A plateau freshwater shallow lake as a significant CO2 sink during the ice-covered period: A case study of Lake Wuliangsuhai, China.

Lakes are essential for estimating the global CO2 budget. However, approximately 50 % of lakes undergo periodic freezing, and there is limited research on the factors influencing the CO2 cycle and ice formation in freshwater lakes located in middle- and high-latitude plateaus during ice-covered periods. Using high-frequency meteorological-flux data collected over six consecutive months during the 2018-2019 freezing period of Lake Wuliangsuhai, this study explored the diurnal variation, daily accumulation, and monthly accumulation of the CO2 cycle and its influencing mechanisms at a half-hour scale. The key findings are as follows. Lakes are CO2 sinks during the ice-covered period, with the fluxes being -1.28 ±â€¯4.79 gCm-2d-1. Net CO2 exchange (NEE), gross primary productivity (GPP), and ecosystem respiration (RECO) during the monitoring period were -116.93 gCm-2,190.36 gCm-2, and 86.09 gCm-2, respectively. The lake ice-air CO2 cycle exhibited significant (p < 0.05) diurnal variation, with daytime contributing 92.89 %, 78.31 %, and 56.91 % to NEE, GPP, and RECO, respectively. From December 2018 to March 2019, the monthly total GPP in the whole lake exceeded 10,000 tons, and high autotrophy was observed in February 2019. During the freezing period, 45.22 % of plant assimilation was consumed by autotrophic and heterotrophic respiration. The capacity of the lake CO2 sink was primarily driven by evaporation, latent heat, radiative sensible heat, and air pressure inhibition, whereas snow cover reduced the CO2 sink capacity by 73.01 %. CO2 capture by surface ice was mainly affected by external factors such as snow cover and solar radiation, whereas bottom ice capture was influenced by internal factors such as acid-base balance, the carbonate pump, and biological primary production. Bubble storage and ice crevice transport significantly affected CO2 migration. In summary, further research should focus on elucidating the CO2 capture mechanisms in seasonally frozen lakes located at middle and high latitudes, within subsequent lake­carbon sink studies.

Surface pCO2 and air-water CO2 fluxes dominated by submerged aquatic vegetation: Implications for carbon flux in shallow lakes.

Inland lakes are crucial for processing, storing, and releasing carbon dioxide (CO2), and they play a significant role in the global carbon cycle and climate change. Studies have shown that inland lakes are mostly supersaturated in CO2, making them significant sources to the atmosphere. However, estimating CO2 fluxes from inland lakes is still challenging due to large variations in surface water CO2 partial pressure (pCO2). Submerged aquatic vegetation (SAV) is widely found in aquatic ecosystems, especially in shallow lakes. However, their role in lake-wide carbonate chemistry has not been thoroughly investigated. Accurately measuring air-water CO2 exchange and understanding the environmental factors that control these fluxes in vegetated ecosystems are essential for reducing uncertainties in global CO2 emission estimates. In this study, high-resolution (3-h interval) field measurements were made along the nearshore of eastern Lake Taihu during the SAV growing seasons to examine their effects on surface water pCO2 and air-water exchange. Our results showed evident daily variations in water chemistry and air-water fluxes. Daytime air-water CO2 exchange switched from sinks in summer to sources in autumn. The vegetation sites were observed to be strong CO2 sources consistently at night. The density of aquatic vegetation was found to be positively correlated with the daily range of pCO2, highlighting their role in regulating surface water carbonate chemistry. Negative correlations were found between water depth and surface pCO2. These results highlight the importance of aquatic vegetation and daily variations in reducing uncertainties in carbon budgets of shallow aquatic systems.

Impacts of shellfish and macroalgae mariculture on the seawater carbonate system and air-sea CO2 flux in Haizhou Bay, China.

