Ecosystem CO2 Exchange Research Articles

Extreme temperature events reduced carbon uptake of a boreal forest ecosystem in Northeast China: Evidence from an 11-year eddy covariance observation.

Boreal forests, the second continental biome on Earth, are known for their massive carbon storage capacity and important role in the global carbon cycle. Comprehending the temporal dynamics and controlling factors of net ecosystem CO2 exchange (NEE) is critical for predicting how the carbon exchange in boreal forests will change in response to climate change. Therefore, based on long-term eddy covariance observations from 2008 to 2018, we evaluated the diurnal, seasonal, and interannual variations in the boreal forest ecosystem NEE in Northeast China and explored its environmental regulation. It was found that the boreal forest was a minor CO2 sink with an annual average NEE of -64.01 (± 24.23) g CO2 m-2 yr-1. The diurnal variation in the NEE of boreal forest during the growing season was considerably larger than that during the non-growing season, and carbon uptake peaked between 8:30 and 9:30 in the morning. The seasonal variation in NEE demonstrated a "U" shaped curve, and the carbon uptake peaked in July. On a half-hourly scale, photosynthetically active radiation and vapor pressure deficit had larger impacts on daytime NEE during the growing season. However, temperature had major control on NEE during the growing season at night and during the non-growing season. On a daily scale, temperature was the dominant factor controlling seasonal variation in NEE. Occurrence of extreme temperature days, especially extreme temperature events, would reduce boreal forest carbon uptake; interannual variation in NEE was substantially associated with the maximum CO2 uptake rate during the growing season. This study deepens our understanding of environmental controls on NEE at multiple timescales and provides a data basis for evaluating the global carbon budget.

Carbon uptake in Eurasian boreal forests dominates the high-latitude net ecosystem carbon budget.

Arctic-boreal landscapes are experiencing profound warming, along with changes in ecosystem moisture status and disturbance from fire. This region is of global importance in terms of carbon feedbacks to climate, yet the sign (sink or source) and magnitude of the Arctic-boreal carbon budget within recent years remains highly uncertain. Here, we provide new estimates of recent (2003-2015) vegetation gross primary productivity (GPP), ecosystem respiration (Reco ), net ecosystem CO2 exchange (NEE; Reco - GPP), and terrestrial methane (CH4 ) emissions for the Arctic-boreal zone using a satellite data-driven process-model for northern ecosystems (TCFM-Arctic), calibrated and evaluated using measurements from >60 tower eddy covariance (EC) sites. We used TCFM-Arctic to obtain daily 1-km2 flux estimates and annual carbon budgets for the pan-Arctic-boreal region. Across the domain, the model indicated an overall average NEE sink of -850 Tg CO2 -C year-1 . Eurasian boreal zones, especially those in Siberia, contributed to a majority of the net sink. In contrast, the tundra biome was relatively carbon neutral (ranging from small sink to source). Regional CH4 emissions from tundra and boreal wetlands (not accounting for aquatic CH4 ) were estimated at 35 TgCH4 -C year-1 . Accounting for additional emissions from open water aquatic bodies and from fire, using available estimates from the literature, reduced the total regional NEE sink by 21% and shifted many far northern tundra landscapes, and some boreal forests, to a net carbon source. This assessment, based on in situ observations and models, improves our understanding of the high-latitude carbon status and also indicates a continued need for integrated site-to-regional assessments to monitor the vulnerability of these ecosystems to climate change.

