Carbon Budget Estimates Research Articles

CO2 variability over a tropical coastal station in India: Synergy of observation and model

Carbon dioxide (CO2), being the prime greenhouse gas, has largest contribution in radiative forcing and global warming due to increased anthropogenic emissions leading to regional and global climate change. We performed CO2 measurements using a continuous monitoring instrument at a tropical coastal station at the southern tip of India (Thumba; 8.54° N, 76.86° E) during 2017–2021. Combining measurements with model simulations and satellite observations, present study aims to understand CO2 variabilities, contribution of various sources and role of dynamics. The observed trend of 2.19 ppm y−1 is well simulated (2.47 ppm y−1) by the chemistry-transport model (MIROC4-ACTM), which is contributed by trends in fossil fuel (4.59 ppm y−1), biosphere (−1.46 ppm y−1) and ocean (−0.66 ppm y−1) tracers of CO2 flux. Mean seasonal cycle with amplitude of about 8 ppm is observed, with a maximum during premonsoon (April) and minimum at the end of the monsoon season (September). The model reproduced the observed seasonal cycle well (r2 = 0.89) and showed that the seasonal peak is predominantly contributed by the land biospheric flux seasonality. Biospheric sink during postmonsoon is counterbalanced by emissions from fossil fuel burning. Biosphere and fossil fuel are major contributors in shaping the observed seasonal pattern with negligible seasonality in oceanic tracer due to its uniform flux patterns in the Indian Ocean. Mean diurnal pattern of CO2 exhibits lower values during daytime (cleaner marine airmass) and higher values during nighttime (continental airmass) with deeper diurnal amplitude in the winter (~50 ppm) than in the summer (~31 ppm). Seasonal pattern in total column CO2 from OCO-2 (Orbiting Carbon Observatory-2) is similar to the surface observations but shows nearly half of the observed seasonal amplitude. Long-term continuous measurements over distinct environments are desirable for further deepening the understanding of CO2 variability, estimation of regional carbon budget and for climate mitigation policies.

Crystalline iron oxide mineral (magnetite) accelerates methane production from petroleum hydrocarbon biodegradation

Methane (CH4) emissions are a factor in climate change; in addition, CH4 production may affect reclamation of fluid fine tailings (FFT) in tailings ponds, and end-pit lakes (EPLs). In laboratory cultures, we investigated the effect of crystalline iron mineral (magnetite) on CH4 production from the biodegradation of hydrocarbons added to FFT collected from methanogenically more and less active sites in a demonstration EPL. Magnetite enhanced CH4 production from both sites, having a greater effect in more active FFT, where it increased the CH4 production rate as much as 48% (from 6.67 μmol d−1 to 9.87 μmol d−1) compared to FFT without magnetite. Correspondingly, magnetite hastened biodegradation of hydrocarbons (monoaromatics, n-alkanes and iso-alkanes), with a pronounced effect on o-xylene, ethylbenzene, m/p-xylenes, n-octane, n-nonane, and 2-methyloctane, where biodegradation rates increased by 46, 117, 11, 45, 28 and 37%, respectively, compared to FFT without magnetite. Little FeII was produced, suggesting that magnetite is not being used as an electron acceptor but rather functions as a conduit for electron transfer. Thus, magnetite may be a suitable amendment to enhance bioremediation of anaerobic environments contaminated with hydrocarbons. Importantly, our observations imply that magnetite may increase CH4 emissions from terrestrial ecosystems, thus affecting carbon budget estimations.

