Net Source Of CO2 Research Articles

Spatial variations in CO2 fluxes in a subtropical coastal reservoir of Southeast China were related to urbanization and land-use types

Carbon dioxide (CO2) emissions from aquatic ecosystems are important components of the global carbon cycle, yet the CO2 emissions from coastal reservoirs, especially in developing countries where urbanization and rapid land use change occur, are still poorly understood. In this study, the spatiotemporal variations in CO2 concentrations and fluxes were investigated in Wenwusha Reservoir located in the southeast coast of China. Overall, the mean CO2 concentration and flux across the whole reservoir were 41.85 ± 2.03 µmol/L and 2.87 ± 0.29 mmol/m2/h, respectively, and the reservoir was a consistent net CO2 source over the entire year. The land use types and urbanization levels in the reservoir catchment significantly affected the input of exogenous carbon to water. The mean CO2 flux was much higher from waters adjacent to the urban land (5.05 ± 0.87 mmol/m2/hr) than other land use types. Sites with larger input of exogenous substance via sewage discharge and upstream runoff were often the hotspots of CO2 emission in the reservoir. Our results suggested that urbanization process, agricultural activities, and large input of exogenous carbon could result in large spatial heterogeneity of CO2 emissions and alter the CO2 biogeochemical cycling in coastal reservoirs. Further studies should characterize the diurnal variations, microbial mechanisms, and impact of meteorological conditions on reservoir CO2 emissions to expand our understanding of the carbon cycle in aquatic ecosystems.

Carbon emissions and removals from forests: new estimates, 1990–2020

Abstract. National, regional and global CO2 emissions and removals from forests were estimated for the period 1990–2020 using as input the country reports of the Global Forest Resources Assessment 2020. The new Food and Agriculture Organization of the United Nations (FAO) estimates, based on a simple carbon stock change approach, update published information on net emissions and removals from forests in relation to (a) net forest conversion and (b) forest land. Results show a significant reduction in global emissions from net forest conversion over the study period, from a mean of 4.3 in 1991–2000 to 2.9 Gt CO2 yr−1 in 2016–2020. At the same time, forest land was a significant carbon sink globally but decreased in strength over the study period, from −3.5 to −2.6 Gt CO2 yr−1. Combining net forest conversion with forest land, our estimates indicated that globally forests were a small net source of CO2 to the atmosphere on average during 1990–2020, with mean net emissions of 0.4 Gt CO2 yr−1. The exception was the brief period 2011–2015, when forest land removals counterbalanced emissions from net forest conversion, resulting in a global net sink of −0.7 Gt CO2 yr−1. Importantly, the new estimates allow for the first time in the literature the characterization of forest emissions and removals for the decade just concluded, 2011–2020, showing that in this period the net contribution of forests to the atmosphere was very small, i.e., a sink of less than −0.2 Gt CO2 yr−1 – an estimate not yet reported in the literature. This near-zero balance was nonetheless the result of large global fluxes of opposite sign, namely net forest conversion emissions of 3.1 Gt CO2 yr−1 counterbalanced by net removals on forest land of −3.3 Gt CO2 yr−1. Finally, we compared our estimates with data independently reported by countries to the United Nations Framework on Climate Change, indicating close agreement between FAO and country emissions and removals estimates. Data from this study are openly available via the Zenodo portal (Tubiello, 2020), with DOI https://doi.org/10.5281/zenodo.3941973, as well as in the FAOSTAT (Food and Agriculture Organization Corporate Statistical Database) emissions database (FAO, 2021a).

