Net Ecosystem CO2 Exchange Research Articles

Cloudy Sky Conditions Promote Net Ecosystem CO2 Exchange in a Subtropical Coniferous Plantation Across Seasons

Dynamic changes in solar radiation have an important influence on ecosystem carbon sequestration, but the effects of changes caused by sky conditions on net ecosystem CO2 exchange (NEE) are unclear. This study analyzed the effects of sunny, cloudy, and overcast sky conditions on NEE using carbon flux and meteorological data for a subtropical coniferous plantation in 2012. Based on one-year data, we found no seasonal variation in the light response curve under various sky conditions. Compared with sunny sky conditions, the apparent quantum yield (α) and potential photosynthetic rate at a light intensity of 150 and 750 W m−2 (P150 and P750) under cloudy sky conditions increased by an average of 82.3%, 217.7%, and 22.5%; α and P150 under overcast sky conditions increased by 118.5% and 301% on average. Moderate radiation conditions were more favorable for maximum NEE, while low radiation conditions inhibited NEE. In most cases, when the sunny NEE was used as a baseline for comparison, the relative change in NEE (%NEE) was positive under cloudy sky conditions and negative under overcast sky conditions. The average maximal %NEE under cloudy sky conditions was 42.4% in spring, 34.1% in summer, 1.6% in autumn and –87.3% in winter. This study indicates that cloudy sky conditions promote photosynthetic rates and NEE in subtropical coniferous plantations.

Changes and regulations of net ecosystem CO2 exchange across temporal scales in the Alxa Desert

Arid ecosystems are an important component of the global carbon cycle. In these ecosystems, plant functional types are particularly important in realizing many ecosystem processes such as the dynamics and regulations of net ecosystem exchange of CO2 (NEE) to the changing environment. Here, we measured the diurnal dynamics of NEE in patches of succulent and non-succulent shrubs over two growing seasons (2012 and 2013) in the Alxa Desert, located in northern China. We find that the interannual difference of NEE was greater for the non-succulent shrub than that for he succulent shrub species. Diurnal NEE changes were similar for all species and were more strongly influenced by soil moisture than by temperature. Nonetheless, these environmental factors had a greater influence on non-succulent shrubs than succulent shrubs. Our findings highlight that species with different life history traits have different NEE dynamics and vary by time scales. These dynamics are strongly tied to water availability and plant type-specific water storage capacity, with plants possessing large water storage organs maintaining their physiological functioning better under stressful conditions. Our findings on the interaction between plant type and environment could be used to improve estimates of terrestrial carbon uptake in critical desert ecosystems.

Memory effects of climate and vegetation affecting net ecosystem CO2 fluxes in global forests.

Forests play a crucial role in the global carbon (C) cycle by storing and sequestering a substantial amount of C in the terrestrial biosphere. Due to temporal dynamics in climate and vegetation activity, there are significant regional variations in carbon dioxide (CO2) fluxes between the biosphere and atmosphere in forests that are affecting the global C cycle. Current forest CO2 flux dynamics are controlled by instantaneous climate, soil, and vegetation conditions, which carry legacy effects from disturbances and extreme climate events. Our level of understanding from the legacies of these processes on net CO2 fluxes is still limited due to their complexities and their long-term effects. Here, we combined remote sensing, climate, and eddy-covariance flux data to study net ecosystem CO2 exchange (NEE) at 185 forest sites globally. Instead of commonly used non-dynamic statistical methods, we employed a type of recurrent neural network (RNN), called Long Short-Term Memory network (LSTM) that captures information from the vegetation and climate’s temporal dynamics. The resulting data-driven model integrates interannual and seasonal variations of climate and vegetation by using Landsat and climate data at each site. The presented LSTM algorithm was able to effectively describe the overall seasonal variability (Nash-Sutcliffe efficiency, NSE = 0.66) and across-site (NSE = 0.42) variations in NEE, while it had less success in predicting specific seasonal and interannual anomalies (NSE = 0.07). This analysis demonstrated that an LSTM approach with embedded climate and vegetation memory effects outperformed a non-dynamic statistical model (i.e. Random Forest) for estimating NEE. Additionally, it is shown that the vegetation mean seasonal cycle embeds most of the information content to realistically explain the spatial and seasonal variations in NEE. These findings show the relevance of capturing memory effects from both climate and vegetation in quantifying spatio-temporal variations in forest NEE.