Impact of the horizontal resolution of GEOS-Chem on land‒ocean and tropic‒extratropic partitioning and seasonal cycle in CO2 inversion

Abstract Credible regional carbon budget estimates from atmospheric CO2 measurements rely on the accuracy of atmospheric transport models (ATMs). However, the atmospheric transport in ATMs is usually simplified and spatiotemporally averaged, leading to systematic biases in simulating the atmospheric CO2 and estimating surface CO2 fluxes. We show that forward simulations of global CO2 using an ATM, GEOS-Chem, at a native resolution of 0.5°×0.625° and a coarse resolution of 4°×5° differ significantly near the surface in the Northern Hemisphere and the polar vortex, mainly because of advection in GEOS-Chem. Comparing observing system simulation experiments that assimilate synthetic observations sampled from the forward simulations, we separate the impact of coarse-resolution GEOS-Chem on regional flux estimates. The results suggest that a significant amount of annual carbon uptake from the ocean and tropics is improperly redistributed to the land and northern and southern extratropics, respectively. In addition, these errors lead to an underestimated seasonal amplitude in the northern extratropical land and a reversed sign of the seasonal cycle in the northern extratropical ocean. The reversed sign of the seasonal cycle has also been observed in a real data assimilation experiment and several state-of-the-art inversions, suggesting that reasonable ocean flux estimates depend strongly on the accuracy of the ATM.

Temporal Variations in Methane Emissions from a Restored Mangrove Ecosystem in Southern China

The role of coastal mangrove wetlands in sequestering atmospheric carbon dioxide has been increasingly investigated in recent years. While studies have shown that mangroves are weak sources of methane (CH4) emissions, measurements of CH4 fluxes from these ecosystems remain scarce. In this study, we examined the temporal variation and biophysical drivers of ecosystem-scale CH4 fluxes in China’s northernmost mangrove ecosystem based on eddy covariance measurements obtained over a 3-year period. In this mangrove, the annual CH4 emissions ranged from 6.15 to 9.07 g C m−2 year−1. The daily CH4 flux reached a peak of over 0.07 g C m−2 day−1 during the summer, while the winter CH4 flux was negligible. Latent heat, soil temperature, photosynthetically active radiation, and tide water level were the primary factors controlling CH4 emissions. This study not only elucidates the mechanisms influencing CH4 emissions from mangroves, strengthening the understanding of these processes but also provides a valuable benchmark dataset to validate the model-derived carbon budget estimates for these ecosystems.

Spatiotemporal heterogeneity of lake carbon dioxide flux leads to substantial uncertainties in regional upscaling estimates

Limited field samplings result in significant uncertainties in regional and global estimates of lake carbon dioxide (CO2) emissions. However, quantitative analysis of uncertainty in regional lake CO2 emission estimates remains unclear. In this study, we utilized satellite data to estimate carbon dioxide flux from 113 eastern China lakes, revealing substantial spatial and temporal variations in flux, averaging 18.07 ± 81.83 mg m−2 d−1. Additionally, satellite-estimated total CO2 effluxes indicated previous upscaling studies had overestimated total CO2 effluxes from these studied lakes by approximately 3–11 times, primarily due to substantial variations in lake CO2 fluxes. Insufficient sampling resolution resulted in considerable uncertainty in upscaling estimations. Temporal variations in carbon dioxide contributed greater upscaling uncertainties than spatial variations in carbon dioxide. To capture the dynamics of lake CO2, increasing the number of sampling points and events is necessary as lake size decreases and trophic state increases. Finally, we propose a prediction for the optimal sampling resolution based on lake area and trophic state, recommending an average of 5 points per lake and bi-monthly sampling as the ideal resolution for similar shallow eutrophic lakes. This approach has been validated as effective in lakes across North America and Europe. We believe that future global-scale lake carbon budget estimates would benefit from field observations conducted at more reasonable sampling points and frequency.

