CO2 Exchange Rate Research Articles

Net Ecosystem Exchange of Carbon Dioxide in Rice Summer – Autumn crop of the lower Mekong delta, Vietnam

This study describes the net ecosystem exchange of CO2 in a rice summer-autumn growth season in the lower Mekong Delta, Vietnam. Eddy Covariance and bio-meteorological systems were used to continually record the net CO2 exchange between the paddy field and the atmosphere, together with environmental variables affecting the exchange rate. Rice plant growth (height, above-, and belowground biomass) was measured every 15 days during the entire crop. Our study paddy field emitted CO2 at the mean rate of 5.06 ± 2.49 µmol m-2 s-1, while the rate of CO2 capturing was 18.65 ± 11.76 µmol m-2 s-1 throughout the entire crop. The paddy field acted as a CO2 sink almost during the crop. The quick increase of leaf area from the 15th to the 60th day led to the rise in CO2 influx. The decrease in influx rate after the 60th day resulted from the senescence and death of the aboveground biomass. The drainage in the last ten days of the crop probably contributed to the increase in CO2 effluxes and hence, turned the paddy field into a CO2 source. Ambient temperature was an important factor affecting the rate of CO2 exchange affects the photosynthesis, plant respiration, photorespiration, and organic matter decomposition.

The effect of rainfall amount and timing on annual transpiration in a grazed savanna grassland

Abstract. The role of precipitation (P) variability with respect to evapotranspiration (ET) and its two components, transpiration (T) and evaporation (E), from savannas continues to draw significant research interest given its relevance to a number of ecohydrological applications. Our study reports on 6 years of measured ET and estimated T and E from a grazed savanna grassland at Welgegund, South Africa. Annual P varied significantly with respect to amount (508 to 672 mm yr−1), with dry years characterized by infrequent early-season rainfall. T was determined using annual water-use efficiency and gross primary production estimates derived from eddy-covariance measurements of latent heat flux and net ecosystem CO2 exchange rates. The computed annual T for the 4 wet years with frequent early wet-season rainfall was nearly constant, 326±19 mm yr−1 (T/ET=0.51), but was lower and more variable between the 2 dry years (255 and 154 mm yr−1, respectively). Annual T and T/ET were linearly related to the early wet-season storm frequency. The constancy of annual T during wet years is explained by the moderate water stress of C4 grasses as well as trees' ability to use water from deeper layers. During extreme drought, grasses respond to water availability with a dieback–regrowth pattern, reducing leaf area and transpiration and, thus, increasing the proportion of transpiration contributed by trees. The works suggest that the early-season P distribution explains the interannual variability in T, which should be considered when managing grazing and fodder production in these grasslands.

Determining whether hydrological processes drive carbon source and sink conversion shifts in a large floodplain-lake system in China

Lake carbon (C) cycling is a key component of the global C cycle and associated C source and sink processes. The partial pressure of carbon dioxide (pCO2) and carbon dioxide (CO2) exchange flux at the lake-air interface (Fc) are controlled by complex physical, chemical, and biological mechanisms. It would be instructively significant to determine whether hydrological processes drive conversion shifts between C sources and sinks in floodplain-lake systems. Findings from this study show that exogenous input and in situ metabolism related to photosynthesis, respiration, and organic matter degradation were the main driving mechanisms of CO2 absorption and release in a large floodplain-lake system (i.e., Lake Poyang). Moreover, the intense and frequent water-level fluctuations inherent to floodplain-lakes may also have a direct or indirect impact on C cycling processes and CO2 exchange rates in floodplain-lake systems via their effect on physical processes, inorganic C transport, in-situ metabolic processes. We confirmed the potential of C source and sink conversion in floodplain-lakes under hydrological fluctuations, and strengthen the understanding of driving mechanisms of C source and sink conversion in floodplain systems.