Net Ecosystem Exchange (NEE) simulation in maize using artificial neural networks

The rate of the net ecosystem exchange of CO2 (NEE) between the atmosphere and vegetated surface plays important role to achieve a better understanding regarding the global carbon budget. The eddy covariance method is known as an accurate and direct approach to measure the CO2 flux in small scale from a hundred meter to several kilometers. Some of resources and practical limitations prevent the utilization of this technique. Thus, simulation methods can be applied to estimate the NEE in the scale of larger than eddy covariance measurement capability. One class of simulation models, Artificial Neural Networks (ANNs), are powerful tools to identify the complicated non-linear relations among the input and output vectors. They used a set of input variables to simulate the measured NEE values by investigation of the relations (hidden rules) among them.In this study, multi-layer perceptron neural network trained by steepest descent Back-Propagation (BP) algorithm was tested to simulate the NEE above two maize sites (rain-fed &irrigated) near Mead, Nebraska. In addition, the sensitivity of NEE with respect to each input was analyzed over the growth stages.Network training and testing was fulfilled using on two different subsets of the hourly data which were selected from days of year (DOY) 169 to 222 for 2001, 2003, 2005, 2007, and 2009.The results showed a high correlation between actual and estimated NEE values (R2 > 0.9620). The RMSE for NEE reached 0.0681 and 0.0642 (mg m−2 s −1) for irrigated and rain-fed sites respectively. Thus, the results indicate the reliability and efficiency of this technique to simulate the NEE. The sensitivity analysis indicated that the most effective inputs on the NEE were identified Rn and LAI for irrigated site and Rn, and VPD for rain-fed site. Furthermore, to achieve the minimal set of input in order to speed up the analysis procedures; the impact intensity of each input on NEE was recognized by deactivation of each input vector in network training.

Herd position habits can bias net CO2 ecosystem exchange estimates in free range grazed pastures

The eddy covariance (EC) technique has been widely used to quantify the net CO2 ecosystem exchange (NEE) of grasslands, which is an important component of grassland carbon and greenhouse gas budgets. In free range grazed pastures, NEE estimations are supposed to also include cattle respiration. However, cattle respiration measurement by an EC system is challenging as animals act as moving points emitting CO2 that are more or less captured by the EC tower depending on their presence in the footprint. Often it is supposed that, over the long term, cattle distribution in the pasture is homogeneous so that fluctuations due to moving sources are averaged and NEE estimates are reasonably representative of cattle respiration.In this study, we test this hypothesis by comparing daily cow respiration rate per livestock unit (LU) estimated by postulating a homogeneous cow repartition over the whole pasture with three other estimates based on animal localization data, animal scale carbon budget and confinement experiments.We applied these methods to an intensively managed free range grassland and showed that the NEE estimate based on a homogeneous cow repartition was systematically lower than the three other estimates. The bias was about 60 g C m–2 yr–1, which corresponded to around 40% of the annual NEE. The sign and the importance of this bias is site specific, as it depends on cow location habits in relation to the footprint of the EC measurements which highlight the importance of testing the hypothesis of homogeneity of cattle distribution on each site.Consequently, in order to allow estimating the validity of this hypothesis but also to improve inter site comparisons, we advocate to compute separately pasture NEE and grazer’s respiration. For the former we propose a method based on cattle presence detection using CH4 fluxes, elimination of data with cattle and gap filling on the basis of data without cattle. For the second we present and discuss three independent methods (animal localization with GPS, animal scale carbon budget, confinement experiments) to estimate the cattle respiration rate.

Plant biomass and soil organic carbon are main factors influencing dry-season ecosystem carbon rates in the coastal zone of the Yellow River Delta.