Sea surface pCO2 variability and air-sea CO2 exchange in the coastal Sudanese Red Sea

The dynamics of sea surface pCO2 (pCO2w) and air–sea CO2 exchange of the Sudanese coastal Red Sea has for the first time been studied over a full annual cycle (October 2014–October​ 2015) based on semi-continuous measurements from moored autonomous sensors. pCO2w showed a seasonal amplitude of approximately 70 μatm, overlaid by a high frequency (3-4 days) signal of around 10 μatm. The highest values, of about 440 μatm occurred during summer and fall, while the lowest values of about 370 μatm occurred during winter. The monthly pCO2w change was primarily driven by temperature, i.e., heating and cooling of the water surface. Additionally, Dissolved Inorganic Carbon (DIC) and Total Alkalinity (AT) contributed significantly to the observed change in pCO2w as a consequence of along-coast advection and upwelling of CO2-rich deep water, and likely biological production, and uptake of atmospheric CO2. The area is a net annual source for atmospheric CO2 of 0.180 ± 0.009 mol CO2 m−2 y−1. Based on a compilation of historic and our new data, altogether covering the years 1977 to 2015, long term trends of pCO2w were determined for the seasons winter–spring (1.75 ± 0.72 μatm y−1) and summer -fall (180 ± 0.41 μatm y−1), both weaker than the atmospheric trend (1.96 ± 0.02 μatm y−1). We are suggesting that the study region has transformed from being a source of CO2 to the atmosphere throughout the year to becoming a sink of CO2 during parts of the year. The long term pCO2w trend was to a large degree driven by increasing DIC, but increasing AT and temperature also played a role.

Simultaneous tree stem and soil greenhouse gas (CO2 , CH4 , N2 O) flux measurements: a novel design for continuous monitoring towards improving flux estimates and temporal resolution.

Tree stems and soils can act as sources and sinks for the greenhouse gases (GHG) carbon dioxide (CO2 ), methane (CH4 ), and nitrous oxide (N2 O). Since both uptake and emission capacities can be large, especially in tropical rainforests, accurate assessments of the magnitudes and temporal variations of stem and soil GHG fluxes are required. We designed a new flexible stem chamber system for continuously measuring GHG fluxes in a French Guianese rainforest. Here, we describe this new system, which is connected to an automated soil GHG flux system, and discuss measurement uncertainty and potential error sources. In line with findings for soil GHG flux estimates, we demonstrated that lengthening the stem chamber closure time was required for accurate estimates of tree stem CH4 and N2 O flux but not tree stem CO2 flux. The instrumented stem was a net source of CO2 and CH4 and a weak sink of N2 O. Our experimental setup operated successfully in situ and provided continuous tree and soil GHG measurements at a high temporal resolution over an 11-month period. This automated system is a major step forward in the measurement of GHG fluxes in stems and the atmosphere concurrently with soil GHG fluxes in tropical forest ecosystems.

Carbon fixation by the phytoplankton community across Lake Winnipeg

Lakes are important sites for carbon fixation and carbon dioxide (CO2) exchange with the atmosphere. Carbon fixation rates have not previously been published for Lake Winnipeg but are important for quantifying the lake’s role in the regional greenhouse gas budget and the lake’s trophic structure and fish habitat. This study measured net ecosystem production (NEP), gross primary production (GPP), and gross respiration (GR) across the lake using a custom-built automated incubator connected to a ship’s water intake during a research cruise between July 31 and August 17, 2018 on Lake Winnipeg. The incubator estimated NEP, GR, and GPP every 60 min while moving along the ship’s track and at anchor, providing high-resolution data that are not obtainable through conventional incubations. The mean NEP for Lake Winnipeg during our survey was −8.4 ± 5.6 g C m−2 d−1, suggesting that the lake was net heterotrophic and thus a net CO2 source to the atmosphere during the 2018 summer cruise. The high-resolution data revealed significant spatiotemporal variability, including short-lived, highly net productive events that preceded remotely sensed chlorophyll a blooms by several days. Conversely, in regions with high chlorophyll a concentrations, we observed strong net heterotrophy and low nutrients, suggesting respiration was fueled by the degradation of mature, nutrient-limited phytoplankton blooms. The incubator system used in this study demonstrated its utility for monitoring rapid changes in NEP over short spatial scales in a lake which shows heightened regional variability in its physical, biogeochemical, and biological make-up.