China is the largest mariculture country, and shellfish and algae output ranks first, showing high carbon sink capacity. In recent years, the single cultivation of macroalgae (Pyropia yezoensis) has been changed to macroalgae-shellfish mariculture in Haizhou Bay to increase the yield of P. yezoensis and improve the water environment quality. In this study, four surveys were carried out in July 2022 during the monoculture period of oyster (Magallana gigas), as well as at different stages of P. yezoensis culture (head-crop period, November 2022, peak growing season, January 2023, and end of harvesting, March 2023) in the mariculture and the surrounding waters of Haizhou Bay. The effects of different stages of culture on the seawater environment and seasonal and spatial variations in the carbonate system were analyzed, and the carbon sink capacity was preliminarily estimated. The results showed that in summer, the calcification of M. gigas and the primary production process of phytoplankton effectively reduced the dissolved inorganic carbon (DIC) level in the culture area. The culture area acts as a CO2 sink, with an average air-sea CO2 flux of -4.5 mmol m-2 d-1. During the polyculture period, the P. yezoensis culture activities maintained the stability of the seawater carbonate system, and the culture area shows strong CO2 sinks, with the average air-sea CO2 flux of -24.10 mmol m-2 d-1, -37.68 mmol m-2 d-1, and -38.99 mmol m-2 d-1, respectively. The absorption of CO2 by large-scale cultured P. yezoensis through the "biological pump" effect is the main factor affecting the CO2 exchange process at the air-sea interface, and the absorption rate of CO2 by P. yezoensis at the mature stage is higher than that at the growth stage before harvesting. The study revealed that macroalgae-shellfish mariculture could promote mutual growth, alleviate environmental pressure, and enhance the carbon sink of the culture area. The relationship between mariculture and the carbon cycle of a mariculture ecosystem is very complicated, and its biochemical process should be given great attention for further study.

Characteristics and regulatory mechanisms of net ecosystem CO2 exchange at the water-air interface in coastal aquaculture ponds.

Coastal aquaculture ponds represented a biogeochemical hotspot in the global carbon cycle. However, there was a limited understanding of their dynamics. In this study, the eddy covariance (EC) technique was applied to quantify the net ecosystem CO2 exchange (NEE) over coastal aquaculture ponds in the Liaohe River estuary in northern China during 2020, aiming to investigate and quantify the carbon exchange characteristics of this region. The results showed that (a) a predominant "U" shaped diurnal NEE pattern throughout the year. During the sea cucumber monoculture phase, the ponds exhibited a consistent daytime carbon sink and nighttime carbon source pattern. In contrast, during the shrimp and sea cucumber polyculture phase, the ponds mostly remained in a net carbon sink state. (b) NEE was negatively correlated with photosynthetically active radiation (PAR), air temperature (Tair), and wind speed (WS), while showing a positive correlation with atmospheric pressure (AP). (c) Overall, the entire study area (complex underlying surfaces) functioned as a carbon sink in 2020, with a total net carbon sequestration of 281.533 g C·m-2. This was approximately four times greater than the restored wetlands that naturally formed from decommissioned coastal aquaculture ponds. Adjusting for surface heterogeneity revealed that the complex surfaces led to a 34.28 % underestimation of the aquaculture region's unit area carbon sequestration capacity. This study was crucial for assessing the carbon cycling and sequestration functions of coastal aquaculture pond ecosystems and provided a scientific basis for related ecological restoration projects.

Distribution and control mechanism of pCO2 and water-air CO2 efflux in the Pearl River Estuary.

Estuaries are generally considered to be important sources of atmospheric CO2. However, the differences between estuaries, and inadequate observations of partial pressure of CO2 in estuarine water (pCO2water) hamper global estuarine CO2 budgeting. In this study, the longitudinal distribution of CO2 in the waters of Modaomen (MSE) and Lingdingyang (LSE), two sub-estuaries of the Pearl River Estuary (PRE), and its influencing mechanism are studied. The change in the distribution of pCO2water along the distance from the upstream estuary to the ocean between LSE and MSE was significantly different. pCO2water at the LSE ranges from 238 to 7267 µatm, whereas the MSE ranges from 406 to 3078 µatm. Stronger microbial respiration and relatively long water retention times were the main influences that led to higher pCO2water at LSE than at MSE. Seasonally, the increase of soil CO2 into the water in the upstream basin caused by precipitation is the potential influencing factor that the water pCO2water in the flood season is higher than in the dry season. PRE was a net source of atmospheric CO2 with an average annual water-air flux of 41.2 ± 33.3 mmol m-2 day-1. Our results suggest that the differences in longitudinal gradients of pCO2water between estuaries in the same region and the effects of different gas transport velocity models on CO2 emission estimates need to be considered in estuarine CO2 emission budgeting.