Effects of nitrogen addition on ecosystem-level carbon fluxes and water use efficiency are more dependent on early growing season precipitation than on annual precipitation in a semi-arid meadow steppe

Precipitation and anthropogenic nitrogen (N) may have profound impacts on ecosystem carbon (C) fluxes and the plant water use efficiency (WUE), particularly in semi-arid salinized and alkalized grasslands that were limited by both N and water availability. To better understand the mechanism of N and precipitation effects on C fluxes and plant WUE, in 2015, we established a randomized block-design N addition experiment, including five N levels (0, 5, 10, 20, 40 g N m−2 yr−1) in a meadow steppe (dominated by Leymus chinensis (Trin.) Tzvel.) of northeast China. Between 2016–2019, we investigated how N addition, combined with different amounts of annual and early growing season precipitation availability, modulated ecosystem C fluxes (NEE, net ecosystem CO2 exchange; ER, ecosystem respiration; GEP, gross ecosystem production), plant WUE, including leaf-scale long-term intrinsic WUE (iWUE) inferred from 13C composition and ecosystem-scale WUE (WG, WG = GEP/ET), as well as related biotic and edaphic variables. We found that: (i) N supply decreased NEE and increased ER, GEP, and WG in a nonlinear pattern; (ii) N addition increased iWUE and thus WG; and (iii) higher early growing season precipitation, in contrast to annual precipitation, magnified the effect of N on NEE, ER, GEP, and WG, suggesting that N effects on NEE, ER, GEP, and WG depend more on early growing season precipitation than annual precipitation. The present study improves our understanding of the coupled mechanisms of future global C, N, and water dynamics shaped by global climate change.

Tropical forests post-logging are a persistent net carbon source to the atmosphere

Logged and structurally degraded tropical forests are fast becoming one of the most prevalent land-use types throughout the tropics and are routinely assumed to be a net carbon sink because they experience rapid rates of tree regrowth. Yet this assumption is based on forest biomass inventories that record carbon stock recovery but fail to account for the simultaneous losses of carbon from soil and necromass. Here, we used forest plots and an eddy covariance tower to quantify and partition net ecosystem CO2 exchange in Malaysian Borneo, a region that is a hot spot for deforestation and forest degradation. Our data represent the complete carbon budget for tropical forests measured throughout a logging event and subsequent recovery and found that they constitute a substantial and persistent net carbon source. Consistent with existing literature, our study showed a significantly greater woody biomass gain across moderately and heavily logged forests compared with unlogged forests, but this was counteracted by much larger carbon losses from soil organic matter and deadwood in logged forests. We estimate an average carbon source of 1.75 ± 0.94 Mg C ha-1 yr-1 within moderately logged plots and 5.23 ± 1.23 Mg C ha-1 yr-1 in unsustainably logged and severely degraded plots, with emissions continuing at these rates for at least one-decade post-logging. Our data directly contradict the default assumption that recovering logged and degraded tropical forests are net carbon sinks, implying the amount of carbon being sequestered across the world's tropical forests may be considerably lower than currently estimated.

Effects of Biofuel Crop Switchgrass (Panicum virgatum) Cultivation on Soil Carbon Sequestration and Greenhouse Gas Emissions: A Review.

Under the macroenvironmental background of global warming, all countries are working to limit climate change. Internationally, biofuel plants are considered to have great potential in carbon neutralization. Several countries have begun using biofuel crops as energy sources to neutralize carbon emissions. Switchgrass (Panicum virgatum) is considered a resource-efficient low-input crop that produces bioenergy. In this paper, we reviewed the effects of switchgrass cultivation on carbon sequestration and greenhouse gas (GHG) emissions. Moreover, the future application and research of switchgrass are discussed and prospected. Switchgrass has huge aboveground and underground biomass, manifesting its huge carbon sequestration potential. The net change of soil surface 30 cm soil organic carbon in 15 years is predicted to be 6.49 Mg ha-1, significantly higher than that of other crops. In addition, its net ecosystem CO2 exchange is about -485 to -118 g C m-2 yr-1, which greatly affects the annual CO2 flux of the cultivation environment. Nitrogen (N) fertilizer is the main source of N2O emission in the switchgrass field. Nitrogen addition increases the yield of switchgrass and also increases the N2O flux of switchgrass soil. It is necessary to formulate the most appropriate N fertilizer application strategy. CH4 emissions are also an important indicator of carbon debt. The effects of switchgrass cultivation on CH4 emissions may be significant but are often ignored. Future studies on GHG emissions by switchgrass should also focus on CH4. In conclusion, as a biofuel crop, switchgrass can well balance the effects of climate change. It is necessary to conduct studies of switchgrass globally with the long-term dimension of climate change effects.