Global Trends in Air‐Water CO2 Exchange Over Seagrass Meadows Revealed by Atmospheric Eddy Covariance

AbstractCoastal vegetated habitats like seagrass meadows can mitigate anthropogenic carbon emissions by sequestering CO2 as “blue carbon” (BC). Already, some coastal ecosystems are actively managed to enhance BC storage, with associated BC stocks included in national greenhouse gas inventories. However, the extent to which BC burial fluxes are enhanced or counteracted by other carbon fluxes, especially air‐water CO2 flux (FCO2) remains poorly understood. In this study, we synthesized all available direct FCO2 measurements over seagrass meadows made using atmospheric Eddy Covariance, across a globally representative range of ecotypes. Of the four sites with seasonal data coverage, two were net CO2 sources, with average FCO2 equivalent to 44%–115% of the global average BC burial rate. At the remaining sites, net CO2 uptake was 101%–888% of average BC burial. A wavelet coherence analysis demonstrated that FCO2 was most strongly related to physical factors like temperature, wind, and tides. In particular, tidal forcing was a key driver of global‐scale patterns in FCO2, likely due to a combination of lateral carbon exchange, bottom‐driven turbulence, and pore‐water pumping. Lastly, sea‐surface drag coefficients were always greater than the prediction for the open ocean, supporting a universal enhancement of gas‐transfer in shallow coastal waters. Our study points to the need for a more comprehensive approach to BC assessments, considering not only organic carbon storage, but also air‐water CO2 exchange, and its complex biogeochemical and physical drivers.

Ecosystem‐Atmosphere Exchange of CO2, Water, and Energy in a Basin Mangrove of the Northeastern Coast of the Yucatan Peninsula

AbstractCoastal settings variations are linked to composition, structural, and functional differences among mangrove ecotypes. Basin mangroves undergo larger flooding and salinity fluctuations, yet remain understudied, compared to other ecotypes. We evaluated the effect of flooding and air temperature (T a) on the surface energy balance and eddy covariance‐derived net CO2 ecosystem exchange (NEE) of a basin mangrove with sporadic freshwater flooding. During the study period (June 2017–November 2018) the site was more frequently not flooded. Under these conditions, in combination with high T a (>27°C), daytime CO2 uptake was significantly lower, while evapotranspiration and sensible heat flux were higher than when flooded. CO2 uptake increased with T a and vapor pressure deficit, but after exceeding a threshold (29°C and 1.4 kPa), uptake declined. Flooding extended this T a threshold by 3°C and increased the radiation saturation point of NEE. The ecosystem is a net sink of CO2 annually (709 ± 09 g C m−2 yr−1), however, it turned a net source of CO2 for 3 months of prolonged rainfall deficit. Most of the precipitation input is returned to the atmosphere (evaporative index: 0.94) and on average, for each gram of atmospheric carbon assimilated into the ecosystem, 2.21 ± 0.50 kg of water was returned to the atmosphere. This ecosystem‐level water‐use efficiency decreased with flooding, but the correlation was not strong. Future temperature increases and lower precipitation (local and regional), combined with lower water table (and/or stronger saline intrusion), imply important losses of primary productivity and stored soil carbon in basin mangroves of northeast Yucatan Peninsula.