Hydrology controls sulfuric acid-mediated weathering in an orogenic regime of southwestern Taiwan.

Chemical weathering is a pivotal geochemical process that shapes the carbon cycling and climates in the critical zone. Among its critical drivers, river discharge holds a particular significance, especially in the orogenic landscapes. Here, we examined the impact of discharge on mineral weathering in southwestern (SW) Taiwan by analyzing river water chemistry across a wide discharge range. Current observations indicated that carbonate contributes significantly to total weathering (50-80 %), with sulfuric acid accounting for one-half to two-thirds of carbonate weathering. A statistically strong correlation between river discharge and sulfuric acid-mediated carbonate weathering was highlighted, while the silicate weathering remained constant. This relationship suggests an increased influx of fresh minerals, such as pyrite, into the weathering regime as water flow increases. Our model identifies a critical discharge threshold of 4.6 m3 s-1, determining whether mineral weathering acts as a net source or sink of CO2. Consequently, mineral weathering in SW Taiwan acts as a net CO2 sink during dry periods but turns into a net source during wet periods. Through analyzing a decade of daily discharge data, we found mineral weathering in SW Taiwan is a net CO2 source, with a 2.6-fold increase in annual mean discharge causing a 3.8-fold increase in net CO2 flux. This pattern is likely to be applicable to other similar minerals containing mountain-building regions, highlighting the significant role of hydrology in determining weathering sources and their potential impact on the carbon cycle balance.

Sea surface carbon dioxide during early summer at the Tuandao nearshore time series site.

To investigate air-sea CO2 flux at the Qingdao nearshore site and its temporal variations, a high-resolution continuous observation of surface carbon dioxide partial pressure (pCO2) was carried out at Zhongyuan Pier near Tuandao from May 25 to July 8, 2019. It was observed that during this period, surface pCO2 varied between ∼490 and ∼690 µatm, mainly associated with sea surface temperature. Surface pCO2 also displayed substantial diurnal variations, with an average amplitude of 64 ± 21 µatm, largely dominated by biological activities. During the observational period, this site acted as a source of atmospheric CO2, releasing 361 mmol CO2 m-2. The notable diurnal variations in air-sea CO2 flux, such as the observed average amplitude of 10.9 mmol m-2 d-1 in this study, pose a challenge for accurately estimating the air-sea CO2 flux in coastal regions without high-resolution observations.

Lignite reduces carbon and nitrogen loss from litter in commercial broiler housing.

The loss of carbon and nitrogen from broiler litter limits nutrient recycling and is damaging to the environment. This study investigated lignite, a low-rank brown coal, as an amendment to reduce the loss of carbon and nitrogen from broiler litter over 3 consecutive grow-out cycles, November 2021 to May 2022, at a commercially operated farm in Victoria, Australia. Lignite-treated litter contained significantly more carbon and nitrogen, with an increase of 70.1 g/bird and 12.6 g/bird for carbon and nitrogen, respectively. Lignite also reduced aerobic microbial respiration, with a 46.0% reduction in CO2 flux recorded in week 7 of the study, resulting in reduced mass loss. It is expected that this is a key mechanism responsible for nutrient retention in litter following treatment with lignite. Furthermore, lignite treatment lowered litter moisture content by 7, 6 and 3 percentage points for grow-out 1, 2 and 3, respectively. These findings present lignite as a beneficial litter amendment for increasing the nutrient value of waste and reducing carbon dioxide emissions. The study highlights the potential of lignite to reduce the environmental impact of poultry production and presents an alternative use for lignite as an existing resource.

The shifting pattern of CO2 source sink in a subtropical urbanizing lightly eutrophic lake.