Biotic and Abiotic Control Over Diurnal CH4 Fluxes in a Temperate Transitional Poor Fen Ecosystem

To understand the variability of methane (CH4) fluxes between a temperate mid-altitude Sphagnum-dominated peatland and the atmosphere, we monitored simultaneously eddy covariance, hydrometeorological and physical parameters between April 2019 and December 2021. The site was a CH4 source for the atmosphere, with a cumulative emission of 23.9 ± 0.6 g C m−2 year−1. At the interannual scale, deeper water table during vegetation growth periods resulted in lower CH4 fluxes (FCH4), and reciprocally. Furthermore, the seasonal temperature variation in the anaerobic peat layer was a good predictor for FCH4. However, while the lowest temperatures occurred between December and February, the lowest FCH4 were observed between March and May, with around 30% of negative FCH4. Indeed, the fastest increase in temperature of the aerobic layer likely stimulated methanotrophy at the expense of methanogenesis. Negative FCH4, systematically observed at midday, were concurrent with high photon flux densities, latent heat fluxes and net negative ecosystem CO2 exchanges, suggesting the control of photosynthesis over CH4 oxidation. Moreover, our results highlighted marked diurnal cycles with FCH4 maximal at night and minimal at midday for all seasons. This diurnal cyclicity is in opposition to what is typically known for peatlands dominated by vascular plants. Physical parameters, such as soil surface temperature and sensible heat fluxes, likely contribute to this diurnal FCH4 cyclicity and require further investigation. Our study thus demonstrates that diurnal variations in FCH4 must be considered before upscaling to seasonal or annual cycles, along with the effect of vegetation on CH4 transfer and oxidation processes.

Net Ecosystem CO2 Exchange in Mountain Grasslands Is Seriously Endangered by the Temperature Increase in the Eastern Pyrenees

Mediterranean mountain grasslands, including the Pyrenees, are highly vulnerable to climate change, due to the increasing temperatures and heat weaves frequency, among other factors. However, the effects of the increased temperatures on CO2 fluxes in those ecosystems have been barley explored. To address this gap of knowledge, we established the FLUXPYR-ECOFUN micrometeorological flux network, which included three eddy covariance flux stations in grasslands along a management and a climatic gradient (montane to subalpine) at the Pyrenees; we aimed at assessing interactions among environmental and phenological drivers on CO2 fluxes, with special attention at the role of temperature as CO2 flux driver under the different climatic and management conditions across the studied gradient. Our results showed that temperature drove CO2 dynamics along the studied gradient in different ways. At the subalpine grassland net CO2 uptake was linearly enhanced by temperature and CO2 fluxes had not reached a temperature shifting point yet (according to the segmented linear models) at which the net uptake would become CO2 emissions. This suggests that in the short term, and under the incoming enhanced temperatures, sub-alpine grasslands in the Pyrenees might increase their net CO2 uptake, although the mid long-term uptake may be compromised. On the contrary, the montane grasslands already presented CO2 emissions at the highest temperatures, most likely driven by a decrease in the greenness and photosynthesis, which suggests that montane grasslands are expected to reduce their CO2 sink capacity under the increasing temperatures. Overall, mountain grasslands in the mid- to long-term in the Pyrenees may experience a reduction in their net CO2 uptake capacity under the current climate change scenario.