Soil respiration and organic carbon changes along a chronosequence of Pinus nigra forest stands

Understanding the trajectory of changes in soil respiration (Rs) and soil organic carbon (SOC) with stand ages of the black pine (Pinus nigra Arnold) forest is essential for forest management and carbon budget estimates. In this research, changes of Rs and SOC were studied with respect to stand age in a chronosequence of three age classes of P. nigra plantations consisting of young (0 to 10-year-olds), middle-aged (11- to 20-year-olds), and pre-mature (35- to 45-year-olds) forest stands. Rs rates, soil temperature, and soil moisture were measured using an automated dynamic survey chamber (Li-8100A) for a year, encompassing summer, fall, winter, and spring seasons. Mean Rs significantly increased from young- to middle-aged and then stabilized, with effluxes ranging from 2.46 to 2.94 µmol CO2 m-2 s-1. Forest litter significantly increased with stand age, but not the SOC in the mineral soil layers. The Rs showed a positive correlation with soil temperature (0.77) and air temperature (0.75) but not with soil moisture (-0.43). The present results highlight the importance of stand age in assessing carbon budget and provide essential information for forest managers and stakeholders in evaluating the potential of P. nigra forests as tools for carbon sequestration and mitigating global warming impacts.

More Frequent Spaceborne Sampling of XCO2 Improves Detectability of Carbon Cycle Seasonal Transitions in Arctic‐Boreal Ecosystems

AbstractSurface, aircraft, and satellite measurements indicate pervasive early cold season (Augut–September) CO2 emissions across Arctic regions, consistent with increased ecosystem metabolism in plants and soils. A key remaining question is whether cold season sources will become large enough to permanently shift the Arctic into a net carbon source. Polar orbiting GHG satellites provide robust estimation of regional carbon budgets but lack sufficient spatial coverage and repeat frequency to track sink‐to‐source transitions in the early cold season. Mission concepts such as the Arctic Observing Mission (AOM) advocate for flying imaging spectrometers in highly elliptical orbits (HEO) over the Arctic to address sampling limitations. We perform retrieval and flux inversion simulation experiments using the AURORA mission concept, leveraging a Panchromatic imaging Fourier Transform Spectrometer (PanFTS) in HEO. Our simulations demonstrate the potential benefits of increased CO2 sampling for detecting emissions during the early cold season.

Potential of using CO2 observations over India in a regional carbon budget estimation by improving the modelling system

Abstract. Devising effective national-level climate action plans requires a more detailed understanding of the regional distribution of sources and sinks of greenhouse gases. Due to insufficient observations and modelling capabilities, India's current carbon source–sink estimates are uncertain. This study uses a high-resolution Lagrangian transport model to examine the potential of available CO2 observations over India for inverse estimation of regional carbon fluxes. We use four different sites in India that vary in the measurement technique, frequency and spatial representation. These observations exhibit substantial seasonal (7.5 to 9.2 ppm) and intra-seasonal (2 to 12 ppm) variability. Our modelling framework, a high-resolution Weather Research and Forecasting Model combined with the Stochastic Time-Inverted Lagrangian Transport model (WRF–STILT), performs better in simulating seasonal (R2=0.50 to 0.96) and diurnal (R2=0.96) variability (for the Mohali station) of observed CO2 than the current-generation global models (CarboScope, CarbonTracker and ECMWF EGG4). The seasonal CO2 concentration variability in Mohali, associated with crop residue burning, is largely underestimated by the models. WRF–STILT captures the seasonal biospheric variability over Nainital better than the global models but underestimates the strength of the CO2 uptake by crops. The choice of emission inventory in the modelling framework alone leads to significant biases in simulations (5 to 10 ppm), endorsing the need for accounting for emission fluxes, especially for non-background sites. Our study highlights the possibility of using the CO2 observations from these Indian stations for deducing carbon flux information at regional (Nainital) and suburban to urban (Mohali, Shadnagar and Nagpur) scales with the help of a high-resolution model. On accounting for observed variability in CO2, the global carbon data assimilation system can benefit from the measurements from the Indian subcontinent.

