Ecosystem-scale CH4 and CO2 fluxes in a seasonally flooded scrub forest of the Brazilian Pantanal

Abstract:

The Pantanal is the largest floodplain in South America, comprised of a mixture of savannah vegetation with patches of semi-deciduous and seasonally flooded forests. In this study we investigated ecosystem-scale methane (CH₄) and carbon dioxide (CO₂) fluxes and the possible factors that control these fluxes, such as the water level soil temperature and the soil redox potential. Trace gas fluxes were measured using an eddy covariance system installed on a 28 m tall tower. The study area was chosen because it is densely vegetated and experiences a seasonal flood pulse of about 6 months, which is typical for the Northern Pantanal. The measurements were performed over two flood cycles, from December to June 2013/2014 and 2014/2015. Methane fluxes showed a seasonal progression, with higher emission rates during the flooding period and near zero fluxes prior to inundation and again after recession. Major peaks of CH₄ (0.30 µmol m^-2 s^-1) were observed after the soil became completely flooded and soil redox values were < -200 mV. The average (± sd) values of CH₄ flux for the 2013/2014 and 2014/2015 seasons were 0.10 ± 0.06 µmol m^-2 s^-1 and 0.14 ± 0.04 µmol m^-2 s^-1, respectively. In contrast, CO₂ fluxes are strongly negative during the flooded period, indicating net CO₂ uptake by the forest, with average (± sd) values of -4.12 ± 3.34 µmol m^-2 s^-1 for 2013/2014 and -4.14 ± 2.62 µmol m^-2 s^-1 for 2014/2015. These data indicate that seasonally flooded forests of the Pantanal are potentially large sinks for CO₂ but strong sources for CH₄, especially during the flood pulse when anaerobic soil conditions concomitantly enhance CH₄ production and limit CO₂ production