Evaluation of Environmental Controls on Terrestrial Net Ecosystem Exchange of CO2: A Global Perspective From the FLUXNET Sites

AbstractNet ecosystem exchange (NEE) of CO2 is a key process modulating carbon exchanges between terrestrial ecosystems and the atmosphere; however, it remains a grand challenge to elucidate how the interactions of NEE with environmental variables vary among ecosystems and climate regimes across timescales. The FLUXNET and AmeriFlux data sets were used to diagnose the environmental controls on NEE. Based on the sites with long‐term observations (≥7 years), the results showed that the couplings between NEE and surrounding environments were stronger at daily and monthly scales than at annual scales, highlighting the temporal dependence of environmental variables influencing NEE. Moreover, the boosted regression tree method was applied to quantify the relative impacts of environmental controls on daily NEE variations. It revealed that leaf area index (LAI) and shortwave radiation (Rs) were the major divers of daily NEE variations at most sites with the average contribution of 35.5% and 27.8%, respectively. Particularly, LAI was the principal control in deciduous broadleaf forested, non‐forested, and arid sites, while Rs was the leading factor in evergreen forested sites. Meanwhile, air temperature (Ta), soil water content (SWC), and vapor pressure deficit (VPD) exerted smaller influences with the average contribution of 14.6%, 13.2%, and 8.9%, respectively. The relative impacts of LAI, Rs, SWC, and VPD also varied with aridity index, and mean annual precipitation and Ta. Furthermore, NEE was more sensitive to vegetation dynamics in drier climate regions. This study provides additional understanding of how environmental factors regulate NEE dynamics across diverse land surface and climatic conditions.

Partitioning evapotranspiration and carbon flux in ungrazed and grazed tallgrass prairie

The effect of cattle grazing on water and carbon cycling in grasslands is not fully understood. The goal of this study was to evaluate how grazing affects carbon and water cycles in a tallgrass prairie. Eddy covariance measurements were carried out from 2003 to 2005 growing seasons (from May to September) in Manhattan, Kansas, U.S., in grazed (GR) and ungrazed (UG) tallgrass prairie paddocks. Net ecosystem CO2 exchange (NEE) was partitioned into gross primary productivity (GPP) and ecosystem respiration (Reco) using the relationship between the air temperature and nighttime NEE data. Evapotranspiration (ET) was partitioned into transpiration (T) and evaporation (E) using an approach based on the concept of underlying water use efficiency. Our results revealed that grazing reduced maximum aboveground biomass (AGB) and green leaf area index (GLAI) by 22.5% and 24.3%, respectively. Grazing also reduced GPP, NEE and T in the middle of the growing seasons but enhanced these flux components at the end of the growing seasons compared to the UG paddock. Grazing reduced the growing season cumulative NEE by 11.9% and enhanced ET by 3.3% compared to the UG paddock. The reduction in NEE at UG was associated with an increase in Reco (9.4%). In turn, changes in ET in response to grazing were driven more by the increase in E (26.6%) than by the decrease in T (5.9%). Despite the reduction in AGB and GLAI caused by grazing, similar cumulative GPP values at the canopy scale at both paddocks suggest that there are compensatory mechanisms that maintain similar CO2 assimilation at UG and GR. Our findings suggest that the grazing regime adopted in this tallgrass prairie impacts the carbon cycle more strongly than the water cycle, given the higher reduction in NEE compared to the slight increase in ET on GR compared to UG. These results can be useful to outline strategies to improve management practices aimed at improving the efficiency of rangelands.

Asynchronous responses of microbial CAZymes genes and the net CO2 exchange in alpine peatland following 5 years of continuous extreme drought events

Peatlands act as an important sink of carbon dioxide (CO2). Yet, they are highly sensitive to climate change, especially to extreme drought. The changes in the net ecosystem CO2 exchange (NEE) under extreme drought events, and the driving function of microbial enzymatic genes involved in soil organic matter (SOM) decomposition, are still unclear. Herein we investigated the effects of extreme drought events in different periods of plant growth season at Zoige peatland on NEE and microbial enzymatic genes of SOM decomposition after 5 years. The results showed that the NEE of peatland decreased significantly by 48% and 26% on average (n = 12, P < 0.05) under the early and midterm extreme drought, respectively. The microbial enzymatic genes abundance of SOM decomposition showed the same decreasing trend under early and midterm extreme drought, but an increasing trend under late extreme drought. The microbial community that contributes to these degradation genes mainly derives from Proteobacteria and Actinobacteria. NEE was mainly affected by soil hydrothermal factors and gross primary productivity but weakly correlated with SOM enzymatic decomposition genes. Soil microbial respiration showed a positive correlation with microbial enzymatic genes involved in the decomposition of labile carbon (n = 18, P < 0.05). This study provided new insights into the responses of the microbial decomposition potential of SOM and ecosystem CO2 sink function to extreme drought events in the alpine peatland.