Increased harvested carbon of cropland in China

Crop harvested carbon (HC) is one of the most important components of the carbon cycle in cropland ecosystems, with a significant impact on the carbon budget of croplands. China is one of the most important crop producers, however, it is still unknown on the spatial and temporal variations of HC. This study collected statistical data on crop production at the province and county levels in China for all ten crop types from 1981 to 2020 and analyzed the magnitude and long-term trend of harvested crop carbon. Our results found a substantial increase of HC in cropland from 0.185 Gt C yr−1 in 1981 to 0.423 Gt C yr−1 in 2020 at a rate of 0.006 Gt C yr−1. The results also highlighted that the average annual carbon sink removal from crop harvesting in China from 1981 to 2020 was 0.32 Gt C yr−1, which was comparable to the net carbon sink of the entire terrestrial ecosystems in China. This study further generated a gridded dataset of HC from 2001 to 2019 in China by using jointly the statistical crop production and distribution maps of cropland. In addition, a model-data comparison was carried out using the dataset and results from seven state-of-the-art terrestrial ecosystem models, revealing substantial disparities in HC simulations in China compared to the dataset generated in the study. This study emphasized the increased importance of HC for estimating cropland carbon budget, and the produced dataset is expected to contribute to carbon budget estimation for cropland ecosystems and the entire China.

Greenhouse gases emissions and carbon budget estimation in horizontal subsurface flow constructed wetlands with different plant species

Constructed wetlands (CWs) are pivotal for wastewater treatment due to their high efficiency and numerous advantages. The impact of plant species and diversity on greenhouse gas (GHG) emissions from CWs requires a more comprehensive evaluation. Moreover, controversial perspectives persist about whether CWs function as carbon sinks or sources. In this study, horizontal subsurface flow (HSSF) CWs vegetated with Cyperus alternifolius, Typhae latifolia, Acorus calamus, and the mixture of these three species were constructed to evaluate pollutant removal efficiencies and GHG emissions, and estimate carbon budgets. Polyculture CWs can stably remove COD (86.79 %), NH4+-N (97.41 %), NO3−-N (98.55 %), and TP (98.48 %). They also mitigated global warming potential (GWP) by suppressing N2O emissions compared with monoculture CWs. The highest abundance of the Pseudogulbenkiania genus, crucial for denitrification, was observed in polyculture CWs, indicating that denitrification dominated in nitrogen removal. While the highest nosZ copy numbers were observed in CWs vegetated with Cyperus alternifolius, suggesting its facilitation of denitrification-related microbes. Selecting Cyperus alternifolius to increase species diversity is proposed for simultaneously maintaining the water purification capacity and reducing GHG emissions. Carbon budget estimations revealed that all four types of HSSF CWs were carbon sinks after six months of operation, with carbon accumulation capacity of 4.90 ± 1.50 (Cyperus alternifolius), 3.31 ± 2.01 (Typhae latifola), 1.78 ± 1.30 (Acorus calamus), and 2.12 ± 0.88 (polyculture) kg C/m2/yr. This study implies that under these operation conditions, CWs function as carbon sinks rather than sources, aligning with carbon peak and neutrality objectives and presenting significant potential for carbon reduction efforts.

Geomorphological changes and sediment carbon accumulation at the bare mudflat-saltmarsh interface: The role of typhoons