Coastal wetlands are considered as a significant sink of global carbon due to their tremendous organic carbon storage. Coastal CO2 and CH4 flux rates play an important role in regulating atmospheric CO2 and CH4 concentrations. However, the relative contributions of vegetation, soil properties, and spatial structure on dry-season ecosystem carbon (C) rates (net ecosystem CO2 exchange, NEE; ecosystem respiration, ER; gross ecosystem productivity, GEP; and CH4) remain unclear at a regional scale. Here, we compared dry-season ecosystem C rates, plant, and soil properties across three vegetation types from 13 locations at a regional scale in the Yellow River Delta (YRD). The results showed that the Phragmites australis stand had the greatest NEE (-1365.4 μmol m-2 s-1), ER (660.2 μmol m-2 s-1), GEP (-2025.5 μmol m-2 s-1) and acted as a CH4 source (0.27 μmol m-2 s-1), whereas the Suaeda heteroptera and Tamarix chinensis stands uptook CH4 (-0.02 to -0.12 μmol m-2 s-1). Stepwise multiple regression analysis demonstrated that plant biomass was the main factor explaining all of the investigated carbon rates (GEP, ER, NEE, and CH4); while soil organic carbon was shown to be the most important for explaining the variability in the processes of carbon release to the atmosphere, i.e., ER and CH4. Variation partitioning results showed that vegetation and soil properties played equally important roles in shaping the pattern of C rates in the YRD. These results provide a better understanding of the link between ecosystem C rates and environmental drivers, and provide a framework to predict regional-scale ecosystem C fluxes under future climate change.

CO2 exchange under different vegetation covers in a coastal wetland of Jiaozhou Bay, China

This study applied the static chamber method to evaluate the CO2 fluxes from December 2013 to October 2014 in the coastal wetland of Jiaozhou Bay, China. The study area was divided into Spartina alterniflora flat, Phragmites australis flat, Suaeda salsa flat and mud flat. Results showed that net ecosystem CO2 exchange (NEE) presented obviously temporal and spatial variations during growing season, and slight fluctuation in non-growing season. Vegetated flats (S. alterniflora flat, P. australis flat and S. salsa flat) acted as CO2 sinks during growing season and CO2 sources during non-growing season; while mud flat acted as a CO2 sink through the entire period. Over the growing season, the estimated total NEE were −1202.83 g CO2 m−2 in S. alterniflora flat, −1129.16 g CO2 m−2 in P. australis flat and −237.43 g CO2 m−2 in S. salsa flat, respectively. Daytime NEE were significantly exponential related to light intensity in S. alterniflora flat and P. australis flat; whereas no overt exponential relationship was found between these two factors in S. salsa flat. In P. australis flat, ecosystem respiration (Reco) and gross primary production (GPP) in growing season were estimated to be 2874.05 and 1744.89 g CO2 m−2, respectively; Reco was significantly exponential related to air temperature and soil temperature at different depths, among which soil temperature at the depth of 5 cm was the most remarkable; temperature sensitivity (Q10) of Reco ranged from 2.72 to 3.67. Aboveground biomass of P. australis over the entire growing season showed non-linear relationships with NEE, GPP, and Reco; while it showed significant linear relationships with these three factors during early and peak growing season. As for the mud flat acting as a CO2 sink, it is supposed to be caused by inorganic process, however the mechanism is not definite. Further research on the long-term and complete observations of CO2 exchange at the mud flat-atmosphere interface is necessary.