CO2 partial pressure and fluxes in the Amazon River plume using in situ and remote sensing data

Estimations of the global carbon budget include a quantitative understanding of the evolving processes that occur along river-to-ocean gradients. However, high spatiotemporal resolution observations of these processes are limited. Here we present in situ measurements of the partial pressure of CO2 (pCO2) made through the Amazon River plume (ARP) during different discharge seasons, from 2010 to 2012. We evaluated the spatiotemporal distribution of pCO2 using Soil Moisture and Ocean Salinity (SMOS) satellite observations for each hydrologic period in the ARP. Regression models were used to estimate pCO2 at the ARP for the period of 2010–2014. From these distributions we calculated sea-air gas exchange of CO2 between the plume waters and the atmosphere (Fco2sea). Intra-annual variability of Fco2sea was related to discharge at the river mouth and ocean currents as well as trade winds in the plume. Climatic events during the study period had a significant impact on the Fco2sea. Including the plume area closer to the river mouth makes the ARP a net source of CO2 with an annual net sea-air flux of 8.6 ± 7.1 Tg C y−1 from 2011 to 2014.

Variations in diurnal and seasonal net ecosystem carbon dioxide exchange in a semiarid sandy grassland ecosystem in China's Horqin Sandy Land

Abstract. Grasslands are major terrestrial ecosystems in arid and semiarid regions, and they play important roles in the regional carbon dioxide (CO2) balance and cycles. Sandy grasslands are sensitive to climate change, yet the magnitudes, patterns, and environmental controls of their CO2 flows are poorly understood for some regions (e.g., China's Horqin Sandy Land). Here, we report the results from continuous year-round CO2 flux measurements for 5 years from a sandy grassland in China's Horqin Sandy Land. The grassland was a net CO2 source at an annual scale with a mean annual net ecosystem CO2 exchange (NEE) of 49 ± 8 gCm-2yr-1 for the years for which a complete dataset was available (2015, 2016, and 2018). Annual precipitation had the strongest effect on annual NEE; grassland carbon sequestration increased with the increasing precipitation since NEE depended on annual precipitation. In the spring, NEE decreased (i.e., C sequestration increased) with increasing magnitude of effective precipitation pulses, total monthly precipitation, and soil temperature (Tsoil). In the summer, NEE was dominated by the total seasonal precipitation and high precipitation pulses (> 20 mm). In the autumn, NEE increased (i.e., C sequestration decreased) with increasing effective precipitation pulses, Tsoil, and near-surface soil water content (SWC) but decreased with increased SWC deeper in the soil. In the winter, NEE decreased with increasing Tsoil and SWC. The sandy grassland was a net annual CO2 source because drought decreased carbon sequestration by the annual plants. Long-term observations will be necessary to reveal the true source or sink intensity and its response to environmental and biological factors.

The effect of tree mortality on CO2 fluxes in an old-growth spruce forest

Long-term (2009–2019) field studies of the CO2 fluxes over the Valday upland in north-western European Russia were performed in an old-growth spruce-dominated forest subject to the combined effects of climate change, bark beetle attacks and windfall events. The annual carbon uptake within the study area decreased from − 300 g C m−2 yr−1 in 2010–2011 to − 95 g C m−2 yr−1 in 2018 as a result of progressive tree mortality. However, mortality did not cause a significant reduction in specific ecosystem respiration. The respiration of the damaged forest was formed by efflux from soil (64.8%), living vegetation (15.5%), hotspots under dry standing trees (12.1%) and decomposition of woody debris (7.6%). High correlations between net ecosystem exchange and dry standing spruce stocks and average soil CO2 efflux were found. The decrease in the carbon sink was followed by a decline in evapotranspiration from 0.0142 ± 0.0003 g H2O m−2 s−1 for May–October 2010 to 0.0116 ± 0.0002 g H2O m−2 s−1 in 2018. We assumed that the decrease in carbon uptake was due to both the reduction in primary tree production and the decrease in the area of the unaffected stands. Our estimates show that an increase in tree mortality up to 27% of a stand area could turn an old-growth spruce forest into a net source of CO2. This should be taken into account when considering human-induced and climate-related effects on boreal forests.