Variation in CO2 and CH4 fluxes among land cover types in heterogeneous Arctic tundra in northeastern Siberia

Abstract. Arctic tundra is facing unprecedented warming, resulting in shifts in the vegetation, thaw regimes, and potentially in the ecosystem–atmosphere exchange of carbon (C). However, the estimates of regional carbon dioxide (CO2) and methane (CH4) budgets are highly uncertain. We measured CO2 and CH4 fluxes, vegetation composition and leaf area index (LAI), thaw depth, and soil wetness in Tiksi (71∘ N, 128∘ E), a heterogeneous site located within the prostrate dwarf-shrub tundra zone in northeastern Siberia. Using the closed chamber method, we determined the net ecosystem exchange (NEE) of CO2, ecosystem respiration in the dark (ER), ecosystem gross photosynthesis (Pg), and CH4 flux during the growing season. We applied a previously developed high-spatial-resolution land cover map over an area of 35.8 km2 for spatial extrapolation. Among the land cover types varying from barren to dwarf-shrub tundra and tundra wetlands, the NEE and Pg at the photosynthetically active photon flux density of 800 µmol m−2 h−1 (NEE800 and Pg800) were greatest in the graminoid-dominated habitats, i.e., streamside meadow and fens, with NEE800 and Pg800 of up to −21 (uptake) and 28 mmol m−2 h−1, respectively. Vascular LAI was a robust predictor of both NEE800 and Pg800 and, on a landscape scale, the fens were disproportionately important for the summertime CO2 sequestration. Dry tundra, including the dwarf-shrub and lichen tundra, had smaller CO2 exchange rates. The fens were the largest source of CH4, while the dry mineral soil tundra consumed atmospheric CH4, which on a landscape scale amounted to −9 % of the total CH4 balance during the growing season. The largest seasonal mean CH4 consumption rate of 0.02 mmol m−2 h−1 occurred in sand- and stone-covered barren areas. The high consumption rate agrees with the estimate based on the eddy covariance measurements at the same site. We acknowledge the uncertainty involved in spatial extrapolations due to a small number of replicates per land cover type. This study highlights the need to distinguish different land cover types including the dry tundra habitats to account for their different CO2 and CH4 flux patterns, especially the consumption of atmospheric CH4, when estimating tundra C exchange on a larger spatial scale.

Persistent Continental Shelf Carbon Sink at the Ieodo Ocean Research Station in the Northern East China Sea

Hourly (2017–2021) to seasonal (2015–2021) inorganic C data were collected at the Ieodo Ocean Research Station (32.07°N and 125.10°E) in the northern East China Sea (ECS), located under the influence of the nutrient-rich Changjiang Diluted Water (CDW). An increase in phytoplankton biomass from April to mid-August (the warming period) equalized much of the temperature-driven increase in the surface pCO2 and thus, made the northern ECS a moderate sink of atmospheric CO2. From November to March (the cooling period), a large pCO2 reduction, driven by a temperature reduction, and a high air–sea CO2 exchange rate, because of high windspeeds, transformed the basin into a substantial CO2 sink, yielding an annual net C uptake of 61.7 g C m–2 yr–1. The effects of biological production and temperature change on seawater pCO2 (and thus, the net air–sea CO2 flux) were decoupled each season and acted in concert to increase the net annual CO2 sink by the region. The present study provided the observational and mechanistic lines of evidence for confirming “continental shelf C pump”—a mechanism in the shallow waters of the continental shelves that accumulate a significant amount of C (via reinforced cooling and promoted biological C uptake) that is transported from the basin surface waters to the interior of the adjacent deep ocean. In the future, an increasing input of anthropogenic nutrients into the northern ECS is likely to make the region a stronger CO2 sink.