Groundwater‐River Water Exchange Enhances Growing Season Evapotranspiration and Carbon Uptake in a Semiarid Riparian Ecosystem

AbstractSemiarid ecosystems play a critical role in determining the interannual variability of the global terrestrial carbon sink. Water availability is a critical driver of productivity in semiarid ecosystems, which often alternate between carbon sink/source functioning during wet/dry years. In this study, we investigate how groundwater availability resulting from groundwater‐river water exchange influences net ecosystem exchange of CO2 (NEE), evapotranspiration (ET), and the surface energy balance at two semiarid ecosystems along the Columbia River in central Washington, USA. We examined 1 year of eddy covariance measurements from an upland sagebrush ecosystem primarily fed by rainfall without groundwater access and a riparian grassland ecosystem with groundwater access during the dry season due to lateral groundwater‐river water exchange. The two sites had distinct seasonal patterns of NEE and ET, driven by differences in water availability. While NEE at the upland sagebrush site was strongly constrained by water availability during the dry months, access to groundwater allowed the riparian site to maintain high NEE magnitude and ET during the same dry months. The riparian site had larger annual gross primary productivity than the upland site (612 vs. 424 gC/m2), which was offset by higher ecosystem respiration (558 vs. 363 gC/m2). Thus, the magnitude of the annual NEE at the upland site was larger than that at the riparian site (−62 vs. −54 gC/m2). Our results demonstrate that groundwater access determined by connectivity between groundwater and surface water can be a critical driver of carbon uptake and ET in semiarid ecosystems.

Carbon Dioxide and Water Exchange between Spruce Forest and Atmosphere in Spring—Summer under Different Weather Conditions

Results of eddy covariance measurements of carbon dioxide and water fluxes above a spruce forest in the middle taiga subzone from April to August 2013 and 2016 are presented. The ecosystem of the spruce forest turned from source to sink CO2 at mean daily air temperatures below zero in late March and early April; in 2016, it was 2 weeks earlier than in 2013. The maximum net ecosystem exchange of CO2 (NEE) was recorded at the end of June and beginning of July. In 2016, the mean daily NEE of spruce forest decreased at a high air temperature and a low amount of precipitation in the beginning of the growing season; turning from the sink CO2 to the source was observed in early August. The net exchange of CO2 between spruce and the atmosphere from April to August 2013 reached 327 g C/m2 and, in 2016, 174 g C/m2. The seasonal evapotranspiration of the spruce forest in these years was 239–247 mm/m2 and the mean value of water-use efficiency for photosynthesis (WUE) in the season was 2.3–3.3 g C/kg H2O. WUE was relatively constant during the growing season, resulting from a close relationship between the exchange of CO2 and water, the main processes that ensure the production of organic matter in the spruce-forest ecosystem.

East Asian summer monsoon substantially affects the inter-annual variation of carbon dioxide exchange in semi-arid grassland ecosystem in Loess Plateau

The net ecosystem CO2 exchange (NEE) of a semi-arid grassland on the boundary area of the East Asian summer monsoon (EASM) was measured continuously by eddy covariance technology in the Loess Plateau from 2007 to 2012. The grassland ecosystem was a weak carbon source with a mean of 17.1 ± 12.1 g C m−2 year-1 during the measure period. The inter-annual variability in NEE was significantly related to the annual precipitation amount. The EASM played an essential role in the CO2 exchange of grassland at the boundary region of monsoon thought dominating climatic variables, such as the annual precipitation amount and seasonal drought. Drought stress suppressed gross primary productivity (GPP) more than ecosystem respiration (Reco). Soil moisture reduction could result in sensitivity weaker of Reco dependence to soil temperature (Ts), and GPP estimated by Reco-Ts exponential growth function might be overestimated under severe soil drought when high Ts. Additionally, warmer spring has been found to be able to enhance the grassland’s carbon uptake, and compensate for the reductions induced by the summer drought. Our results contribute to understanding of the specific driving mechanism for inter-annual variability of carbon fluxes in the Loess Plateau, and enhancing the terrestrial ecosystem model’s performance in carbon fluxes estimate under the drought stress environment.