Seasonal and Interannual Variations of CO2 Fluxes Over 10 Years in an Alpine Wetland on the Qinghai‐Tibetan Plateau

AbstractAlpine wetlands play a sensitive function in global carbon cycle during the ongoing climate warming, yet the temporal patterns of carbon dynamics from in situ ground‐based long‐term observations remain unclear. Here, we analyzed the continuous net ecosystem CO2 exchange (NEE) measured with the eddy covariance technique over an alpine peatland on the northeastern Qinghai‐Tibetan Plateau from 2007 to 2016. The wetland acted as a net CO2 source with a positive NEE (120.4 ± 34.8 gC m−2 year−1, Mean ± SD), with the mean annual gross primary productivity (GPP) of 500.3 ± 59.4 gC m−2 year−1 and annual ecosystem respiration (RES) of 620.7 ± 74.2 gC m−2 year−1. At the seasonal scale, the classification and regression trees (CART) analysis showed that aggregated growing season degree days (GDDs) were the predominant determinant on variations in monthly NEE and monthly GPP. Variations in monthly RES were determined by soil temperature (Ts). Furthermore, nongrowing season Ts had a significant positive correlation with the following year annual GPP (p < 0.05). Nongrowing season RES only accounted for about 25% of annual RES but had significant correlation with annual RES and annual NEE (p < 0.05). The further partial correlation analysis showed that nongrowing season air temperature (Ta, p = 0.05), rather than precipitation (PPT, p = 0.25) was a predominant determinant on variations in annual NEE. Our results highlighted the importance in carbon dynamics of climate fluctuations and CO2 emission from the nongrowing season in alpine wetlands. We speculated that the vast peadlands would positively feedback to climate change on the Tibetan plateau where the nongrowing season warming was significant.

Water temperature control on CO2 flux and evaporation over a subtropical seagrass meadow revealed by atmospheric eddy covariance

AbstractSubtropical seagrass meadows play a major role in the coastal carbon cycle, but the nature of air–water CO2 exchanges over these ecosystems is still poorly understood. The complex physical forcing of air–water exchange in coastal waters challenges our ability to quantify bulk exchanges of CO2 and water (evaporation), emphasizing the need for direct measurements. We describe the first direct measurements of evaporation and CO2 flux over a calcifying seagrass meadow near Bob Allen Keys, Florida. Over the 78‐d study, CO2 emissions were 36% greater during the day than at night, and the site was a net CO2 source to the atmosphere of 0.27 ± 0.17 μmol m−2 s−1 (x̅ ± standard deviation). A quarter (23%) of the diurnal variability in CO2 flux was caused by the effect of changing water temperature on gas solubility. Furthermore, evaporation rates were ~ 10 times greater than precipitation, causing a 14% increase in salinity, a potential precursor of seagrass die‐offs. Evaporation rates were not correlated with solar radiation, but instead with air–water temperature gradient and wind shear. We also confirm the role of convective forcing on night‐time enhancement and day‐time suppression of gas transfer. At this site, temperature trends are regulated by solar heating, combined with shallow water depth and relatively consistent air temperature. Our findings indicate that evaporation and air–water CO2 exchange over shallow, tropical, and subtropical seagrass ecosystems may be fundamentally different than in submerged vegetated environments elsewhere, in part due to the complex physical forcing of coastal air–sea gas transfer.