Radiocarbon in the Atmosphere and Seawater in the South China Sea: Flux, Inventory and Air‐Sea CO2 Exchange Rate Tracing

AbstractΔ14C values of the atmosphere and seawater dissolved inorganic carbon (DIC) were measured during a cruise in the South China Sea (SCS) in September 2015, in order to determine the 14C flux and bomb 14C‐based air‐sea CO2 exchange rates for this region. The background atmospheric Δ14C value (13.8 ± 5.0‰) for the SCS during that period was lower than that (35.4 ± 3.4‰) of surface seawater (5 m) DIC, and a net transfer of 14C from the sea to the atmosphere (7.4 ± 5.0 × 1011 atoms m−2 yr−1) was determined at the wind speed of 5.2 ± 1.7 m s−1. Seawater DIC Δ14C profiles showed the highest value (37.9 ± 3.7‰) at a depth of 100 m, a rapid decrease below that depth to −220.3 ± 3.2‰ at 1,500 m, and nearly constant values below 1,500 m. The average mean penetration depth of bomb 14C was 585.5 ± 99.2 m, and a value of 8.2 ± 1.0 × 109 atoms cm−2 was obtained for the bomb 14C inventory in this region. Based on this inventory, a long‐term (1954–2015) average air‐sea CO2 exchange rate of 20.2 ± 2.8 mol m−2 yr−1 was traced for the SCS. Combined with the pCO2 measurements in this region, a net CO2 flux rate of 0.54 ± 0.08 mol m−2 yr−1 was yielded for the SCS, which is comparable to the cruise measured flux (0.44 ± 0.62 mol m−2 yr−1) obtained from a synthesis study (Li et al., 2020, https://doi.org/10.1016/j.pocean.2020.102272). Our study highlights the importance of continued atmospheric and seawater 14C observations on determining the air‐sea flux in this region.

Changes in morpho-physiological traits of rice cultivars upon different fertilization regimes

Improving crop yield is one of the main challenges in lands with intensive agriculture. In order to study the nutritional treatment effects (macro and micro elements) on agronomic and morpho-physiologic traits on three different rice cultivars, a field experiment was conducted at Mazandaran province of Iran as a factorial arrangement based on a randomized complete block design with three replications. Three different fertilizer treatments including complete (NPK + micro elements), macro (NPK) and micro (N + Mn + Fe + Zn), along with a control (nutrient free) were applied. The results showed that distinctive cultivar’s 1000-grain weight and biomass, unlike other traits, was not significantly affected amongst the treatments. Fertilizer treatments, however, had significant effect on the studied agro-morphological, biochemical and physiological parameters of rice plants, except the number of panicles per plant and biomass. Furthermore, among yield components, interaction effect of cultivar and fertilizer on panicle per plant was significant. Among cultivars investigated, “Fajr” showed the highest panicle length, number of grains per panicle and panicle per plant, leaf area, chlorophyll content, photosynthesis rate, respiration, stomatal conductance, harvest index and grain yield, whereas it had the shortest overall plants height. NPK + micro fertilizer treatment, also, had the most positive effect on production units in rice plant particularly chloroplast function, which eventually resulted in grain yield. Moreover, the combination of micro and macro nutrients facilitated the rate of CO2 and O2 exchange. Therefore, “Fajr” cultivar with more leaf area showed better efficiency in absorption of receipted lights and finally, because of high harvest index, transferred more assimilates into grains.

Evaluating the Arabian Sea as a regional source of atmospheric CO<sub>2</sub>: seasonal variability and drivers