Blue carbon stored in coastal wetlands can be highly impacted by extreme events, such as typhoons. However, the sediment carbon accumulation capacity in this circumstance, remains unclear. In this study, a field site crossing the bare mudflat-saltmarsh interface was established in Southern Hangzhou Bay, to investigate the geomorphological and subsequent sediment carbon accumulation changes during the typhoon season (2 months) and the succeeding calm weather period (10 months). Surface Elevation Tables (SETs) were used, together with GPS-RTK and UAV-LiDAR surveys, to observe surface elevation changes at different spatial scales. The elevation and sediment organic carbon density data were used to estimate the gain/loss of sediment carbon pool under the impact of two successive typhoons, “In-Fa” and “Chanthu”. This revealed that during the typhoon season, 6.3 cm of surface erosion occurred on the bare mudflat whilst rapid depositions of 6.1 and 12.3 cm occurred in Scirpus mariqueter and Spartina alterniflora habitats, respectively. In the gap between the two typhoon events, the bare mudflat experienced deposition of 4.2 cm, signalling recovery from erosion, but deposition within the saltmarsh front slowed. During the calm weather period, sediment deposition increased landward, with magnitudes of 13.1 cm, 16.4 cm, and 18.2 cm. Sediment carbon budget estimates indicated that during the typhoon season, the bare mudflat lost ~3.4 Mg C ha−1 but saltmarshes gained ~4.9 Mg C ha−1 in their sediment organic carbon pools. These changes in sediment carbon accumulation rates over the typhoon season accounted for >30 % of annual change in the study area, and may be associated with tidal creek dynamics. This study suggests the important role of typhoons in mediating the sediment carbon pool across the mudflat-saltmarsh interface. With future climate change, increasingly frequent catastrophic events could cause considerable changes in coastal blue carbon sequestration capacity.

Substantial reductions in non-CO2 greenhouse gas emissions reductions implied by IPCC estimates of the remaining carbon budget

Carbon budgets are quantifications of the total amount of carbon dioxide that can ever be emitted while keeping global warming below specific temperature limits. However, estimates of these budgets for limiting warming to 1.5 °C and well-below 2 °C include assumptions about how much warming can be expected from non-CO2 emissions. Here, we uncover the non-CO2 emissions assumptions that underlie the latest remaining carbon budget estimates by the Intergovernmental Panel on Climate Change and quantify the implication of the world pursuing alternative higher or lower emissions. We consider contributions of methane, nitrous oxide, fluorinated gases, and aerosols and show how pursuing inadequate methane emission reductions causes remaining carbon budgets compatible with the Paris Agreement temperature limits to be exhausted today, effectively putting achievement of the Paris Agreement out of reach.

A constraint on historic growth in global photosynthesis due to rising CO2

Theory predicts that rising CO2 increases global photosynthesis, a process known as CO2 fertilization, and that this is responsible for much of the current terrestrial carbon sink. The estimated magnitude of the historic CO2 fertilization, however, differs by an order of magnitude between long-term proxies, remote sensing-based estimates and terrestrial biosphere models. Here we constrain the likely historic effect of CO2 on global photosynthesis by combining terrestrial biosphere models, ecological optimality theory, remote sensing approaches and an emergent constraint based on global carbon budget estimates. Our analysis suggests that CO2 fertilization increased global annual terrestrial photosynthesis by 13.5 ± 3.5% or 15.9 ± 2.9 PgC (mean ± s.d.) between 1981 and 2020. Our results help resolve conflicting estimates of the historic sensitivity of global terrestrial photosynthesis to CO2 and highlight the large impact anthropogenic emissions have had on ecosystems worldwide.

Ikaite formation in streams affected by steel waste leachate: First report and potential impact on contaminant dynamics

Highly alkaline (pH 9–12) waters can arise from a range of globally significant and environmentally impactful industrial processes such as lime, steel and cement production, alumina refining and energy generation (e.g. combustion ashes). Such residue storage sites are often characterized by extreme geochemical conditions that can be hazardous to aquatic life but are quickly becoming a critical focus for resource recovery and carbon capture initiatives. The very high rates of mineral precipitation at these sites can give rise to the formation of transient minerals that are not currently well understood. As such our estimates of carbon budgets and understanding of trace metal dynamics at highly alkaline sites is currently limited. This study provides a significant advancement in the basis for characterising hyperalkaline carbonate systems through identification and chemical analysis of transient minerals forming in sites receiving high pH (>11) steel slag leachate in northern England. Whilst most of the secondary deposits at the study sites appear to be dominated by calcite, this study provides the first account of ikaite (CaCO3.6H2O) crystallization within steel-slag leachate, using novel field (Fourier Transform Infra-Red) supported by rapid laboratory (X-Ray Diffraction) validation. This study suggests that ikaite is a secondary mineral with a primary phase being amorphous calcium carbonate (ACC). Trace element analysis of ikaite forming in these steel-slag leachate affected waters is demonstrates its strong affinity to incorporate relatively large inventories of potentially harmful metals (e.g. lead and cadmium). Importantly, ikaite is only stable at low temperatures (−4 to 8 °C) and thus is of significant concern given its potential to release hazardous pulses of contamination during warming events in the spring. The findings provide an improved understanding of carbonate precipitation processes at highly alkaline sites which in turn should influence future research endeavours around mineral carbonation, trace metal dynamics and environmental remediation at these sites globally.