Globally, numerous freshwater lakes exist, and rapid urbanization has impacted carbon biogeochemical cycling at the interface where water meets air in these bodies. However, there is still a limited understanding of CO2 absorption/emission in eutrophic urbanizing lakes. This study therefore involved biweekly in-situ monitoring to evaluate fluctuations in the partial pressure (pCO2) and flux (fCO2) of CO2 and associated parameters from January to September 2020 (7:00-17:00 CST) in an urbanizing lake in southwestern China. Our study revealed that during the daylight hours of the 11 sampling days, both pCO2 and fCO2 consistently demonstrated decreasing trends from the early morning period to the late afternoon period, with notable increases on May 7th and August 15th, respectively. Interestingly, unlike our previous findings, an nonsignificant difference (p > 0.05) in mean pCO2 and fCO2 was observed between the morning period and the afternoon period (n = 22). Furthermore, the mean pCO2 in January (~105 µatm; n = 4) and April (133-212 µatm; n = 8) was below the typical atmospheric CO2 level (C-sink), while that in the other months surpassed 410 µatm (C-source), although the average values (n = 44) of pCO2 and fCO2 were 960 ± 841 µatm and 57 ± 85 mmol m-2 h-1, respectively. Moreover, the pCO2 concentration was significantly greater in summer (May to August, locally reaching 1087 µatm) than in spring (January to April at 112 µatm), indicating a seasonal shift between the C-sink (spring) and the C-source (summer). In addition, a significant positive correlation in pCO2/fCO2 with chlorophyll-a/nitrate but a negative correlation in dissolved oxygen and total phosphorus were recorded, suggesting that photosynthesis and respiration were identified as the main drivers of CO2 absorption/emissions, while changes in nitrate and phosphorus may be attributed to urbanization. Overall, our investigations indicated that this lightly eutrophic lake demonstrated a distinct shifting pattern of CO2 source-sink variability at daily and seasonal scales.

Seasonal CO2 fluxes from alpine river influenced by freeze-thaw in the Qinghai Lake basin, northeastern of the Qinghai-Tibet Plateau.

River CO2 emissions, which contribute 53 % of the basin's overall carbon emissions, are essential parts of the global and regional carbon cycles. Previous CO2 flux calculates are mostly based on single samples collected during ice-free periods; however, little is known about the effects of freeze-thaw cycles on the river CO2 flux (FCO2) of inland rivers in alpine regions. Based on one year-round monthly continuous field sampling, we quantified the FCO2 and determined their driving factors in typical rivers during different freeze-thaw periods in the Qinghai Lake Basin (QLB) using the thin boundary layer model (TBL) and the path analysis method. The findings indicated that (1) the average FCO2 in the typical rivers was 184.98 ± 329.12 mmol/m2/d, acting as a carbon source during different freeze-thaw periods, and showed a decreasing trend with completely thawed periods (CTP, 303.15 ± 376.56 mmol/m2/d) > unstable freezing periods (UFP, 189.44 ± 344.08 mmol/m2/d) > unstable thawing periods (UTP, 62.35 ± 266.71 mmol/m2/d); (2) pH, surface water temperature (Tw) and total alkalinity (TA) were the dominant controlling factors during different freeze-thaw periods. Interestingly, they significantly affected FCO2 more before completely frozen than after frozen, with Tw and TA changing from having promoting effects to having limiting effects; (3) in addition, dissolved carbon components indirectly affected FCO2, primarily through the indirect effects of pH and Tw in the UTP; wind speed (U) directly promoted FCO2 in the CTP; and Ca2+ and dissolved inorganic carbon (DIC) were susceptible to indirect effects, which promoted/limited the release of FCO2 in the UFP, respectively. Our results reveal the changes of FCO2 and the factors influencing it in inland rivers within alpine regions during different freeze-thaw periods, thereby offering valuable support for carbon emission-related studies in alpine regions.

Wet canopy photosynthesis in a temperate Japanese cypress forest.

This study aimed to reveal the mechanism and significance of wet canopy photosynthesis during and after rainfall in temperate coniferous ecosystems by evaluating the influence of abaxial leaf interception on wet canopy photosynthesis. We used the eddy covariance method in conjunction with an enclosed-path gas analyser to conduct continuous ecosystem CO2 flux observations in a Japanese cypress forest within the temperate Asian monsoon area over 3 years. The observation shows that wet-canopy CO2 uptake predominantly occurred during the post-rainfall canopy-wet period rather than the during-rainfall period. Then, the measured canopy-wet net ecosystem exchange was compared with the soil-vegetation-atmosphere transfer multilayer model simulations under different parameter settings of the abaxial (lower) leaf surface wet area ratio. The multilayer model predicted net ecosystem exchange most accurately when it assumed the wet area ratio of the abaxial surface was 50% both during and after rainfall. For the wet canopy both during and after rainfall, the model overestimated CO2 uptake when it assumed no abaxial interception in the simulation, but underestimated CO2 uptake when it assumed that the entire abaxial leaf surface was wet. These results suggest that the abaxial surface of the Japanese cypress leaf is only partly wet to maintain stomatal openness and a low level of photosynthesis. These results allow for an evaluation of the effect of rainfall on forest carbon circulation under a changing climate, facilitating an improvement of ecosystem carbon exchange models.