Abstract. The Arabian Sea (AS) was confirmed to be a net emitter of CO2 to the atmosphere during the international Joint Global Ocean Flux Study program of the 1990s, but since then few in situ data have been collected, leaving data-based methods to calculate air–sea exchange with fewer and potentially out-of-date data. Additionally, coarse-resolution models underestimate CO2 flux compared to other approaches. To address these shortcomings, we employ a high-resolution (1/24∘) regional model to quantify the seasonal cycle of air–sea CO2 exchange in the AS by focusing on two main contributing factors, pCO2 and winds. We compare the model to available in situ pCO2 data and find that uncertainties in dissolved inorganic carbon (DIC) and total alkalinity (TA) lead to the greatest discrepancies. Nevertheless, the model is more successful than neural network approaches in replicating the large variability in summertime pCO2 because it captures the AS's intense monsoon dynamics. In the seasonal pCO2 cycle, temperature plays the major role in determining surface pCO2 except where DIC delivery is important in summer upwelling areas. Since seasonal temperature forcing is relatively uniform, pCO2 differences between the AS's subregions are mostly caused by geographic DIC gradients. We find that primary productivity during both summer and winter monsoon blooms, but also generally, is insufficient to offset the physical delivery of DIC to the surface, resulting in limited biological control of CO2 release. The most intense air–sea CO2 exchange occurs during the summer monsoon when outgassing rates reach ∼ 6 molCm-2yr-1 in the upwelling regions of Oman and Somalia, but the entire AS contributes CO2 to the atmosphere. Despite a regional spring maximum of pCO2 driven by surface heating, CO2 exchange rates peak in summer due to winds, which account for ∼ 90 % of the summer CO2 flux variability vs. 6 % for pCO2. In comparison with other estimates, we find that the AS emits ∼ 160 Tg C yr−1, slightly higher than previously reported. Altogether, there is 2× variability in annual flux magnitude across methodologies considered. Future attempts to reduce the variability in estimates will likely require more in situ carbon data. Since summer monsoon winds are critical in determining flux both directly and indirectly through temperature, DIC, TA, mixing, and primary production effects on pCO2, studies looking to predict CO2 emissions in the AS with ongoing climate change will need to correctly resolve their timing, strength, and upwelling dynamics.

Air-sea CO2 exchange rate in the northern Indian Ocean based on coral radiocarbon records

Corals growing in shallow oceanic regions record the radiocarbon variations in ocean surface waters. Tracing bomb radiocarbon in various carbon reservoirs, can help understand the pathways and rates of carbon exchange between the different reservoirs. The northern Indian Ocean comprising two adjoining basins the Arabian Sea and the Bay of Bengal demonstrates contrasting behaviour both in its hydrological condition and CO2 sequestration. In this study, coral based bomb radiocarbon records from the Lakshadweep Islands and the Andaman Islands in the northern Indian Ocean have been analysed. The estimates of air-sea CO2 exchange rate based on bomb radiocarbon for the Lakshadweep is 13.4 ± 2.1 mol m−2 yr−1 and for the northern Andaman is 8.8 ± 1.3 mol m−2 yr−1. The bomb radiocarbon based air-sea CO2 exchange over the Indian Ocean follow the trend of empirical relationship with wind speed. Using the air-sea CO2 exchange rates, the net regional CO2 fluxes over the Lakshadweep and the northern Andaman region has been determined.

Estimation of CO2 and Carbon Exchange Rate of Strawberry in Greenhouse Cultivation with 1D Convolutional Neural Network and Temperature for Application to Agriculture

Estimates of CO2 and Carbon Exchange Rate (CER) is vital tasks for agriculture challenges and hydrological and climate science studies. In the situation that farmers decrease and climate change, applying artificial intelligence techniques to agriculture challenges are desired as innovative approaches to overcome these difficulties in Society.  This study aim to approach following four agriculture challenges (1)limited available data (2) real-time estimate (3) reliable estimate (4) user-friendly, using deep learning models and environment datasets of strawberry in greenhouse cultivation. In this study, using deep learning models, 1D Convolutional Neural Network(1D CNN), Long-Short-Term-Memory(LSTM) and 1D CNN+LSTM, the estimation of CO2 and Carbon Exchange Rate of Strawberry in greenhouse cultivation at one minutes or ten minutes time duration are evaluated. 1D CNN performed the best with R2 0.67 for CO2 in one minute time duration from Temperature, Vapour Pressure Deficit. The applying 1D CNN for estimate CER in ten minutes duration performed with R2 0.77 from Temperature. These results suggest that applying 1D CNN success for estimate CO2 and CER of strawberry in greenhouse cultivation with limited available data and its applicability in agriculture system in society. 

