Growing season variability in carbon dioxide exchange of irrigated and rainfed soybean in the southern United States

CO2 flux in a wheat-soybean succession in subtropical Brazil: A carbon sink

The subtropical region of Brazil is home to 33% of the soybean [Glycine max (L.) Merr.] growing area and 90% of the wheat (Tritucum aestivum L.) growing area of this country. A soybean-wheat succession with fallow between crops is used in about 11% of the cultivated area. No study has quantified CO2 fluxes in annual soybean-wheat succession in this region. Hence, this study analyzed the seasonality of CO2 exchange (net ecosystem exchange [NEE]) in a 2015/2016 wheat-soybean succession in a commercial farm located in Carazinho, Rio Grande do Sul State, Brazil. The eddy covariance method was used to estimate the annual C balance of this system. The NEE was partitioned between gross primary productivity and ecosystem respiration to understand the dynamics of these fluxes during a year of wheat-soybean succession. Considering the net ecosystem balance between photosynthesis and respiration during the growing season, both soybean and wheat absorbed CO2 from the atmosphere (NEE wheat: -347 ± 4 g C m-2 ; NEE soybean: -242 ± 3 g C m-2 ). The fallow periods between growing seasons, however, acted as a source of 156 ± 2 g C m-2 , reducing the C absorbed by the crops by 27%. For 1 yr, the net biome productivity was -50 g C m-2  yr-1 . The results obtained here demonstrate that the wheat-soybean succession was a net C sink under these specific climatic conditions and field management practices and that the long fallow period between crops limited the agroecosystem from becoming a more efficient CO2 sink.

Effects of clouds and aerosols on ecosystem exchange, water and light use efficiency in a humid region orchard

Investigating the patterns of water and carbon dynamics in agro-ecosystems in response to clouds and aerosols can shed new insights in understanding the biophysical impacts of climate change on crop productivity and water consumption. In this study, the effects of clouds and aerosols as well as other environmental factors on ecosystem water and carbon fluxes were examined based on three-year eddy covariance measurements under different sky conditions (quantified as the clearness index, K_t, i.e., the ratio of global solar radiation to extraterrestrial solar radiation) in a kiwifruit plantation in the humid Sichuan Basin of China. Results showed that evapotranspiration (ET) and canopy transpiration (T_c, measured by sap flow sensors) increased, while ecosystem light use efficiency (eLUE) and ecosystem water use efficiency (eWUE) decreased with increasing K_t. GPP presented a parabolic relationship with increasing K_t. The path analysis revealed that surface conductance (G_s) and canopy conductance (G_c) were the most dominant variables directly regulated carbon (GPP) and water (ET and T_c) fluxes. The effect path of K_t on ET and T_c was converted from through diffuse photosynthetic active radiation (PAR_dif) to direct PAR (PAR_dir) when the sky became clearer. The effect path of K_t on GPP was primarily through PAR_dif under different sky conditions. The declined eWUE with increasing K_t was caused by the different responses of GPP and ET to PAR_dir under clear skies. The declined eLUE resulted from the sharp decrease in GPP/PAR_dir, which surpassed the slight increase of GPP/PAR_dif with increasing PAR. The Priestley-Taylor Jet Propulsion Laboratory ET model (PT-JPL) incorporating K_t with an exponential function produced more reliable T_c estimates but minor improvement in ET. Further, the LUE-GPP model incorporating K_t with a linear function obtained much better GPP estimates. Our study shed light on how sky conditions modulate water and carbon dynamics between the biosphere and atmosphere, highlighting the necessity of the inclusion of sky conditions for better modeling regional water and carbon budgets.

Net ecosystem CO2 exchange from jute crop (Corchorus olitorius L.) and its environmental drivers in tropical Indo-Gangetic plain using open-path eddy covariance technique

Present study is a maiden attempt to assess net ecosystem exchange (NEE) of carbon dioxide (CO₂) flux from jute crop (Corchorus olitorius L.) in the Indo-Gangetic plain by using open-path eddy covariance (EC) technique. Diurnal variations of NEE were strongly influenced by growth stages of jute crop. Daytime peak NEE varied from - 5 µmol m^-2 s^-1 (in germination stage) to - 23 µmol m^-2 s^-1 (in fibre development stage). The ecosystem was net CO₂ source during nighttime with an average NEE value of 5-8 μmol m^-2 s^-1. Combining both daytime and nighttime CO₂ fluxes, jute ecosystem was found to be a net CO₂ sink on a daily basis except the initial 9 days from date of sowing. Seasonal and growth stage-wise NEEs were computed, and the seasonal total NEE over the jute season was found to be - 268.5 gC m^-2 (i.e. 10.3 t CO₂ ha^-1). In different jute growth stages, diurnal variations of NEE were strongly correlated (R² > 0.9) with photosynthetic photon flux density (PPFD). Ecosystem level photosynthetic efficiency parameters were estimated at each growth stage of jute crop using the Michaelis-Menten equation. The maximum values of photosynthetic capacity (P_max, 63.3 ± 1.15 µmol CO₂ m^-2 s^-1) and apparent quantum yield (α, 0.072 ± 0.0045 µmol CO₂ µmol photon^-1) were observed during the active vegetative stage, and the fibre development stage, respectively. Results of the present study would significantly contribute to understanding of the carbon flux from the Indian agro-ecosystems, which otherwise are very sparse.