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.

Effect of warming on the carbon flux of the alpine wetland on the Qinghai–Tibet Plateau

Under the scenario of global warming, the response of greenhouse gas emissions from alpine wetlands remains unclear. In this study, fluxes of CO2 and CH4 were measured during daytime for the microtopographic features of hollows and hummocks in a wetland in the Tibetan Plateau under two elevated temperatures, increments of ∼1°C (T1 treatment) and ∼2°C (T2 treatment), during the growing season in 2019. The results showed that warming significantly increased the cumulative net ecosystem CO2 exchanges (NEE) for both microtopographic features in the wetland compared to the control due to a combination of the increased gross primary production (GPP) with an increase in ecosystem respiration (ER). Similarly, warming also increased cumulative CH4 emission significantly. The effect was stronger for T2 than that for T1 for all component fluxes (GPP, ER, NEE, and CH4). Generally, NEE and CH4 fluxes both rose at first and then decreased. NEE peaked at the end of July for both hollows and hummocks, while CH4 emissions peaked in the middle of August. The cumulative CH4 emissions from the hummocks were significantly higher than those of the hollows, and CH4 emissions under illumination were significantly higher than those in darkness, which may be caused by the irradiation-sensitive vegetable internal convective gas transport system which diffuses CH4 from the pedosphere. This study revealed that warming strengthened the function of the CO2 sink but also increased CH4 emissions from the alpine wetlands on the Qinghai–Tibet Plateau.

Seasonal dynamics of carbon dioxide and water fluxes in a rice-wheat rotation system in the Yangtze-Huaihe region of China

Investigating variations in the characteristics and influencing factors on carbon dioxide and water fluxes in cultivated lands are crucial for improving crop yield and water use efficiency. As a critical cropping system, rice-wheat rotations are widely distributed in South and East Asia. However, few studies have reported on the long-term changes in carbon dioxide and water fluxes in rice-wheat rotation systems using the eddy covariance method. In this study, a typical rice-wheat rotation field located in Anhui province, China, was chosen for our study. We analyzed seasonal dynamics and influencing factors of net ecosystem CO2 exchange (NEE), gross primary production (GPP), ecosystem respiration (Re), and evapotranspiration (ET) based on 5-year observations and boosted regression trees method. The main results were as follows: (1) daily NEE, GPP, Re, and ET displayed a bimodal curve during the year, with two peaks in the rice and wheat growing seasons, respectively. (2) The cumulative values of NEE were negative in the rice and wheat growing seasons, indicating the cultivated land acted as a carbon sink during the study period. Furthermore, cumulative values of NEE, GPP, Re, and ET were slightly higher in the rice-growing seasons than the wheat-growing seasons. (3) The relative importance analysis revealed the leaf area index (LAI) was the primary control factor of NEE and GPP, whereas temperature and LAI essentially controlled Re. In comparison, net radiation was the most crucial controller of ET. However, their contributions varied in different growing seasons. The findings have important implications for parameterizing crop models to improve simulating carbon and water fluxes for rice-wheat rotation systems.

Interannual characteristics and driving mechanism of CO2 fluxes during the growing season in an alpine wetland ecosystem at the southern foot of the Qilian Mountains.