Peatland protection and restoration are key for climate change mitigation

Peatlands cover only about 3% the global land area, but store about twice as much carbon as global forest biomass. If intact peatlands are drained for agriculture or other human uses, peat oxidation can result in considerable CO2 emissions and other greenhouse gases (GHG) for decades or even centuries. Despite their importance, emissions from degraded peatlands have so far not been included explicitly in mitigation pathways compatible with the Paris Agreement. Such pathways include land-demanding mitigation options like bioenergy or afforestation with substantial consequences for the land system. Therefore, besides GHG emissions owing to the historic conversion of intact peatlands, the increased demand for land in current mitigation pathways could result in drainage of presently intact peatlands, e.g. for bioenergy production. Here, we present the first quantitative model-based projections of future peatland dynamics and associated GHG emissions in the context of a 2 °C mitigation pathway. Our spatially explicit land-use modelling approach with global coverage simultaneously accounts for future food demand, based on population and income projections, and land-based mitigation measures. Without dedicated peatland policy and even in the case of peatland protection, our results indicate that the land system would remain a net source of CO2 throughout the 21st century. This result is in contrast to the outcome of current mitigation pathways, in which the land system turns into a net carbon sink by 2100. However, our results indicate that it is possible to reconcile land use and GHG emissions in mitigation pathways through a peatland protection and restoration policy. According to our results, the land system would turn into a global net carbon sink by 2100, as projected by current mitigation pathways, if about 60% of present-day degraded peatlands would be rewetted in the coming decades, next to the protection of intact peatlands.

A Bornean peat swamp forest is a net source of carbon dioxide to the atmosphere.

Tropical peat forests are a globally important reservoir of carbon, but little is known about CO2 exchange on an annual basis. We measured CO2 exchange between the atmosphere and tropical peat swamp forest in Sarawak, Malaysia using the eddy covariance technique over 4years from 2011 to 2014. The CO2 fluxes varied between seasons and years. A small carbon uptake took place during the rainy season at the beginning of 2011, while a substantial net efflux of >600gC/m2 occurred over a 2month period in the middle of the dry season. Conversely, the peat ecosystem was a source of carbon during both the dry and rainy seasons in subsequent years and more carbon was lost during the rainy season relative to the dry season. Our results demonstrate that the forest was a net source of CO2 to the atmosphere during every year of measurement with annual efflux ranging from 183 to 632gCm-2 year-1 , noting that annual flux values were sensitive to gap filling methodology. This is in contrast to the typical view of tropical peat forests which must have acted as net C sinks over time scales of centuries to millennia to create the peat deposits. Path analyses revealed that the gross primary productivity (GPP) and ecosystem respiration (RE) were primarily affected by vapour pressure deficit (VPD). Results suggest that future increases in VPD could further reduce the C sink strength and result in additional net CO2 losses from this tropical peat swamp forest in the absence of plant acclimation to such changes in atmospheric dryness.

Seasonal and spatial variability in surface pCO2 and air–water CO2 flux in the Chesapeake Bay

AbstractInteractions between riverine inputs, internal cycling, and oceanic exchange result in dynamic variations in the partial pressure of carbon dioxide (pCO2) in large estuaries. Here, we report the first bay‐wide, annual‐scale observations of surface pCO2 and air–water CO2 flux along the main stem of the Chesapeake Bay, revealing large annual variations in pCO2 (43–3408 μatm) and a spatial‐dependence of pCO2 on internal and external drivers. The low salinity upper bay was a net source of CO2 to the atmosphere (31.2 mmol m−2 d−1) supported by inputs of CO2‐rich Susquehanna River water and the respiration of allochthonous organic matter, but part of this region was also characterized by low pCO2 during spring and fall phytoplankton blooms. pCO2 decreased downstream due to CO2 ventilation supported by long water residence times, stratification, mixing with low pCO2 water masses, and carbon removal by biological uptake. The mesohaline middle bay was a net CO2 sink (−5.8 mmol m−2 d−1) and the polyhaline lower bay was nearly in equilibrium with the atmosphere (1.0 mmol m−2 d−1). Although the main stem of the bay was a weak CO2 source (3.7 ± 3.3 × 109 mol C) during the dry hydrologic (calendar) year 2016, our observations showed higher river discharge could decrease CO2 efflux. In contrast to many other estuaries worldwide that are strong sources of CO2 to the atmosphere, the Chesapeake Bay and potentially other large estuaries are very weak CO2 sources in dry years, and could even turn into a CO2 sink in wet years.