Emission of CO2 and its related carbonate system dynamics in a hotspot area during winter and summer: The Changjiang River estuary.

The carbonate chemistry in river-dominated marginal seas is highly heterogeneous, and there is ongoing debate regarding the definition of atmospheric CO2 source or sink. On this basis, we investigated the carbonate chemistry and air-sea CO2 fluxes in a hotspot estuarine area: the Changjiang Estuary during winter and summer. The spatial characteristics of the carbonate system were influenced by water mixing of three end-members in winter, including the Changjiang freshwater with low total alkalinity (TA) concentration, the less saline Yellow Sea Surface Water with high TA, and the saline East China Sea (ECS) offshore water with moderate TA. While in summer with increased river discharge, the carbonate system was regulated by simplified two end-member mixing between the Changjiang freshwater and the ECS offshore water. By performing the end-member mixing model on DIC variations in the river plume region, significant biological addition of DIC was found in winter with an estimation of -120 ± 113 µmol kg-1 caused by wintertime organic matter remineralization from terrestrial source. While this biological addition of DIC shifted to DIC removal due to biological production in summer supported by the increased nutrient loading from Changjiang River. The pCO2 dynamics in the river plume and the ECS offshore were both subjected to physical mixing of freshwater and seawater, whether in winter and summer. In the inner estuary without horizontal mixing, the pCO2 dynamics were mainly influenced by biological uptake in winter and temperature in summer. The inner estuary, the river plume, and the ECS offshore were sources of atmospheric CO2, with their contributions varying seasonally. The Changjiang runoff enhanced the inner estuary's role as a CO2 source in summer, while intensive biological uptake reduced the river plume's contribution.

Thermal structure regulates the dynamics of carbon dioxide flux in alpine saline lake on the Qinghai-Tibet Plateau, China.

Thermal stratification and mixing play important roles in the physicochemical composition of lakes and affect the geochemical cycle. However, the regulation of lake carbon exchange at the water-air interface by seasonal thermal structures remains unclear, especially for alpine saline lake on the Qinghai-Tibet Plateau (QTP). Based on continuous field sampling, carbon dioxide flux (FCO2) at the water-air interface in Qinghai Lake during the ice-free period was quantitatively analyzed by thin boundary layer model, as well as the driving factors of the change in FCO2 at the water-air interface. The findings revealed that the FCO2 was -22.16 ± 11.73 mmol m-2d-1 during the stratification period, and - 45.32 ± 29.67 mmol m-2d-1 during the mixing period. We found that thermal stratification limits the matter-energy exchange between the upper and bottom water columns, and carbonate precipitation results in a higher FCO2 than during mixing stage. However, the mixing process reduces the limiting effect of thermal stratification. During the carbonate process, water with higher salinity and pH at the bottom of the water column enters the upper part of the water column, reducing the partial pressure of carbon dioxide (pCO2) in the water column and causing the absorption of CO2 by the lake. Thermal stratification affects the vertical material-energy exchange and atmospheric CO2 uptake of lake. The present study further explains the possible underlying regulation of CO2 uptake in saline lake on the QTP involving the varied thermal structure.

Experimental warming promotes phytoplankton species sorting towards cyanobacterial blooms and leads to potential changes in ecosystem functioning.

A positive feedback loop where climate warming enhances eutrophication and its manifestations (e.g., cyanobacterial blooms) has been recently highlighted, but its consequences for biodiversity and ecosystem functioning are not fully understood. We conducted a highly replicated indoor experiment with a species-rich subtropical freshwater phytoplankton community. The experiment tested the effects of three constant temperature scenarios (17, 20, and 23 °C) under high-nutrient supply conditions on community composition and proxies of ecosystem functioning, namely resource use efficiency (RUE) and CO2 fluxes. After 32 days, warming reduced species richness and promoted different community trajectories leading to a dominance by green algae in the intermediate temperature and by cyanobacteria in the highest temperature treatments. Warming promoted primary production, with a 10-fold increase in the mean biomass of green algae and cyanobacteria. The maximum RUE occurred under the warmest treatment. All treatments showed net CO2 influx, but the magnitude of influx decreased with warming. We experimentally demonstrated direct effects of warming on phytoplankton species sorting, with negative effects on diversity and direct positive effects on cyanobacteria, which could lead to potential changes in ecosystem functioning. Our results suggest potential positive feedback between the phytoplankton blooms and warming, via lower net CO2 sequestration in cyanobacteria-dominated, warmer systems, and add empirical evidence to the need for decreasing the likelihood of cyanobacterial dominance.