The carbon process of the alpine ecosystem is complex and sensitive in the face of continuous global warming. However, the long-term dynamics of carbon budget and its driving mechanism of alpine ecosystem remain unclear. Using the eddy covariance (EC) technique-a fast and direct method of measuring carbon dioxide (CO2) fluxes, we analyzed the dynamics of CO2 fluxes and their driving mechanism in an alpine wetland in the northeastern Qinghai-Tibet Plateau (QTP) during the growing season (May-September) from 2004-2016. The results show that the monthly gross primary productivity (GPP) and ecosystem respiration (Re) showed a unimodal pattern, and the monthly net ecosystem CO2 exchange (NEE) showed a V-shaped trend. With the alpine wetland ecosystem being a carbon sink during the growing season, that is, a reservoir that absorbs more atmospheric carbon than it releases, the annual NEE, GPP, and Re reached -67.5 ± 10.2, 473.4 ± 19.1, and 405.9 ± 8.9 gCm-2, respectively. At the monthly scale, the classification and regression tree (CART) analysis revealed air temperature (Ta) to be the main determinant of variations in the monthly NEE and GPP. Soil temperature (Ts) largely determined the changes in the monthly Re. The linear regression analysis confirmed that thermal conditions (Ta, Ts) were crucial determinants of the dynamics of monthly CO2 fluxes during the growing season. At the interannual scale, the variations of CO2 fluxes were affected mainly by precipitation and thermal conditions. The annual GPP and Re were positively correlated with Ta and Ts, and were negatively correlated with precipitation. However, hydrothermal conditions (Ta, Ts, and precipitation) had no significant effect on annual NEE. Our results indicated that climate warming would be beneficial to the improvement of GPP and Re in the alpine wetland, while the increase of precipitation can weaken this effect.

Microbially enhanced methane uptake under warming enlarges ecosystem carbon sink in a Tibetan alpine grassland.

The alpine grasslands of the Tibetan Plateau store 23.2 Pg soil organic carbon, which becomes susceptible to microbial degradation with climate warming. However, accurate prediction of how the soil carbon stock changes under future climate warming is hampered by our limited understanding of belowground complex microbial communities. Here, we show that 4 years of warming strongly stimulated methane (CH4 ) uptake by 93.8% and aerobic respiration (CO2 ) by 11.3% in the soils of alpine grassland ecosystem. Due to no significant effects of warming on net ecosystem CO2 exchange (NEE), the warming-stimulated CH4 uptake enlarged the carbon sink capacity of whole ecosystem. Furthermore, precipitation alternation did not alter such warming effects, despite the significant effects of precipitation on NEE and soil CH4 fluxes were observed. Metagenomic sequencing revealed that warming led to significant shifts in the overall microbial community structure and the abundances of functional genes, which contrasted to no detectable changes after 2 years of warming. Carbohydrate utilization genes were significantly increased by warming, corresponding with significant increases in soil aerobic respiration. Increased methanotrophic genes and decreased methanogenic genes were observed under warming, which significantly (R2 =.59, p < .001) correlated with warming-enhanced CH4 uptakes. Furthermore, 212 metagenome-assembled genomes were recovered, including many populations involved in the degradation of various organic matter and a highly abundant methylotrophic population of the Methyloceanibacter genus. Collectively, our results provide compelling evidence that specific microbial functional traits for CH4 and CO2 cycling processes respond to climate warming with differential effects on soil greenhouse gas emissions. Alpine grasslands may play huge roles in mitigating climate warming through such microbially enhanced CH4 uptake.

СЕЗОННАЯ ИЗМЕНЧИВОСТЬ ПОТОКОВ ДИОКСИДА УГЛЕРОДА, ЯВНОГО И СКРЫТОГО ТЕПЛА В СЕВЕРОТАЕЖНОМ ЛИСТВЕННИЧНОМ ЛЕСУ СРЕДНЕЙ СИБИРИ ПО ДАННЫМ ПУЛЬСАЦИОННЫХ ИЗМЕРЕНИЙ

