Influence of transient flooding on methane fluxes from subtropical pastures

Abstract

Seasonally flooded subtropical pastures are major methane (CH 4 ) sources, where transient flooding drives episodic and high-magnitude emissions from the underlying landscape. Understanding the mechanisms that drive these patterns is needed to better understand pasture CH 4 emissions and their response to global change. We investigated belowground CH 4 dynamics in relation to surface fluxes using laboratory water table manipulations and compared these results to field-based eddy covariance measurements to link within-soil CH 4 dynamics to ecosystem fluxes. Ecosystem CH 4 fluxes lag flooding events, and this dynamic was replicated in laboratory experiments. In both cases, peak emissions were observed during water table recession. Flooding of surface organic soils and precipitation driven oxygen pulses best explained the observed time lags. Precipitation oxygen pulses likely delay CH 4 emissions until groundwater dissolved oxygen is consumed, and emissions were temporally linked to CH 4 production in surface soil horizons. Methane accumulating in deep soils did not contribute to surface fluxes and is likely oxidized within the soil profile. Methane production rates in surface organic soils were also orders of magnitude higher than in deep mineral soils, suggesting that over longer flooding regimes CH 4 produced in deep horizons is not a significant component of surface emissions. Our results demonstrate that distinct CH 4 dynamics may be stratified by depth and flooding of surface organic soils drives CH 4 fluxes from subtropical pastures. These results suggest that small changes in pasture water table dynamics can drive large changes in CH 4 emissions if surface soils remain saturated over longer time scales.Methane fluxes from flooded ecosystems are the product of CH 4 production, consumption, and transport within soils and water. Here CH 4 is produced exclusively by Archaea (methanogens) that convert end products of fermentation, most notably acetate or CO 2 and H 2 , to CH 4 for energy. Methanogens are most active in anaerobic and highly reduced environments, and their activity is generally limited by high oxygen (O 2 ) concentrations, substrate (C) availability, and the presence of alternative electron acceptors used for respiration [Conrad, 2007]. In contrast, CH 4 consuming bacteria (methanotrophs) oxidize CH 4 for energy in the presence of O 2 . Methanotrophs are primarily aerobic, and their activity is often limited by low CHAMBERLAIN ET AL.TRANSIENT FLOODING AND METHANE FLUXES 965 PUBLICATIONS

Seasonal Controls of CO2 and CH4 Dynamics in a Temporarily Flooded Subtropical Wetland

“…Average annual precipitation (1980 to 2015) was 1,310 mm, with two thirds of total annual precipitation falling during the wet season (DayMet database; Thornton et al, ). By quantifying the biophysical controls of CO 2 and CH 4 fluxes from a subtropical wetland and the direct and indirect influence of these controls on C fluxes by combining high‐resolution greenhouse gas (GHG) data with structural equation modeling, this study complements previous work from MAERC that investigated the effect of grazing on C fluxes of subtropical pastures (Gomez‐Casanovas, DeLucia, Bernacchi, et al, ; Gomez‐Casanovas, DeLucia, Bernacchi, et al, ) and the effect of water management (Chamberlain et al, , ) on CH 4 emissions from a subtropical pasture monoculture. Other studies in the area include Chamberlain et al () that investigated the contribution of CH 4 emissions from ruminant fermentation to CH 4 fluxes in a wetland‐pasture landscape mosaic and DeLucia et al () that evaluated the impact of surrounding land use on CH 4 emissions from subtropical wetlands.…”

“…Fluxes of CH 4 from the subtropical wetland were well within the range of values found in tropical mangroves and swamp forests (Table ; Bartlett et al, ; Harriss et al, ; Sjögersten et al, ). However, they were higher than emissions from grazed subtropical and tropical pastures (Chamberlain et al, , ; Gomez‐Casanovas, DeLucia, Bernacchi, et al, ; Gomez‐Casanovas, DeLucia, Hudiburg, et al, ) indicating that although these systems cover an ~3% of global land area, they can contribute significantly to regional CH 4 emissions (Melton et al, ).…”

The impact of water management practices on subtropical pasture methane emissions and ecosystem service payments

“…Pasture CH 4 emissions were driven by landscape flooding and were primarily correlated to water table fluctuations as described in detail in Chamberlain et al. (, ). Ecosystem CH 4 emissions peaked in the wet season during periods of extended pasture flooding, and appreciable emissions were not observed when the water table reached the surface for 1 d or less (Fig.…”

“…We were interested in how long this soil horizon is flooded because, in this region, flooding of the top 15 cm of soil controls ecosystem CH 4 emissions (Chamberlain et al. ). We then estimated total CH 4 emissions due to water retention ( F WR ) by multiplying the annual difference in surface flooding duration ( D ) by the mean CH 4 emission rate when the water table was within the 0–15 cm surface soil horizon ( F SH ), as estimated from our eddy covariance CH 4 flux data and accounting for flux uncertainty.…”

“…Understanding the influence of water retention practices on pasture GHG emissions is important, given the high CH 4 production and emission rates observed from south Florida pasture soils during flooding events (Chamberlain et al. ).…”