Annual dynamics of carbon dioxide fluxes over a rainfed alfalfa field in the U.S. Southern Great Plains

A thorough investigation of annual dynamics of carbon dioxide (CO2) fluxes with respect to major controlling factors and harvest management is lacking for rainfed alfalfa (Medicago sativa L.), a high quality perennial legume forage. To address this knowledge gap, this study reports two years (April 2016 - March 2018) of eddy covariance measurements of CO2 fluxes over a rainfed alfalfa field in central Oklahoma, USA. Alfalfa yields were strongly regulated by amount and timing of rainfall. As a result, cumulative dry forage yield was ∼7.5 t ha−1 (four harvests) in 2016 (dry year) and ∼10 t ha−1 (five harvests) in 2017 (wet year). An optimum air temperature (Ta) and vapor pressure deficit (VPD) for net ecosystem CO2 exchange (NEE) was approximately 25 °C and 2.2 kPa, respectively. The response of gross primary production (GPP) to photosynthetically active radiation (PAR) varied with growth stage of alfalfa and climatic conditions (i.e., dry or normal/wet periods). Daily (8-day averages) NEE and gross primary production (GPP) reached -8.17 and 16.69 g C m-2 d−1, respectively. Magnitudes of GPP (GPPMOD) derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) were < 50% of tower-derived GPP, most likely due to a smaller value (0.15 g C mol−1 PAR) for light use efficiency in the GPPMOD algorithm. The observed 8-day composite ecosystem light use efficiency (ELUE) was up to 0.36 g C mol−1 PAR in this study. The rainfed alfalfa field with 4–5 hay harvests per year showed large carbon uptake potential (e.g., cumulative NEE of -454 g C m-2 in 2017) at an annual scale. The GPP and ELUE showed a strong correspondence with MODIS-derived vegetation indices, indicating the potential of applying satellite remote sensing to upscale site-level observations of CO2 fluxes for alfalfa to larger spatial scales.

Disentangling the carbon budget of a vineyard: The role of soil management

The environmental sustainability of viticulture can be enhanced with the application of conservative management practices (e.g. resident vegetation or cover crop on the inter-row), which can lead to an increase of soil carbon (C) sequestration. However, studies disentangling the vineyard C budget are still very scarce. In this context, comparing vineyard net ecosystem CO2 exchange (NEE) with soil fluxes is especially useful. From January 2015 to August 2016, we continuously monitored vineyard NEE with the eddy covariance method and ground CO2 fluxes with an automated chamber system in a commercial vineyard in North Eastern Italy. At the site, inter-rows are covered with resident herbaceous vegetation, however, due to low soil permeability, soil cultivation (ripping or tillage) was performed on alternate alleys in autumn and, sometimes, spring in order to improve water infiltration. Measured annual soil respiration was comparable, but lower, to values estimated by previous studies in vineyards and the net uptake of the grass cover laid in the middle of yearly C budget range reported in the literature for grasslands. At the end of the measurement period, the vineyard ecosystem showed to be a net sink of CO2, absorbing around −233 gC m−2. However, the C sequestration could have been much greater if no soil cultivation had been applied. Indeed, the ground compartment was a source of CO2, but without inter-row cultivation it could have been a net sink, with an overall vineyard C budget of about −421 gC m−2. This confirms that grass cover of the inter-rows can play an important role in the C budget of woody crops. Additionally, the pattern of C fluxes reveals that the activity of herbaceous vegetation in summer decreased well before vines, thus reducing water competition during dry periods. These results provide important information for the tuning of management practices aimed at improving the environmental sustainability of viticulture.

Biochar improves soil quality and N2-fixation and reduces net ecosystem CO2 exchange in a dryland legume-cereal cropping system