A global synthesis and conceptualization of the magnitude and duration of soil carbon losses in response to forest disturbances.

Forest disturbances are increasing around the globe due to changes in climate and management, deteriorating forests' carbon sink strength. Estimates of global forest carbon budgets account for losses of plant biomass but often neglect the effects of disturbances on soil organic carbon (SOC). Here, we aimed to quantify and conceptualize SOC losses in response to different disturbance agents on a global scale. Global. 1983-2022. Forest soils. We conducted a comprehensive global analysis of the effects of harvesting, wildfires, windstorms and insect infestations on forest SOC stocks in the surface organic layer and top mineral soil, synthesizing 927 paired observations from 151 existing field studies worldwide. We further used global mapping to assess potential SOC losses upon disturbance. We found that both natural and anthropogenic forest disturbances can cause large SOC losses up to 60 Mg ha-1. On average, the largest SOC losses were found after wildfires, followed by disturbances from windstorms, harvests and insects. However, initial carbon stock size, rather than disturbance agent, had the strongest influence on the magnitude of SOC losses. SOC losses were greatest in cold-climate forests (boreal and mountainous regions) with large accumulations of organic matter on or near the soil surface. Negative effects are present for at least four decades post-disturbance. In contrast, forests with small initial SOC stocks experienced quantitatively lower carbon losses, and their stocks returned to pre-disturbance levels more quickly. Our results indicate that the more carbon is stored in the forest's organic layers and top mineral soils, the more carbon will be lost after disturbance. Robust estimates of forest carbon budgets must therefore consider disturbance-induced SOC losses, which strongly depend on site-specific stocks. Particularly in cold-climate forests, these disturbance-related losses may challenge forest management efforts to sequester CO2.

Carbon budget response to climate change varies with grassland type in Qilian Mountains, China

Carbon budget estimation is crucial for scientifically assessing the ecosystem’s carbon sequestration capacity. Grasslands in the Qilian Mountains are widely recognized as vulnerable and sensitive to ongoing climate change. However, it is still unclear how the carbon budget of different grassland types responds to climate change in this area. Here, we evaluated the net primary productivity (NPP), soil heterotrophic respiration (RH), and net ecosystem productivity (NEP) of grassland ecosystems in the Qilian Mountains. Correlation analysis, linear regression, and standardized coefficient methodologies were employed to explore the response of NPP, RH, and NEP in various grassland types to changes in temperature, precipitation, and solar radiation over the past two decades. The results showed that the alpine meadow has the highest NPP and NEP while the temperate typical steppe has the highest RH. Grassland NPP, RH, and NEP increased significantly from 2001 to 2020, with a distinct spatial pattern of being low in the northwest and high in the southeast of the Qilian Mountains. NPP and NEP correlated with precipitation and solar radiation more closely than with temperature. Precipitation and solar radiation, respectively, explained 43.7% and 52.9% of grassland NEP alterations, and there was a threshold effect on the response of NEP and NPP to climate factors. Specifically, the NPP and NEP increased at first and then decreased as the precipitation, temperature, and solar radiation increased. The highest NEP and NPP occurred when precipitation ranged from 415 to 455 mm, along with temperatures between − 0.1–1.5 ℃ and solar radiation reaching 7685–7905MJ/m2. The warming and humidification of climate is conducive to the increase of NPP and NEP in alpine meadows and alpine steppes, but when the precipitation exceeds 415 mm and the temperature is greater than 1.5 ℃, the NPP and NEP begin to decrease. Solar radiation dominated changes in NPP and NEP of alpine meadows, alpine steppes, and temperate desert steppes. With the increase of solar radiation, NPP and NEP of these three grasslands first increased and then decreased. However, the changes in NPP and NEP of the temperate typical steppe were most affected by precipitation, and NPP and NEP first increase and then decrease with the increase of precipitation The higher the temperature, the higher the NPP, RH, and NEP of the temperate typical steppe. This study helps better understand how climate change impacts the carbon budget components of different grassland types in the Qilian Mountains and provides more theoretical thoughts on managing local grasslands sustainably.

