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AbstractThe eddy covariance technique was employed with a tunable diode laser spectrometer to quantify methane flux from a prairie marsh dominated by Phragmites australis in north-central Nebraska, USA. The observations spanned the entire growing season (April to October) and a wide range of weather conditions, allowing a quantitative assessment of the physical and biological controls of methane emission in this ecosystem. Diel patterns in methane emission varied markedly depending on plant growth stage. Prior to plant emergence above water, the rate of methane emission from the marsh was fairly constant throughout the day. After emergence above water, there was a gradual increase in methane emission after sunrise with a peak in late afternoon. Significant changes in diel patterns were observed after tillering. Then, the diel pattern was characterized by a mid- to late-morning peak and a 2-to 4-fold increase in methane emissions from night to daytime. In early stages of plant growth, molecular diffusion through dead/live plants and the standing water column seemed to be the primary transport mechanism. After tillering, a transition occurred in the transport mechanism from a molecular diffusion to a convective throughflow, which is a rapid and active gas transport driven by pressure differences. The role of convective throughflow became less important as the plants senesced. Integrated methane emission over the six-month measurement period (April–October) was about 64 g CH4 m–2. On an annual basis, we estimate the annual methane emission from this ecosystem to be ≈ 80 g CH4 m–2 and that about 80% of the total methane emission occurred between late April and late October. [ABSTRACT FROM AUTHOR]

Abstract: Drought and fire are common disturbances to grassland ecosystems. We report two years of eddy covariance ecosystem–atmosphere fluxes and biometric variables measured in nearby burned and unburned pastures in the US Southern Great Plains. Over the course of the experiment, annual precipitation (∼600mmyr−1) was lower than the long term mean (∼860mmyr−1). Soil moisture decreased from productive conditions in March 2005 dry, unproductive conditions during the growing season starting in March 2006. Just prior to the burn in early March 2005, burned and unburned pastures contained 520±60 and 360±40gCm−2 of total above ground biomass (AGB) and litter, respectively. The fire removed approximately 200gCm−2 of litter and biomass. In the 2005 growing season following the burn, maximum green AGB was 450±60 and 270±40gCm−2, with corresponding cumulative annual net ecosystem carbon exchange (NEE) of −330 and −150gCm−2 for the burned and unburned pastures, respectively. In contrast to NEE, cumulative mean sensible heat and water fluxes were approximately equal in both pastures during the growing season, suggesting either an increase in water use efficiency or a decrease in evaporation in the burned relative to the unburned pasture. In the 2006 growing season, dry conditions decreased carbon uptake and latent heat, and increased sensible heat fluxes. Peak AGB was reduced to 210±30gCm−2 and 140±30gCm−2 in the burned and unburned pastures, respectively, while NEE was near zero. These results suggest that the lack of precipitation was responsible for most of the interannual variation in carbon exchange for these un-irrigated prairie pastures. [Copyright &y& Elsevier]

Johnson grass (Sorghum halepense (L.) Pers.) is rapidly spreading throughout the continental United States (U.S.). Thus, determining magnitudes and seasonal dynamics of carbon dioxide (CO 2) and water vapor (H 2 O) fluxes in Johnson grass is crucial to understand regional changes in hydrology and carbon balance. Using eddy covariance (EC), CO 2 and H 2 O fluxes were measured from June 2017 to October 2019 over a rainfed Johnson grass field in central Oklahoma. Hay was harvested from late May to early July each year, with biomass yield ~7.5 t ha−1. Weekly averaged daily integrated net ecosystem CO 2 exchange (NEE), gross primary production (GPP), and evapotranspiration (ET) reached −8.28 ± 0.76 g C m−2, 20.02 ± 1.62 g C m−2, and 5.42 ± 0.26 mm, respectively. Ecosystem water use efficiency (EWUE) and ecosystem light use efficiency (ELUE) ranged from 3.22 to 3.93 g C mm−1 ET and 0.34 to 0.41 g C mol−1 PAR (photosynthetically active radiation), respectively, during peak growths. Based on aggregated fluxes for each month over the three years (2017–2019), cumulative annual NEE was −434 ± 112 g C m−2, indicating a carbon gain by the Johnson grass field. Cumulative annual ET (858 ± 72 mm) was ~86% of the average annual rainfall (996 ± 100 mm). Results showed Johnson grass could be a carbon sink from May to September in the U.S. Southern Great Plains. Both NEE and ET did not decline up to air temperature (T a) of ~33 °C and vapor pressure deficit (VPD) of ~2 kPa, suggesting optimum T a of ≥33 °C and VPD of ≥2 kPa for the fluxes. Results indicated that Johnson grass might be well suited for dryland production in the region. Additionally, these findings provide initial baseline information on CO 2 fluxes and ET for Johnson grass relative to other forage species in the region. Unlabelled Image • First work to thoroughly study dynamics of CO 2 and H 2 O fluxes in Johnson grass. • Johnson grass harvested for hay showed large carbon uptake potential at annual scales. • Johnson grass could be a carbon sink in the southern plains during May–Sept period. • CO 2 uptake did not decrease up to air temperature of ~33 °C. • Cumulative annual ET (858 ± 72 mm) was ~86% of the average annual rainfall. [ABSTRACT FROM AUTHOR]