Flux of carbon and greenhouse gases in a pasture ecosystem constituted by Panicum maximum Jacq. and Stylosanthes capitata Voegel cultivated under elevated CO2 and warming in a FACE/T-FACE system

Abstract

Climatic changes such as global warming and changes in rainfall patterns associates to rising in atmospheric concentration of greenhouse gases (GHG) such as carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) in the atmosphere caused by human activities are happening at a pace never experienced by the planet. Recently, the global atmospheric CO2 concentration has already exceeded 400 umol mol-1, which shows that the increase of GHG is occurring at higher rates than those estimated by predictive models. Despite the existence of several forecasts, the effects of climate change on ecosystems are not well elucidated. Regarding to pasture ecosystems, it is expected that the increased CO2 concentration and temperature will cause: 1) an increased amount of carbon entering the ecosystem through photosynthesis; 2) changes in the amount of CO2 leaving ecosystem by autotrophic (Ra) and heterotrophic (Rh) respiration; 3) changes in the gross (GPP) and net (NPP) primary productivity of ecosystems; 4) changes in the emission of methane (CH4), both by the anaerobic decomposition of organic matter in the soil, as well as by the enteric fermentation of forage consumed by livestock. 5) It is possible that the increase in temperature will increases the emission of nitrous oxide (N2O) from nitrogen fertilizers and by increased activity of soil microbes. In Brazil, it is estimated that pastures for livestock to produce meat and milk, including areas previously occupied by native forests, occupy approximately 160 million hectares. Thus, the effects of climate change on the flow of carbon dioxide and other GHG, as well as its effects on the productivity of grassland ecosystems will be important to estimate the role of pastures as carbon sinks or instead as sources of greenhouse gases. This research proposal aims to quantify the flow of carbon and the emission of methane and nitrous oxide in a cultivated pasture system simulating a conservative scenario (B1) climate change. Thus, two forage species: Panicum maximum Jacq. cv Mombasa (C4 grass) and Stylosanthes capitata Voegel (Legumes C3), will be grown in field under high CO2 concentration (600 µmol mol-1) and high temperature (+2 ° C) in a FACE (Free-air carbon dioxide combined system enrichment) and T-FACE (Free-air temperature controlled enhancement), to simulate future conditions of elevated CO2 and elevated temperature, respectively. In this study will be evaluated, 1) The net primary productivity (NPP, gC m-2 day-1); 2) The net ecosystem CO2 exchange (NEE, gC m-2 day-1) measured in daytime (NEEday) and night (NEEnight) using open-top chambers installed inside the FACE/T-FACE system. 3) The soil (Rs), autotrophic (Ra) and heterotrophic (Rh) respiration as well as the ecosystem respiration (Re), expressed as gC m-2 day-1; 4) The release of methane (CH4) produced by soil (gC m-2 day-1) and the CH4 produced by enteric fermentation of forage biomass (gCH4 g MS-1); 5) The emission of N2O from the soil (µg m-2 h-1); 6) The primary ecosystem productivity (NEP, gC m-2 day-1) estimated by the equation: NEP = NPP - Re - CH4; 7)The gross primary productivity (GPP, gC m-2 day-1) will be estimated by modeling. The heterotrophic soil respiration will be estimated by the carbon isotope discrimination released from the soil as CO2. This work will contribute to understanding the dynamics of the flow of carbon, emission of GHG as CH4 and N2O and adaptation of a tropical pastures to future climate change scenario, as well as to understand the role of pastures in carbon sequestration and mitigation, generating scientific basis that could contribute to making decisions more suitable for grazing management in tropical regions like Brazil. (AU)