Seasonal dynamics of the COS and CO<sub>2</sub> exchange of a managed temperate grassland

Abstract. Gross primary productivity (GPP), the CO2 uptake by means of photosynthesis, cannot be measured directly on the ecosystem scale but has to be inferred from proxies or models. One newly emerged proxy is the trace gas carbonyl sulfide (COS). COS diffuses into plant leaves in a fashion very similar to CO2 but is generally not emitted by plants. Laboratory studies on leaf level gas exchange have shown promising correlations between the leaf relative uptake (LRU) of COS to CO2 under controlled conditions. However, in situ measurements including daily to seasonal environmental changes are required to test the applicability of COS as a tracer for GPP at larger temporal scales. To this end, we conducted concurrent ecosystem-scale CO2 and COS flux measurements above an agriculturally managed temperate mountain grassland. We also determined the magnitude and variability of the soil COS exchange, which can affect the LRU on an ecosystem level. The cutting and removal of the grass at the site had a major influence on the soil flux as well as the total exchange of COS. The grassland acted as a major sink for CO2 and COS during periods of high leaf area. The sink strength decreased after the cuts, and the grassland turned into a net source for CO2 and COS on an ecosystem level. The soil acted as a small sink for COS when the canopy was undisturbed but also turned into a source after the cuts, which we linked to higher incident radiation hitting the soil surface. However, the soil contribution was not large enough to explain the COS emission on an ecosystem level, hinting at an unknown COS source possibly related to dead plant matter degradation. Over the course of the season, we observed a concurrent decrease in CO2 and COS uptake on an ecosystem level. With the exception of the short periods after the cuts, the LRU under high-light conditions was rather stable and indicated a high correlation between the COS flux and GPP across the growing season.

Effects of agricultural activities coupled with karst structures on riverine biogeochemical cycles and environmental quality in the karst region

The karst region in southwestern China is a typical region with fragile ecological environments, and the coordination of regional agricultural development and environmental protection faces enormous challenges. Based on the soil and hydrological characteristics of karst in southwestern China, this study summarized water-related environmental issues caused by agricultural activities in this karst region. Agriculture in the karst hills is more likely to cause soil and nutrient losses due to the fast hydrological flow through special karst structures with high permeability. Thus, this review emphasized the impacts of agricultural development on the riverine biogeochemical cycles of elements based on previous studies. Meanwhile, the carbon cycle is also strongly impacted by agricultural activities in this karst region due to enhanced carbonate weathering by nitric acid from the nitrification of ammonium. This weathering mechanism represents a net source of atmospheric CO2 and might impact regional and global carbon cycling in the Anthropocene. Based on the results summarized in this study, we advocate that in the future, better management of agricultural land, improvement of fertilizer use efficiency, and boosting of nutrient recycling rate should be taken into account for reducing nutrient losses and water quality deterioration. Targeted management of local agricultural practices along with guidance from scientific research results is needed to be devoted to sustainable development of agriculture and economies while protecting water environment.

The CO2 -equivalent balance of freshwater ecosystems is non-linearly related to productivity.

Eutrophication of fresh waters results in increased CO2 uptake by primary production, but at the same time increased emissions of CH4 to the atmosphere. Given the contrasting effects of CO2 uptake and CH4 release, the net effect of eutrophication on the CO2 -equivalent balance of fresh waters is not clear. We measured carbon fluxes (CO2 and CH4 diffusion, CH4 ebullition) and CH4 oxidation in 20 freshwater mesocosms with 10 different nutrient concentrations (total phosphorus range: mesotrophic 39µg/L until hypereutrophic 939µg/L) and planktivorous fish in half of them. We found that the CO2 -equivalent balance had a U-shaped relationship with productivity, up to a threshold in hypereutrophic systems. CO2 -equivalent sinks were confined to a narrow range of net ecosystem production (NEP) between 5 and 19mmolO2 m-3 day-1 . Our findings indicate that eutrophication can shift fresh waters from sources to sinks of CO2 -equivalents due to enhanced CO2 uptake, but continued eutrophication enhances CH4 emission and transforms freshwater ecosystems to net sources of CO2 -equivalents to the atmosphere. Nutrient enrichment but also planktivorous fish presence increased productivity, thereby regulating the resulting CO2 -equivalent balance. Increasing planktivorous fish abundance, often concomitant with eutrophication, will consequently likely affect the CO2 -equivalent balance of fresh waters.