The seasonal and daily variability of sensible and latent heat and carbon dioxide (CO2) fluxes, as well as the sensitivity of the fluxes to changing environmental conditions were derived by the eddy covariance flux measurements in an undisturbed northern-taiga larch forest on continuous permafrost in Central Siberia from the end of May to the end of September in 2013 and 2015. The measurements showed a high seasonal and daily variability of CO2, sensible and latent heat fluxes. It was found that whereas dry and sunny weather conditions of June and July in 2013 and 2015 resulted in lower daily latent heat fluxes as compared to sensible ones, mostly cloudy and rainy weather in the second half of the summer led to the reduced net radiation and increased latent heat fluxes. The daily values of gross primary production from the beginning of June to mid-August in 2013 and 2015 exceeded the values of the ecosystem respiration, and the forest ecosystem was the sink of CO2 from the atmosphere. The maximum CO2 uptake was observed in late June and early July, primarily due to high photosynthesis rates influenced by high incoming solar radiation, optimal air temperature, and soil moisture conditions. Since mid-August, the balance between the CO2 uptake and release was close to zero. The total net ecosystem CO2 exchange during the growing season of 2013 was -76.5 g C/m2, while it was slightly higher (-103.6 g C/m2) during the growing season of 2015, primarily due to higher air temperatures in June and greater precipitation in the summer months of 2015.

Rainfed cotton crop in central India is a strong net CO2 sink: An eddy covariance-based analysis of ecosystem fluxes

The present study reports ecosystem CO2, H2O exchanges (at diurnal, daily, and seasonal-scale) from a rainfed (dryland) cotton crop (Gossypium sp) grown in deep black soil of tropical central India during 2019–20 and 2020–21, using an Eddy Covariance (EC) system. The half-hourly CO2 fluxes were found to vary significantly across the growth stages of cotton crop with peak values during first boll opening to first picking stage [mean Net Ecosystem CO2 Exchange (NEE), −16 to −18 µmol m-2 s-1; mean Gross Primary production (GPP), 20–22 µmol m-2 s-1; and mean Ecosystem Respiration (Reco), 5–6 µmol m-2 s-1]. The response of daytime GPP to Photosynthetic Photon Flux Density (PPFD) was found to be poor during 1–30 days after sowing (DAS) with R2∼0.4. It improved during 30–150 DAS (R2∼0.6), while deteriorating further as the crop started senescing. The rainfed cotton crop was found to be a strong net CO2 sink, with seasonal NEE, GPP, Reco, and evapotranspiration (ET) of − 333.66 gC m-2, 990.16 gC m-2, 656.50 gC m-2 and 468.32 mm during 2019–20, respectively. The cotton season of 2020–21 had seasonal NEE, GPP, Reco, and ET of − 391.85 gC m-2, 1063.89 gC m-2, 672.04 gC m-2, and 545.25 mm respectively. The seasonal Ecosystem Water Use Efficiency (EWUE) was estimated to be 1.9 and 2.1 gC kg-1 H2O during 2019–20 and 2020–21, respectively. Maximum values of crop coefficient (Kc) were observed to be 1–1.2 during 30–75 DAS, signifying favourable conditions for crop growth. The inter-seasonal differences in the CO2 and H2O exchanges were mainly driven by the differences in incoming PPFD due to cloudiness. Results of the present study will help in understanding the ecosystem processes and exchanges from a rainfed cotton-based cropping system, which in turn can significantly contribute to the global carbon and moisture budget.

Unraveling the effects of management and climate on carbon fluxes of U.S. croplands using the USDA Long-Term Agroecosystem (LTAR) network

Understanding the carbon fluxes and dynamics from a broad range of agricultural systems has the potential to improve our ability to increase carbon sequestration while maintaining crop yields. Short-term, single-location studies have limited applicability, but long-term data from a network of many locations can provide a broader understanding across gradients of climate and management choices. Here we examine eddy covariance measured carbon dioxide (CO2) fluxes from cropland sites across the United States Department of Agriculture's Long-Term Agroecosystem Research (LTAR) network. The dataset was collected between 2001 and 2020, spanning 13 sites for a total of 182 site-years. Average seasonal patterns of net ecosystem CO2 exchange (NEE), gross primary productivity (GPP), and ecosystem respiration (Reco) were determined, and subsequent regression analysis on these “flux climatologies” was used to identify relationships to mean annual temperature (MAT), mean annual precipitation (MAP), cropping systems, and management practices. At rainfed sites, carbon fluxes were better correlated with MAP (r2 ≤ 0.5) than MAT (r2 ≤ 0.22). Net carbon balance was different among cropping systems (p < 0.001), with the greatest net carbon uptake occurring in sugarcane (Saccharum spp. hybrids) and the least in soybean (Glycine max) fields. Crop type had a greater effect on carbon balance than irrigation management at a Nebraska site. Across cropping systems, grain crops often had higher GPP and were more likely to have net uptake when compared to legume crops. This multi-site analysis highlights the potential of the LTAR network to further carbon flux research using eddy covariance measurements.