Biochar, a product of the pyrolysis of organic material, has received wide attention as a means to improve soil fertility, crop productivity, and carbon sequestration to mitigate climate change. This study was conducted in an arid zone agricultural region to investigate the impacts of biochar on soil quality, N2-fixation, and net ecosystem CO2 exchange (NEE) for a legume-cereal crop rotation (2013-15). Biochar was produced by pyrolysis of sugarcane bagasse and applied at rates of 0%, 0.25%, and 0.5% carbon (C) equivalent, with and without chemical fertilizer (23 kg N, 45 kg P and 25 kg K ha−1). In terms of soil quality, biochar applications increased soil organic carbon (SOC), total nitrogen (TN), and decreased soil bulk density. The yield of mash bean and wheat crop was significantly enhanced in response to biochar treatment with and without chemical fertilizer during both the years of study. N2-fixation in mash bean increased by 83% in biochar-only (0.5% C) amended soil and by 127% with chemical fertilizer. In the first year of mash bean, no difference was observed in NEE, while a significant decrease was observed in the second year attributed to priming effect of fresh biochar. Wheat NEE decreased by 144% and 200% in the first year, and by 283% and 265% in the second year, in unfertilized soil amended with 0.25 and 0.5% biochar, respectively, as compared to the controls. In treatments with chemical fertilizer, wheat NEE decreased by 311% and 344% in the first year, and by 293% and 292% in the second year, as compared to their respective controls. Biochar treatment increased biomass yield (BY) and grain yield (GY) in both mash bean and wheat crops during both the years with and without chemical fertilizer. These results indicate that sugarcane-bagasse biochar applications in arid soils with low organic carbon may increase crop productivity and N2-fixation of legumes while reducing the NEE of legume-cereal cropping systems.

Changes in plant biomass induced by soil moisture variability drive interannual variation in the net ecosystem CO2 exchange over a reclaimed coastal wetland

Changes in the timing and magnitude of precipitation is a threat to agricultural productivity and farmland carbon stocks. However, the relationship between inter-annual variations in precipitation and net ecosystem CO2 exchange (NEE) remains to be clarified, particularly when combined with water-salt transport in reclaimed coastal wetland. Here, based on the eddy-covariance technique, we investigated the interannual variation in carbon dioxide exchange and its control mechanism over a reclaimed coastal wetland of the Yellow River Delta from 2010 to 2014. The coastal wetland functioned as a strong sink for atmospheric CO2, with the annual NEE of −229, −175, −142, −92 and −80 g C m−2 in the 5 years from 2010 to 2014, respectively. Surprisingly, we find that large annual variation in net ecosystem exchange (NEE) can be predicted accurately using plant biomass. Plant biomass was driven by soil water content (SWC), with about 48%–80% seasonal variation of biomass attributed to SWC. During the early growing stage, high SWC accompanied with low salinity promoted plant biomass and NEE. While high SWC accompanied with increased waterlogged stress inhibited plant biomass and NEE during the middle growing stage. The same results were also observed in a field manipulation experiment over a nearby natural coastal wetland. Our study indicated that extreme climate accompanied with extreme drought and flooding may decrease carbon sequestration capacity of the reclaimed coastal wetland due to the increase in salinity.

Effect of straw return on soil respiration and NEE of paddy fields under water-saving irrigation.

Straw return (SR) and rice water-saving irrigation (WSI) affect the greenhouse gas emission of paddy fields. However, studies on CO2 exchange between paddy fields and the atmosphere with joint regulation of SR and WSI are few. We conducted a two-year field experiment to investigate the effects of SR on soil respiration and net ecosystem exchange of CO2 (NEE) in paddy fields under controlled irrigation (CI), which is a typical WSI technique. The rice yields, irrigation water use efficiency, seasonal variations in soil respiration, NEE, and soil organic carbon content were measured. Compared with the control (flooding irrigation and traditional chemical fertilizer), a significant increase in rice yield and irrigation water use efficiency in the paddy fields under CI and SR joint management (CS) was observed. CS increased the soil respiration rate during most of the rice growth stage and increased the net CO2 absorption rate before approximately 80 days after transplanting; afterward, the pattern reversed. Total CO2 emissions through soil respiration in CS paddy fields increased by 43.7% and 182% compared with the control in 2014 and 2015, respectively. However, CS also caused an increase in the total net CO2 absorption by 18.1% and 30.1% in these two years, respectively. The acceleration in the consumption and decomposition of soil organic carbon induced by frequent alternate wet–dry cycles of the CI paddy fields increased the soil respiration and decreased the net CO2 absorption. SR promoted soil respiration but also improved rice growth, increasing the net CO2 absorption. The soil organic carbon content of the CS paddy fields after harvesting increased by 23.2% compared with that before transplanting. The present study concluded that joint regulation of WSI and SR is an effective measure for maintaining yield, increasing irrigation water use efficiency, mitigating CO2 emission, and promoting paddy soil fertility.

