65). The relationship between uWUE approach and ML E measurements showed a Pearson correlation coefficient (r) of 0.42 and a root mean square error (RMSE) of 0.58 mm d -1 . Air temperature was the main environmental driver of T/ET during the wet growing seasons, while the subsurface soil moisture (0.45 m) was the main driver of T/ET during the dry growing season. Grazing did not affect GPP, but increased R eco (9.4%) and reduced NEE (11.9%). T/ET was reduced by grazing (7.0%), while ET increased by 3.3% due to the increase in E (26.6%). Our findings demonstrate that the precipitation variability not only has a direct impact on the ET components but also modulates the response of those components to other environmental drivers. In turn, grazing regime adopted in this tallgrass prairie impacts the carbon cycle more strongly than the water cycle. Keywords: Eddy covariance. Grasslands. Transpiration. Evaporation. Underlying water use efficiency.