CO2, H2O and energy fluxes from chickpea crop grown under residual soil moisture condition in rainfed peninsular India using eddy covariance techniques

Present study is the first to report the ecosystem CO2, H2O and energy exchanges (at diurnal, daily and seasonal-scale) from rainfed winter chickpea crop (Cicer arietinum L.), grown under residual soil moisture over semi-arid peninsular India, using eddy covariance system. The available energy to the ecosystem was found to be predominantly partitioned into sensible heat flux (H), except during the peak vegetative stage. Asymmetric response of the daytime (forenoon and afternoon) CO2 and energy fluxes were observed due to limited soil moisture condition. The peak vegetative stage of the crop i.e., 40–60 Days after Sowing (DAS) showed highest rate of Net Ecosystem CO2 Exchange (NEE; −9 to −11 μmol m−2 s−1) and Latent Heat Flux (LE; ∼ 300 W m−2). The response of daytime Gross Primary Production (GPP) to light intensity was found to be poor during 1–16 DAS, improved steadily in 17−34 DAS, remained high in 35–69 DAS, and deteriorated thereafter till harvesting (70–106 DAS). The chickpea crop was net CO2 sink on daily basis during 17–69 DAS. Peak daily NEE of −2.52 gC m−2 day−1, GPP of 4.2 gC m−2 day-1 and Evapotranspiration (ET) of 2.38 mm day−1 were observed during the growing season. Over the entire season, rainfed chickpea was found to be a weak carbon sink with cumulative NEE of -32.39 ± 4.21 gC m−2; GPP of 163.03 ± 15.04 gC m−2; Ecosystem Respiration (Reco) of 127.45 ± 12.85 gC m−2; ET of 177 ± 18.13 mm. The maximum ecosystem water use efficiency (GPP/ET) and crop coefficient were found to be 1.7 gC m−2 mm−1 and 0.6 respectively at peak vegetative stage, which are sub-optimal compared to well managed chickpea crop. The results help us to understand the ecosystem processes and exchanges from rainfed winter legume-based cropping system, which intern would significantly contribute in global carbon and moisture budget.

Eutrophication overcoming carbonate precipitation in a tropical hypersaline coastal lagoon acting as a CO2 sink (Araruama Lagoon, SE Brazil)

The carbonate chemistry was investigated in the semiarid eutrophic Araruama Lagoon (Brazil), one of the largest hypersaline coastal lagoons in the world. Spatial surveys during winter and summer periods were performed, in addition to a diurnal sampling in summer. The hypersaline waters have higher concentrations of total alkalinity (TA) and dissolved inorganic carbon (DIC) than the seawater that feed the lagoon, due to evaporation. However, TA and DIC concentrations were lower than those expected from evaporation. Calcium carbonate (CaCO3) precipitation partially explained these deficits. The negative correlation between the partial pressure of CO2 (pCO2) and chlorophyll a (Chl a) indicated that DIC was also consumed by primary producers. The uptake by photosynthesis contributes to 57–63% of DIC deviation from evaporation, the remaining credited to CaCO3 precipitation. Marked pCO2 undersaturation was prevalent at the innermost region with shallow, confined, and phytoplankton-dominated waters, with a strong enrichment of heavier carbon isotope (δ13C-DIC up to 5.55‰), and highest pH (locally counter-acting the process of ocean acidification). Oversaturation was restricted to an urbanized region, and during night-time. The lagoon behaved as a marked CO2 sink during winter (− 15.32 to − 10.15 mmolC m−2 day−1), a moderate sink during summer (− 5.50 to − 4.67 mmolC m−2 day−1), with a net community production (NCP) of 93.7 mmolC m−2 day−1 and prevalence of net autotrophic metabolism. A decoupling between CO2 and O2 exchange rate at the air–water interface was attributed to differences in gas solubility, and high buffering capacity. The carbonate chemistry reveals simultaneous and antagonistic actions of CaCO3 precipitation and autotrophic metabolism on CO2 fluxes, and could reflect future conditions in populated and semiarid coastal ecosystems worldwide.