Multi-year net ecosystem carbon balance of a restored peatland reveals a return to carbon sink.

Peatlands after drainage and extraction are large sources of carbon (C) to the atmosphere. Restoration, through re-wetting and revegetation, aims to return the C sink function by re-establishing conditions similar to that of an undrained peatland. However, the time needed to re-establish C sequestration is not well constrained due to the lack of multi-year measurements. We measured over 3years the net ecosystem exchange of CO2 (NEE), methane ( ), and dissolved organic carbon (DOC) at a restored post-extraction peatland (RES) in southeast Canada (restored 14years prior to the start of the study) and compared our observations to the C balance of an intact reference peatland (REF) that has a long-term continuous flux record and is in the same climate zone. Small but significant differences in winter respiration driven by temperature were mainly responsible for differences in cumulative NEE between years. Low growing season inter-annual variability was linked to constancy of the initial spring water table position, controlled by the blocked drainage ditches and the presence of water storage structures (bunds and pools). Half-hour at RES was small except when Typha latifolia-invaded drainage ditches were in the tower footprint; this effect at the ecosystem level was small as ditches represent a minor fraction of RES. The restored peatland was an annual sink for CO2 (-90±18gCm-2 year-1 ), a source of CH4 (4.4±0.2gCm-2 year-1 ), and a source of DOC (6.9±2.2gCm-2 year-1 ), resulting in mean net ecosystem uptake of 78±17gCm-2 year-1 . Annual NEE at RES was most similar to wetter, more productive years at REF. Integrating structures to increase water retention, alongside re-establishing key species, have been effective at re-establishing the net C sink rate to that of an intact peatland.

Study of the Carbon Budget of a Temperate-Climate Vineyard: Inter-Annual Variability of CO2 Flux

Agriculture can improve the global carbon budget by reducing emissions and increasing soil carbon sequestration. Woody crops like vineyards present biological, structural, and management peculiarities, such as perennial structure, abundant pruning debris, and interrow cover cropping, which could potentially sequester a significant amount of CO2. However, such ecosystems are still poorly understood. Here, we present results from a long-term study from northeastern Italy on the carbon budget of a vineyard (cv. Sauvignon blanc, Guyot pruned, vertical shoot-positioned trellised, with grass-covered alleys). We used the eddy covariance technique to continuously monitor the net ecosystem CO2 exchange (NEE), and here we present the first three years of data (May 2014 to April 2017). The vineyard acted as a moderate carbon sink, with an average annual net carbon uptake of 134 g C/ m2. However, the inter-annual variability of NEE was high, and environmental conditions during the growing season had a strong impact. The annual carbon uptake was greatest in 2014 (207 g C/m2), when there was abundant rain in summer. In contrast, in 2015, several heat waves decreased the annual carbon uptake to 69 g C/m2. Elevated air temperature and low soil moisture in summer significantly reduced net carbon flux during times when the vineyard activity was at maximum. Nevertheless, we show that a vineyard can behave as a carbon sink in the medium-to-long term, which should be factored into agricultural and environmental policies, complementing the standard life cycle assessment of the wine production chain.

Aboveground biomass and root/shoot ratio regulated drought susceptibility of ecosystem carbon exchange in a meadow steppe