Unknown bacterial species lead to soil CO2 emission reduction by promoting lactic fermentation in alpine meadow on the Qinghai-Tibetan Plateau

Highly variable soil microbial respiration among grasslands has been identified as a major cause of uncertainty in regional carbon (C) budget estimation in the Qinghai-Tibetan Plateau; microbial metabolism mechanisms might explain this variation, but remain elusive. Therefore, we investigated soil CO2 production in incubated soils and detected the associated functional genes at four sampling sites from two major alpine grasslands on the Qinghai-Tibetan Plateau. The results showed that the cumulative CO2 emissions from alpine meadow soils were 71 %–83 % lower than those from alpine steppe soils. Both the enriched genes abundance encoding fermentation and glycolysis (Embden-Meyerhof pathway (EMP)) and the diminished genes encoding tricarboxylic acid cycle (TCA) and phosphate pentose pathway (PPP) explained the CO2 emission reduction in the alpine meadow soils. The EMP: PPP and fermentation: TCA cycle ratios in alpine meadow soils were 1.45- and 1.50-fold higher than those in alpine steppe soils, respectively. Such shifts in metabolic pathways were primarily caused by the increasing dominance of an unknown species of Desulfobacteraceae with high glycolytic potential, carrying a higher abundance of ldh genes during fermentation. These unknown species were promoted by warmer temperatures and higher precipitation in the alpine meadows. Further studies on the unknown species would enhance our understanding and predictability of C cycling in alpine grasslands.

Contribution of water erosion to organic carbon and total nitrogen loads in agricultural discharge from boreal mineral soils

While organic carbon (OC) in agricultural mineral soils is widely studied in terms of soil carbon sequestration and gaseous emissions, discharge-induced OC loss from soil is still poorly understood and estimations of boreal soil OC loads within water erosion are lacking. Loss of organic matter from arable soils is a concern for surface water quality, climate change and soil productivity. The main aim of this study was to quantify the role of water erosion in total OC and nitrogen (N) loads exported in agricultural discharge from boreal mineral soils under various cultivation practices. Surface water and subsurface drainage were collected near-continually over 2 years in two clayey and one sandy soil in Finland. Eroded sediment was mechanically separated by centrifugation from all discharge samples to detect sediment OC% and N% by dry-combustion method. Dissolved OC and N concentrations in selected discharge samples were measured with high-temperature catalytic oxidation of unfiltered supernatant. A multiple linear regression model was used to study the significant factors affecting dissolved, sediment and total OC loads. In the clayey soils, the sediment OC (2–24 kg ha−1 y−1) and N (0.2–1.1 kg ha−1 y−1) export accounted for up to 35 % and 20 % of the annual discharge-induced total loads of OC (19–85 kg ha−1) and N (2–8 kg ha−1), respectively. In the sandy soil, erosion was negligible and dissolved loads of 17–35 kg OC ha−1 y−1and 4–7 kg N ha−1 y−1 were detected. Subsurface drainage exported most of the sediment-associated OC and N loads from clayey soils. For the total OC loads, the distribution varied between the discharge routes, while the total N loads were mostly exported in subsurface drainage in both soil types. Sediment OC and N exports were related to soil plowing and discharge intensity, while dissolved OC loss was promoted by high surface soil OC%. Our results also indicated that a single cultivation practice may affect sediment and dissolved loads in opposite ways. These findings can be used to complement carbon budget estimations for mineral agricultural soils, and to assess soil management effects on terrestrial organic matter loading to boreal surface waters.