Effects of rainfall amount and frequency on carbon exchange in a wet meadow ecosystem on the Qinghai-Tibet Plateau

Extreme rainfall amount and frequency can alter carbon dioxide (CO2) exchange between the soil and vegetation in wetlands. Seasonal redistribution of extreme rainfall events is one component of a hydrological regime that affects soil water content (SWC), but its impacts on ecosystem CO2 exchange in the Qinghai-Tibet Plateau (QTP) wetlands remain unclear. Here, we conducted field simulation experiments to understand the effects of extreme rainfall with high to low frequency once every-one, two, three, and four weeks on CO2 exchange of wet meadow ecosystem in QTP. The ecosystem absorbed more CO2 with high-frequency extreme rainfall (−1468.52 mg C m-2h−1) than with current natural amounts and frequency of rainfall (−1162.51 mg C m-2h−1) throughout local vegetation growing seasons. However, under low-frequency extreme rainfall (−916.72 mg C m-2h−1) the wetland absorbed less CO2 than under natural rainfall frequency. This difference comes from the changes in net ecosystem productivity (NEP) and ecosystem respiration (ER) in wet meadows due to soil temperature (ST) and soil nutrient content. In the rapid growth stage of plants, ER and NEP exhibited single-peak curves due to increases in ST and SWC, and they reached their maximum values in August. Our results indicate that the distribution pattern of extreme rainfall events plays a critical role in regulating ecosystem CO2 exchange in the wet meadows of the QTP. More research on the related changes between contents of soil nutrient contents and extreme rainfall events could improve the accuracy of C budget estimation in wet meadows. The predicted regional impacts of future climate extremes of rainfall will rely on changes in their frequency and timing.

Mowing mitigated the sensitivity of ecosystem carbon fluxes responses to heat waves in a Eurasian meadow steppe

The heat waves (HW) will be more frequent and intense in the future with increased human activity and uncertain implications for ecosystem carbon fluxes. The semi-arid Eurasian grassland is sensitive to climate change and under frequent HWs attacks. Mowing as one of the most common human practices in this region, combining with HW can have comprehensive effects on plant communities, biomass, and nutrient cycling. Hence, a 3-year (2019–2021) field manipulation experiment was conducted to assess how mowing influenced the carbon cycling under HWs, and the interactions between HWs and mowing on carbon fluxes at the community and ecosystem levels in a Eurasian meadow steppe. Over the three years, HW significantly reduced net ecosystem CO2 exchange (NEE) and gross ecosystem production (GEP) by 28 % and 8 % (P < 0.05), respectively, whereas ecosystem respiration (Re) did not show significant changes. Moderate mowing (stubble height was set at 6–8 cm) for harvest effectively mitigated ecosystem sensitivity to HWs and significantly increased ecosystem carbon fluxes (NEE, Re, and GEP), biomass and the number of species. Mowing reduced the negative impact of HWs on ecosystem carbon fluxes by about 15 % compared to HWs alone, contributing to the invasion of species such as Thalictrum squarrosum and Vicia amoena, and increased the indirect effect of HW on NEE in the structural equation model. In addition, the higher soil water content (SWC) was another effective way to reduce the impact of HWs. Therefore, mowing and higher SWC would be effective ways to counteract the negative effects of HWs on carbon fluxes in future grassland management.