Integrating Aquatic Metabolism and Net Ecosystem CO2 Balance in Short- and Long-Hydroperiod Subtropical Freshwater Wetlands

How aquatic primary productivity influences the carbon (C) sequestering capacity of wetlands is uncertain. We evaluated the magnitude and variability in aquatic C dynamics and compared them to net ecosystem CO2 exchange (NEE) and ecosystem respiration (Reco) rates within calcareous freshwater wetlands in Everglades National Park. We continuously recorded 30-min measurements of dissolved oxygen (DO), water level, water temperature (Twater), and photosynthetically active radiation (PAR). These measurements were coupled with ecosystem CO2 fluxes over 5 years (2012–2016) in a long-hydroperiod peat-rich, freshwater marsh and a short-hydroperiod, freshwater marl prairie. Daily net aquatic primary productivity (NAPP) rates indicated both wetlands were generally net heterotrophic. Gross aquatic primary productivity (GAPP) ranged from 0 to − 6.3 g C m−2 day−1 and aquatic respiration (RAq) from 0 to 6.13 g C m−2 day−1. Nonlinear interactions between water level, Twater, and GAPP and RAq resulted in high variability in NAPP that contributed to NEE. Net aquatic primary productivity accounted for 4–5% of the deviance explained in NEE rates. With respect to the flux magnitude, daily NAPP was a greater proportion of daily NEE at the long-hydroperiod site (mean = 95%) compared to the short-hydroperiod site (mean = 64%). Although we have confirmed the significant contribution of NAPP to NEE in both long- and short-hydroperiod freshwater wetlands, the decoupling of the aquatic and ecosystem fluxes could largely depend on emergent vegetation, the carbonate cycle, and the lateral C flux.

Variation of carbon partitioning in newly expanded maize leaves and plant adaptive growth under extended darkness

Plants must maintain a balance between their carbon (C) supply and utilization during the day–night cycle for continuous growth since C starvation often causes irreversible damage to crop production. It is not well known how C fixation and allocation in the leaves of crops such as maize adapt to sudden environmental changes. Here, to quantify primary C fixation and partitioning in photosynthetic maize leaves under extended darkness and to relate these factors to plant growth, maize seedlings were subjected to extended darkness (ED) for three successive days at the 6th leaf fully expanded stage (V6). ED reduced plant growth and leaf chlorophyll levels but not the rate of net CO2 exchange. As a result of the reduction in photoassimilates, the accumulation of starch and total soluble carbohydrates (TSC) in mature leaves also decreased under ED. However, the percentage of the daily C fixation reserved in mature leaves increased. These transient C pools were largely composed of TSC and were mainly used for consumption by increased nocturnal respiration rather than for transport. As the days went on, both the amount of C accumulated and the percentage of the daily fixed C that was reserved in leaves decreased, which could be largely accounted for by the attenuated starch synthesis in all treatments. The activities of ADP-glucose pyrophosphorylase and soluble starch synthase decreased significantly over time. Therefore, this study concluded that both starch and TSC are involved in the coordination of the C supply and plant growth under a sudden C shortage but that they may be involved in different ways. While the ratio of reserved C to daily fixed C increased to maintain blade function under acute C starvation, both the amount and the proportion of C reserved in mature leaves decreased as plant growth continued in order to meet the growth demands of the plant.

Drivers of Atmosphere-Ocean CO2 Flux in Northern Norwegian Fjords

High-latitude fjords and continental shelves are shown to be sinks for atmospheric CO2, yet large spatial-temporal variability and poor regional coverage of sea-air CO2 flux data, especially from fjord systems, makes it difficult to scale our knowledge on how they contribute to atmospheric carbon regulation. The magnitude and seasonal variability of atmosphere-sea CO2 flux was investigated in high-latitude northern Norwegian coastal areas over 2018 and 2019, including four fjords and one coastal bay. The aim was to assess the physical and biogeochemical factors controlling CO2 flux and partial pressure of CO2 in surface water via correlation to physical oceanographic and biological measurements. The results show that the study region acts as an overall atmospheric CO2 sink throughout the year, largely due to the strong undersaturation of CO2 relative to atmospheric concentrations. Wind speed exerted the strongest influence on the instantaneous rate of sea-air CO2 exchange, while exhibiting high variability. We concluded that the northernmost fjords (Altafjord and Porsangerfjord) showed stronger potential for instantaneous CO2 uptake due to higher wind speeds. We also found that fixation of CO2 was likely a significant factor controlling ΔpCO2 from April to June, which followed phenology of spring phytoplankton blooms at each location. Decreased ΔpCO2 and the resulting sea-air CO2 flux was observed in autumn due to a combined reduction of the mixed layer with entrain of high CO2 subsurface water, damped biological activity and higher surface water temperatures. This study provides the first measurements of atmospheric CO2 flux in these fjord systems and therefore an important new baseline for gaining a better understanding on how the northern Norwegian coast and characteristic fjord systems participate in atmosphere carbon regulation.