Carbon dioxide reduction by photosynthesis undetectable even during phytoplankton blooms in two lakes

Lakes located in the boreal region are generally supersaturated with carbon dioxide (CO2), which emerges from inflowing inorganic carbon from the surrounding watershed and from mineralization of allochthonous organic carbon. While these CO2 sources gained a lot of attention, processes that reduce the amount of CO2 have been less studied. We therefore examined the CO2 reduction capacity during times of phytoplankton blooms. We investigated partial pressure of CO2 (pCO2) in two lakes at times of blooms dominated by the cyanobacterium Gloeotrichia echinulata (Erken, Sweden) or by the nuisance alga Gonyostomum semen (Erssjön, Sweden) during two years. Our results showed that pCO2 and phytoplankton densities remained unrelated in the two lakes even during blooms. We suggest that physical factors, such as wind-induced water column mixing and import of inorganic carbon via inflowing waters suppressed the phytoplankton signal on pCO2. These results advance our understanding of carbon cycling in lakes and highlight the importance of detailed lake studies for more precise estimates of local, regional and global carbon budgets.

Seasonal and interannual variability of the pelagic ecosystem and of the organic carbon budget in the Rhodes Gyre (eastern Mediterranean): influence of winter mixing

Abstract. The Rhodes Gyre is a cyclonic persistent feature of the general circulation of the Levantine Basin in the eastern Mediterranean Sea. Although it is located in the most oligotrophic basin of the Mediterranean Sea, it is a relatively high primary production area due to strong winter nutrient supply associated with the formation of Levantine Intermediate Water. In this study, a 3D coupled hydrodynamic–biogeochemical model (SYMPHONIE/Eco3M-S) was used to characterize the seasonal and interannual variability of the Rhodes Gyre's ecosystem and to estimate an annual organic carbon budget over the 2013–2020 period. Comparisons of model outputs with satellite data and compiled in situ data from cruises and Biogeochemical-Argo floats revealed the ability of the model to reconstruct the main seasonal and spatial biogeochemical dynamics of the Levantine Basin. The model results indicated that during the winter mixing period, phytoplankton first progressively grow sustained by nutrient supply. Then, short episodes of convection driven by heat loss and wind events, favoring nutrient injections, organic carbon export, and inducing light limitation on primary production, alternate with short episodes of phytoplankton growth. The estimate of the annual organic carbon budget indicated that the Rhodes Gyre is an autotrophic area, with a positive net community production in the upper layer (0–150 m) amounting to 31.2 ± 6.9 gCm-2yr-1. Net community production in the upper layer is almost balanced over the 7-year period by physical transfers, (1) via downward export (16.8 ± 6.2 gCm-2yr-1) and (2) through lateral transport towards the surrounding regions (14.1 ± 2.1 gCm-2yr-1). The intermediate layer (150–400 m) also appears to be a source of organic carbon for the surrounding Levantine Sea (7.5 ± 2.8 gCm-2yr-1) mostly through the subduction of Levantine Intermediate Water following winter mixing. The Rhodes Gyre shows high interannual variability with enhanced primary production, net community production, and exports during years marked by intense heat losses and deep mixed layers. However, annual primary production appears to be only partially driven by winter vertical mixing. Based on our results, we can speculate that future increase of temperature and stratification could strongly impact the carbon fluxes in this region.