Eight years of variations in ecosystem respiration over a residue-incorporated rotation cropland and its controlling factors

The carbon dioxide emissions from cropland play important roles in the regional carbon budget. In this study, continuous measurements of the ecosystem respiration (RE) were obtained using the eddy covariance technique in a winter wheat-summer maize double cropping agroecosystem mainly between 2004 and 2012 in order to identify the among-year variations in RE and the related factors responsible. The annual RE, estimated by Lloyd and Taylor model, which was the most accurate, was 1866.4 ± 105.75 g C m^-2 year^-1 and it ranged from 1650.68 g C m^-2 year^-1 to 1945.57 g C m^-2 year^-1 during the eight years. The seasonal RE values were 867.98 ± 125.24 g C m^-2 year^-1 and 890.55 ± 131.34 g C m^-2 year^-1 for wheat and maize, respectively. Additionally, crop residue carbon ranged from 322.73 g C m^-2 year^-1 in 2012 and 453.49 g C m^-2 year^-1 in 2007. Correlation analysis indicated that the interannual variations in wheat and maize RE were correlated with the seasonal mean soil water content (W-W_s) and maximum leaf area index (W-LAI_max) of wheat, and seasonal mean air temperature of maize (S-T_a), respectively. A rest method was attempted to investigate whether these relationships were occasional or inevitable. The rests of RE, i.e. the difference between simulated and observed RE values, were significantly influenced by LAI of wheat and hourly T_a of maize season but not by hourly W_s of maize season, indicating that the influence of W-LAI_max and S-T_a on RE were inevitable outcomes and that of W-W_s on wheat RE was occasional. So we suggested that one should not confirm the controlling factors of interannual variations in carbon fluxes just from simple relationships, which may be statistical coincidences and do not correlated with biotical processes.

Carbon dioxide fluxes in a farmland ecosystem of the southern Chinese Loess Plateau measured using a chamber-based method

Background

Farmland accounts for a relatively large fraction of the world’s vegetation cover, and the quantification of carbon fluxes over farmland is critical for understanding regional carbon budgets. The carbon cycle of farmland ecosystems has become a focus of global research in the field of carbon dynamics and cycling. The objectives of this study are to monitor the temporal variation in the net ecosystem exchange (NEE) and soil respiration in a spring maize (Zea mays L.) farmland ecosystem of the southern Loess Plateau of China.

Methods

A fully automated temperature-controlled flux chamber system was adopted in this study. The system contained nine chambers for CO₂ flux measurements, and three treatments were conducted: with and without maize plants in the chamber, as well as a bare field. Observations were conducted from June to September 2011. This time period covers the seedling, jointing, heading, grain filling, and ripening stages of spring maize. Other factors, such as air temperature (Ta), soil temperature (Ts), soil water content (SWC), photosynthetically active radiation (PAR), and precipitation (P), were simultaneously monitored.

Results

There was observed diurnal variation in the NEE of the maize ecosystem (NEE-maize). A short “noon break” occurred when the PAR intensity was at its maximum, while soil respiration rates had curves with a single peak. During the overall maize growth season, the total NEE-maize was –68.61 g C m⁻², and the soil respiration from the maize field (SR-maize) and bare field (SR-bare field) were 245.69 g C m⁻² and 114.08 g C m⁻², respectively. The temperature sensitivity of soil respiration in the maize field exceeded that in the bare field. Significant negative correlations were found between the NEE, PAR, and temperature (all p-values < 0.01), with both Ta and PAR being the primary factors that affected the CO₂ fluxes, collectively contributing 61.7%, 37.2%, and 56.8% to the NEE-maize, SR-maize, and SR-bare field, respectively. It was therefore concluded that both meteorological factors and farming practices have an important impact on the carbon balance process in corn farmland ecosystems. However, it is necessary to conduct long-term observational studies, in order to get a better understanding of the driving mechanism.