Carbon Dioxide (CO2) Fluxes From Terrestrial and Aquatic Environments in a High‐Altitude Tropical Catchment

AbstractHigh‐altitude tropical grasslands, known as “páramos,” are characterized by high solar radiation, high precipitation, and low temperature. They also exhibit some of the highest ecosystem carbon stocks per unit area on Earth. Recent observations have shown that páramos may be a net source of CO2 to the atmosphere as a result of climate change; however, little is known about the source of this excess CO2 in these mountainous environments or which landscape components contribute the most CO2. We evaluated the spatial and temporal variability of surface CO2 fluxes to the atmosphere from adjacent terrestrial and aquatic environments in a high‐altitude catchment of Ecuador, based on a suite of field measurements performed during the wet season. Our findings revealed the importance of hydrologic dynamics in regulating the magnitude and likely fate of dissolved carbon in the stream. While headwater catchments are known to contribute disproportionately larger amounts of carbon to the atmosphere than their downstream counterparts, our study highlights the spatial heterogeneity of CO2 fluxes within and between aquatic and terrestrial landscape elements in headwater catchments of complex topography. Our findings revealed that CO2 evasion from stream surfaces was up to an order of magnitude greater than soil CO2 efflux from the adjacent terrestrial environment. Stream carbon flux to the atmosphere appeared to be transport limited (i.e., controlled by flow characteristics, turbulent flow, and water velocity) in the upper reaches of the stream, and source limited (i.e., controlled by CO2 and carbon availability) in the lower reaches of the stream. A 4‐m waterfall along the channel accounted for up to 35% of the total evasion observed along a 250‐m stream reach. These findings represent a first step in understanding ecosystem carbon cycling at the interface of terrestrial and aquatic ecosystems in high‐altitude, tropical, headwater catchments.

Eutrophication Drives Extreme Seasonal CO2 Flux in Lake Ecosystems

Lakes process a disproportionately large fraction of carbon relative to their size and spatial extent, representing an important component of the global carbon cycle. Alterations of ecosystem function via eutrophication change the balance of greenhouse gas flux in these systems. Without eutrophication, lakes are net sources of CO2 to the atmosphere, but in eutrophic lakes this function may be amplified or reversed due to cycling of abundant autochthonous carbon. Using a combination of high-frequency and discrete sensor measurements, we calculated continuous CO2 flux during the ice-free season in 15 eutrophic lakes. We found net CO2 influx over our sampling period in 5 lakes (− 47 to − 1865 mmol m−2) and net efflux in 10 lakes (328 to 11,755 mmol m−2). Across sites, predictive models indicated that the highest efflux rates were driven by nitrogen enrichment, and influx was best predicted by chlorophyll a concentration. Regardless of whether CO2 flux was positive or negative, stable isotope analyses indicated that the dissolved inorganic carbon pool was not derived from heterotrophic degradation of terrestrial organic carbon, but from degradation of autochthonous organic carbon, mineral dissolution, and atmospheric uptake. Optical characterization of dissolved organic matter revealed an autochthonous organic matter pool. CO2 influx was correlated with autochthony, while efflux was correlated with total nitrogen and watershed wetland cover. Our findings suggest that CO2 uptake by primary producers during blooms can contribute to continuous CO2 influx for days to months. Conversely, eutrophic lakes in our study that were net sources of CO2 to the atmosphere showed among the highest rates reported in the literature. These findings suggest that anthropogenic eutrophication has substantially altered biogeochemical processing of carbon on Earth.