Dynamics of CO2 and H2O fluxes in Johnson grass in the U.S. Southern Great Plains

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 (CO₂) and water vapor (H₂O) fluxes in Johnson grass is crucial to understand regional changes in hydrology and carbon balance. Using eddy covariance (EC), CO₂ and H₂O 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 CO₂ 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 CO₂ fluxes and ET for Johnson grass relative to other forage species in the region.

Net ecosystem exchange of CO2 and H2O fluxes from irrigated grain sorghum and maize in the Texas High Plains

Net ecosystem exchange (NEE) of carbon dioxide (CO₂) and water vapor (H₂O) fluxes from irrigated grain sorghum (Sorghum bicolor L. Moench) and maize (Zea mays L.) fields in the Texas High Plains were quantified using the eddy covariance (EC) technique during 2014-2016 growing seasons and examined in terms of relevant controlling climatic variables. Eddy covariance measured evapotranspiration (ET_EC) was also compared against lysimeter measured ET (ET_Lys). Daily peak (7-day averages) NEE reached approximately -12 g C m^-2 for sorghum and -14.78 g C m^-2 for maize. Daily peak (7-day averages) ET_EC reached approximately 6.5 mm for sorghum and 7.3 mm for maize. Higher leaf area index (5.7 vs 4-4.5 m² m^-2) and grain yield (14 vs 8-9 t ha^-1) of maize compared to sorghum caused larger magnitudes of NEE and ET_EC in maize. Comparisons of ET_EC and ET_Lys showed a strong agreement (R² = 0.93-0.96), while the EC system underestimated ET by 15-24% as compared to lysimeter without any corrections or energy balance adjustments. Both NEE and ET_EC were not inhibited by climatic variables during peak photosynthetic period even though diurnal peak values (~2-weeks average) of photosynthetic photon flux density (PPFD), air temperature (T_a), and vapor pressure deficit (VPD) had reached over 2000 μmol m^-2 s^-1, 30 °C, and 2.5 kPa, respectively, indicating well adaptation of both C₄ crops in the Texas High Plains under irrigation. However, more sensitivity of NEE and H₂O fluxes beyond threshold T_a and VPD for maize than for sorghum indicated higher adaptability of sorghum for the region. These findings provide baseline information on CO₂ fluxes and ET for a minimally studied grain sorghum and offer a robust geographic comparison for maize outside the United States Corn Belt. However, longer-term measurements are required for assessing carbon and water dynamics of these globally important agro-ecosystems.

Land use affects the net ecosystem CO(2) exchange and its components in mountain grasslands

Changes in land use and management have been strongly affecting mountain grassland, however, their effects on the net ecosystem exchange of CO(2) (NEE) and its components have not yet been well documented. We analysed chamber-based estimates of NEE, gross primary productivity (GPP), ecosystem respiration (R) and light use efficiency (LUE) of six mountain grasslands differing in land use and management, and thus site fertility, for the growing seasons of 2002 to 2008. The main findings of the study are that: (1) land use and management affected seasonal NEE, GPP and R, which all decreased from managed to unmanaged grasslands; (2) these changes were explained by differences in leaf area index (LAI), biomass and leaf-area-independent changes that were likely related to photosynthetic physiology; (3) diurnal variations of NEE were primarily controlled by photosynthetically active photon flux density and soil and air temperature; seasonal variations were associated with changes in LAI; (4) parameters of light response curves were generally closely related to each other, and the ratio of R at a reference temperature/ maximum GPP was nearly constant across the sites; (5) similarly to our study, maximum GPP and R for other grasslands on the globe decreased with decreasing land use intensity, while their ratio remained remarkably constant. We conclude that decreasing intensity of management and, in particular, abandonment of mountain grassland lead to a decrease in NEE and its component processes. While GPP and R are generally closely coupled during most of the growing season, GPP is more immediately and strongly affected by land management (mowing, grazing) and season. This suggests that management and growing season length, as well as their possible future changes, may play an important role for the annual C balance of mountain grassland.

Contribution of root respiration to soil respiration in a C3/C4 mixed grassland

The spatial and temporal variations of soil respiration were studied from May 2004 to June 2005 in a C3/C4 mixed grassland of Japan. The linear regression relationship between soil respiration and root biomass was used to determine the contribution of root respiration to soil respiration. The highest soil respiration rate of 11.54 micro mol m-2 s-1 was found in August 2004 and the lowest soil respiration rate of 4.99 micro mol m-2 s-1 was found in April 2005. Within-site variation was smaller than seasonal change in soil respiration. Root biomass varied from 0.71 kg m-2 in August 2004 to 1.02 in May 2005. Within-site variation in root biomass was larger than seasonal variation. Root respiration rate was highest in August 2004 (5.7 micro mol m-2 s-1) and lowest in October 2004 (1.7 micro mol m-2 s-1 ). Microbial respiration rate was highest in August 2004 (5.8 micro mol m-2 s-1 ) and lowest in April 2005 (2.59 micro mol m-2 s-1 ). We estimated that the contribution of root respiration to soil respiration ranged from 31% in October to 51% in August of 2004, and from 45% to 49% from April to June 2005.