Quantifying CO2 Uptakes Over Oceans Using LIDAR: A Tentative Experiment in Bohai Bay

AbstractOceans are widely regarded as major offsets for anthropogenic carbon emissions, leading to an evident lower measured atmospheric CO2 concentration than expected. It is thus of great significance to develop effective means to monitor CO2 fluxes over oceans globally. In this work, we utilized observations obtained by an airborne CO2‐IPDA LIDAR to evaluate the potential of such means in estimating sea‐air CO2 flux. During a flight experiment in 2019, we have estimated the CO2 exchange rate, −1.5 ± 0.18 mmol/m2/h, between the Bohai Bay and the atmosphere using equilibrium atmospheric boundary layer theory. These findings indicated that the forthcoming space‐borne CO2‐IPDA LIDAR is capable of identifying CO2 uptakes over oceans qualitatively, which would be a novel means and a basis for monitoring CO2 fluxes over global oceans.

Abiotic and biotic factors contribute to CO2 exchange variation at the hourly scale in a semiarid maize cropland

Understanding the variables influencing the carbon budget in agricultural ecosystems is crucial for the prediction of future carbon dynamics. The purpose of this study was to identify the biotic and abiotic determinants of the net ecosystem CO2 exchange (NEE) and net assimilation rate (NPP) in a semiarid maize cropland. The CO2 exchange (NEE and NPP) was measured at different growth stages of maize plants using an improved chamber methodology. Heat map clustering of the correlation coefficients between CO2 exchange and its driving factors demonstrated that soil temperature and air humidity were positively correlated with CO2 emissions regardless of daytime or nighttime, while other factors affecting CO2 exchange were negatively correlated with emissions during daytime yet positively correlated during nighttime. The machine learning algorithm random forest (RF) and structural equation modeling (SEM) were used to analyze the effects of different factors on CO2 exchange. The RF analysis results indicated that for CO2 exchange in the daytime, photosynthetically active radiation (PAR) was the most important variable and presented an importance score of 0.574 for NEE and 0.558 for NPP. The SEM results indicated that in the daytime PAR exerted significant direct and indirect effects on both NEE and NPP, and the standardized direct and indirect effects were −0.668 and 0.022, respectively, for NEE, and the effects were 0.655 and −0.011, respectively for NPP. Like PAR, soil water content also exerted significant direct and indirect effects on both NEE and NPP, but the remaining factors affecting CO2 exchange only have one of the direct or indirect effects, sometimes neither. For CO2 exchange at night, the leaf area was the most important variable and presented an importance score of 0.72 for NEE and 0.45 for NPP. At night, both the direct and indirect effects of most abiotic factors on NEE and NPP were significant.

Rapid Light-Response Curve of Chlorophyll Fluorescence in Terrestrial Plants: Relationship to CO2 Exchange among Five Woody and Four Fern Species Adapted to Different Light and Water Regimes.