Response of Tallgrass Prairie to Management in the U.S. Southern Great Plains: Site Descriptions, Management Practices, and Eddy Covariance Instrumentation for a Long-Term Experiment

Understanding the consequences of different management practices on vegetation phenology, forage production and quality, plant and microbial species composition, greenhouse gas emissions, and water budgets in tallgrass prairie systems is vital to identify best management practices. As part of the Southern Plains Long-Term Agroecosystem Research (SP-LTAR) grassland study, a long-term integrated Grassland-LivestOck Burning Experiment (iGLOBE) has been established with a cluster of six eddy covariance (EC) systems on differently managed (i.e., different burning and grazing regimes) native tallgrass prairie systems located in different landscape positions. The purpose of this paper is to describe this long-term experiment, report preliminary results on the responses of differently managed tallgrass prairies under variable climates using satellite remote sensing and EC data, and present future research directions. In general, vegetation greened-up and peaked early, and produced greater forage yields in burned years. However, drought impacts were greater in burned sites due to reductions in soil water availability by burning. The impact of grazing on vegetation phenology was confounded by several factors (e.g., cattle size, stocking rate, precipitation). Moreover, prairie systems located in different landscapes responded differently, especially in dry years due to differences in water availability. The strong correspondence between vegetation phenology and eddy fluxes was evidenced by strong linear relationships of a greenness index (i.e., enhanced vegetation index) with evapotranspiration and gross primary production. Results indicate that impacts of climate and management practices on vegetation phenology may profoundly impact carbon and water budgets of tallgrass prairie. Interacting effects of multiple management practices and inter-annual climatic variability on the responses of tallgrass prairie highlight the necessity of establishing an innovative and comprehensive long-term experiment to address inconsistent responses of tallgrass prairie to different intensities, frequencies, timing, and duration of management practices, and to identify best management practices.

Effects of sky conditions on net ecosystem productivity of a subtropical coniferous plantation vary from half-hourly to daily timescales

The dynamic changes of solar radiation have received wide attention in global change studies, but there are controversies about the influence of diffuse radiation on ecosystem carbon sequestration. Using eddy covariance measurements from 2010 to 2012, the effects of sky conditions extracted from adjacent sunny, cloudy, and overcast days on net ecosystem productivity (NEP) of a subtropical coniferous plantation were examined from half-hourly to daily scales. Half-hourly NEP responded to the changing radiation more efficiently on overcast days compared to sunny days, but such response did not differ obviously between cloudy and sunny days. Compared with sunny conditions, apparent quantum yield (α) under overcast (cloudy) conditions changed 282.4% (41.7%) in spring, 140.3% (-4.2%) in summer, 218.5% (38.9%) in autumn, and 146.2% (0.5%) in winter, respectively; annually, α under overcast (cloudy) conditions increased by 225.9% (19.8%) in 2010, 189.8% (6.0%) in 2011, and 159.5% (21.4%) in 2012, respectively. Moreover, the potential NEP at the light intensity of 150 and 750 W m^-2 was improved due to increased diffuse fraction. However, both daytime NEP and daily NEP were significantly lower under overcast skies than under sunny and cloudy skies. Compared with sunny days, daily NEP on overcast days decreased by 127.7% in spring, 126.4% in summer, 121.8% in autumn, and 100.6% in winter, respectively; annually, daily NEP decreased by 122.5% in 2010, 141.7% in 2011, and 109.9% in 2012, respectively. Diurnal patterns of daily NEP were quite similar between sunny and cloudy days. Both path analysis and multiple regression showed that solar radiation, especially diffuse radiation, was responsible for the variations of NEP under different skies across seasons, but this effect may be weakened by seasonal droughts. This study implies that the effects of sky conditions on NEP are timescale dependent and should be paid more attention in ecosystem carbon cycle study.

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.

Higher assimilation than respiration sensitivity to drought for a desert ecosystem in Central Asia

Responses of ecosystem assimilation and respiration to global climate change vary considerably among terrestrial ecosystems constrained by both biotic and abiotic factors. In this study, net CO₂ exchange between ecosystem and atmosphere (NEE) was measured over a 4-year period (2013-2016) using eddy covariance technology in a desert ecosystem in Central Asia. Ecosystem assimilation (gross primary production, GPP) and respiration (R_eco) were derived from NEE by fitting light response curves to NEE data based on day- and nighttime data, and their responses to soil water content (SWC) and evaporative fraction (EF) were assessed during the growing season. Results indicated that both GPP and R_eco linearly decreased with declining SWC, with the sensitivity of GPP to SWC being 3.8 times higher than that of R_eco during the entire growing season. As a result, ecosystem CO₂ sequestration capacity decreased from 4.00μmolm^-2s^-1 to 1.00μmolm^-2s^-1, with increasing soil drought_. On a seasonal scale, significant correlation between GPP and SWC was only found in spring while that between R_eco and SWC was found in all growing seasons with the sensitivity increasing steadily from spring to autumn. EF had a low correlation with SWC, GPP and R_eco (R²=0.03, 0.02, 0.05, respectively), indicating that EF was not a good proxy for soil drought and energy partitioning was not tightly coupled to ecosystem carbon exchanges in this desert ecosystem. The study deepens our knowledge of ecosystem carbon exchange and its response to drought as well as its coupling with ecosystem energy partitioning in an extreme dry desert. The information is critical for better assessing carbon sequestration capacity in dryland, and for understanding its feedback to climate change.