Nitrogen deposition and altered precipitation regime are likely to change plant growth, biomass allocation and community structure, which may influence susceptibility of ecosystem functions (i.e. ecosystem carbon exchange) to extreme climatic events, such as drought. In a meadow steppe, we deployed a drought treatment on a long-term water and nitrogen addition experiment to investigate resource abundance changes induced variation in the sensitivity of ecosystem carbon exchange to extreme drought. Compared to the control plots, long-term water and nitrogen addition caused a strong increase in biomass, and a reduction in diversity and root/shoot ratio. Net ecosystem CO2 exchange (NEE) in water and nitrogen addition plots were more sensitive to drought stress than the control plots. The enhanced NEE drought sensitivity (SNEE) in nitrogen fertilization habitat is associated with changes in aboveground biomass and root/shoot ratio, rather than variation in species diversity, while SNEE in the unfertilized plots was controlled by root/shoot ratio. Compared to the water and nitrogen addition plots, the control plots had the highest percentage recovery of ecosystem carbon exchange (RNEE) during the rehydration period. RNEE is likely determined by aboveground biomass and level of damage in the photosynthetic organ. These findings suggest that long-term changes in precipitation regimes and nitrogen deposition may significant alter the susceptibility of key ecosystem processes to drought stress.

Net ecosystem carbon and greenhouse gas budgets in fiber and cereal cropping systems

To assess the contributions of fiber and cereal production on climate change, the net ecosystem exchange of carbon dioxide (CO2), main exchanges of non-CO2 carbon, and methane (CH4) and nitrous oxide (N2O) fluxes were continuously monitored throughout two year-round crop cycles (Y1 and Y2: 1st and 2nd year-round crop cycles, respectively) using eddy covariance, biometric observation, and static chamber methods in typical cotton and wheat–maize rotational cropping systems in China. The evaluation of net ecosystem carbon budgets (NECBs: considering net ecosystem CO2 exchange and non-CO2 carbon exchanges by fertilization, seeding, and harvest) and greenhouse gas budgets (GHGBs: adding CH4 and N2O fluxes to the NECBs based on CO2 equivalents) showed that the cotton cropping system persistently functioned as an intensive carbon (−1527 and −974 kg C ha−1 yr−1) and greenhouse gas (GHG) source (5618 and 3591 kg CO2-eq ha−1 yr−1) because of the large CO2 emissions during the long fallow periods (5748 and 5160 kg CO2 ha−1 in Y1 and Y2, respectively). The wheat–maize cropping system had high net ecosystem production (NEP) and low harvest index and therefore, served as a notable carbon sink (1461 kg C ha−1 yr−1 in Y2). Although high irrigation water and chemical fertilizer inputs stimulated N2O emissions, the wheat–maize cropping system still behaved as an important GHG sink (−4257 kg CO2-eq ha−1 yr−1 in Y2) because of the tremendous net carbon sequestration. However, in Y1 incidental wind damage lowered the NEP and turned the wheat–maize cropping system into a GHG source (2144 kg CO2-eq ha−1 yr−1). The NEP, NECBs, and GHGBs of the double cropping system generally exceeded those of the single cropping system. The traditional rotation between double and single cropping systems should be restored to maintain soil carbon storage and alleviate the radiative forcing effects of cotton production.

The relationship between leaf and ecosystem CO2 exchanges in a maize field

The relationship between leaf photosynthetic rate (A) in a vegetation canopy and the net ecosystem CO2 exchange (NEE) over an entire ecosystem is not well understood. The aim of the present study is to assess the coordinated changes in NEE derived with eddy covariance, A measured in leaf cuvette, and their associations in a rainfed maize field. The light response-curves were estimated for the carbon assimilation rate at both the leaf and ecosystem scales. NEE and A synchronically changed throughout the day and were greater around noon and persisted longer during rapid growth periods. The leaf A had a similar pattern of daytime changes in the top, middle, and bottom leaves. Only severe leaf ageing led to a significant decline in the maximum efficiency of photosystem II (PSII) photochemistry. The greater maximum NEE was associated with a higher ecosystem quantum yield. NEE was positively and significantly correlated with the leaf A averaged based on the vertical distribution of leaf area. The finding highlights the feasibility of assessing NEE by leaf CO2 exchange because of most of experimental data obtained with leaf cuvette methods; and also implies that simultaneously enhancing leaf photosynthetic rate, electron transport rate, net carbon assimilation at whole ecosystem might play a critical role for the enhancement of crop productivity.