The rapid light response of electron transport rate (ETRR), obtained from chlorophyll fluorescence parameters by short illumination periods (10–30 s) at each light level, can provide a rapid and easy measurement of photosynthetic light response in plants. However, the relationship between ETRR and the steady-state light response of CO2 exchange rate (AS) of terrestrial plants has not been studied in detail. In this study, we compared the ETRR and AS for five woody and four fern species with different light and/or water adaptations. Under well-watered conditions, a constant temperature (25 °C) and with stomatal conductance (gs) not being a main limiting factor for photosynthesis, ETRR and AS were closely related, even when merging data for regression analysis for a species grown under different light conditions and measured under different light intensity and air humidity. However, when Alnus formosana was treated with low soil water and air humidity, because of the decrease in AS mainly due to stomatal closure, the ETRR–AS relation was not so close. In addition, at both 100 and 2000 μmol m−2 s−1 photosynthetic photon flux density (PPFD), ETRR and AS were significantly correlated within a plant group (i.e., woody plants and ferns) regardless of the broad difference in AS due to different species or environmental factors. The results indicate that the relationship between the ETRR and AS is varied by species. We concluded that 1) ETRR could reflect the variation in AS at each irradiance level within a species under well-watered conditions and 2) ETRR at 100 μmol m−2 s−1 PPFD (as the efficiency of light capture) or 2000 μmol m−2 s−1 PPFD (as a maximum photosynthetic parameter) could be used to compare the photosynthetic capacity within a plant group, such as woody plants and ferns.

Placental gas exchange during amniotic carbon dioxide insufflation in sheep.

Insufflation of the amniotic cavity with carbon dioxide (CO2 ) is used clinically to improve visibility during complex fetoscopic surgery. Insufflation with heated, humidified CO2 has recently been shown to reduce fetal hypercapnia and acidosis in sheep, compared with use of cold and dry CO2 , but the underlying mechanisms are unclear. The aim of this study was to investigate whether differences in placental CO2 and oxygen (O2 ) exchange during insufflation with heated and humidified vs cold and dry CO2 could explain these findings. Thirteen fetal lambs at 105 days of gestation (term, 146 days) were exteriorized partially, via a midline laparotomy and hysterotomy, and instrumented with an umbilical artery catheter, an umbilical vein catheter and a common umbilical vein flow probe. Arterial and venous catheters and flow probes were also inserted into the maternal uterine circulation. Six ewes were insufflated with cold, dry CO2 (22°C; 0-5% humidity) and seven with heated, humidified CO2 (40°C; 95-100% humidity) at 15 mmHg for 180 min. Blood-flow recordings and paired arterial and venous blood gases were sampled from uterine and umbilical vessels. Rates of placental CO2 and O2 exchange were calculated. After 180 min of insufflation, fetal survival was 33% (2/6) using cold, dry CO2 and 71% (5/7) using heated, humidified CO2 . By 120 min, fetuses insufflated with heated, humidified CO2 had lower arterial CO2 levels and higher arterial pH compared to those insufflated with cold, dry gas. Insufflation decreased significantly placental gas exchange in both groups, as measured by rates of both (i) fetal CO2 clearance and O2 uptake and (ii) maternal O2 delivery and CO2 uptake from the fetal compartment. Lower arterial CO2 and higher pH levels in fetuses insufflated with heated and humidified, compared to cold and dry, CO2 could not be explained by differences in placental gas exchange. Instead, heated and humidified insufflation appeared to reduce fetal CO2 absorption from the uterus, supporting its use in preference to cold, dry CO2 . © 2019 International Society of Ultrasound in Obstetrics and Gynecology.

Vertical distribution of gas exchanges and their integration throughout the entire canopy in a maize field.

Fluxes of carbon and water along a vertical profile within a canopy, particularly the associations between canopy and ecosystem levels, are not well studied. In this study, gas exchange along the vertical profile in a maize canopy was examined. The relationships between leaf- and ecosystem-level carbon and water fluxes were compared. The results from research conducted over two growing seasons showed that during vegetative growth, the top and middle leaf layers in the canopy contribute most to the carbon and water fluxes of the entire canopy. During the grain-filling stage, gasexchange processes were performed mostly in the middle leaves with and near the ears. Significant relationships were observed between the net ecosystem CO2 exchange rate (NEE) plus soil respiration and the assumed canopy levels (Acanopy) and between evapotranspiration rates at the ecosystem (ET) and assumed canopy levels (Ecanopy). This highlights the close associations between these parameters by integrating the leaf gasexchange rates measured in a conventional leaf cuvette and those at the ecosystem level via the eddy covariance technique. These results improve our understanding of how carbon assimilation varies vertically within a canopy, highlighting the critical role of ear leaves.