High-frequency dynamics of CO2 emission flux and its influencing factors in a subtropical karst groundwater-fed reservoir, south China.

Revealing the magnitude, dynamics, and influencing factors of CO2 emissions across the water-air interface in karst water with high frequency is crucial for accurately assessing the carbon budget in a karst environment. Due to the limitations of observation methods, the current research is still very insufficient. To solve the above problems and clarify the main influencing factors of CO2 emission in karst water, this study selected Dalongdong (DLD) Reservoir, located in the typical karst peak and valley area in southwest China, to carry out a multi-parameter high-frequency monitoring study from January to December 2021, and used the thin boundary model method to estimate the CO2 flux across the water-air interface (CF). The average annual flux of DLD reservoir is 84.48 mmol·(m2·h)-1, which represents a CO2 source overall. However, during the stratification period in August, there is a transient carbon sink due to negative CO2 emission. The alteration of thermal stratification in water is crucial in regulating the seasonal variation of CF. Meanwhile, the diurnal variation is significantly influenced by changes in hydrochemical parameters during the thermal stratification stage. Compared to low wind speeds (<3 m/s), high wind speeds (≥3 m/s) have a greater impact on the CO2 flux. Furthermore, high-frequency continuous data revealed that the reservoir triggered a CO2 pulse emission during the turnover process, primarily at night, leading to unusually high CO2 flux values. It is of great significance to monitor and reveal the process, flux, and control factors of CO2 flux in land water at a high-frequency strategy. They will help improve the accuracy of regional or watershed carbon budgets and clarify the role of global land water in the global carbon budget.

Enhancement of carbon sink in the main marginal sea ice zone by cold season Arctic cyclones.

The Arctic Ocean, as a significant carbon sink, is attracting increased attention within the scientific community. This study focused on the main marginal sea ice zone, which has been the most sensitive to environmental changes in recent decades. Using data from reanalysis, models, and on-site observations, the changes in air-sea CO2 flux (FCO2) were analyzed during the influence of Arctic cyclones (ACs) in 2021-2022. Results indicated that the passage of ACs tended to increase the average carbon sink in the main marginal ice zone, with a more pronounced effect during the cold season. During ACs, the average FCO2 could reach -6.95 mmolC m-2 d-1. This was mainly associated with the stronger and more concentrated distribution of ACs where there was lower pCO2 (air-sea gradient of CO2 partial pressure) in the cold season. Additionally, the change in FCO2 during ACs was primarily affected by the sea surface wind and sea-ice concentration in the cold season, while it was influenced by a variety of environmental factors in the warm season, including the sea surface wind, sea-ice concentration, and ecological factors.

Effects of cultural practices on soil respiration in hazelnut orchards and a comparison with an adjacent natural oak forest.

Soil respiration (RS) is one of the largest terrestrial sources of CO2 causing global warming and may vary according to land use and vegetation type. Türkiye is in the first place in the world in terms of area of hazelnut orchards that are generally converted from natural forests. The aim of this study was the comparison of the effects of cultural practices (pruning, fertilizing, and pruning+fertilizing) on RS in hazelnut orchards and that of the adjacent natural oak forest. Every trial site had a statistically similar annual mean RS, which ranged from 0.15 to 1.55 g C m-2 day-1. The RS on the sites was different only in the spring season and was similar in the other seasons. The RS of the pruned and fertilized hazelnut orchard (Hpf) in the spring was 58% greater than the unmaintained hazelnut orchard (Hc) and oak forest and 28% greater than the only fertilized hazelnut orchard (Hf). The RS of Hpf was also greater than other sites in most monthly measurements. While the positive correlation between soil moisture and RS was on an annual basis (r = 0.44), it was higher in summer (r = 0.61) and autumn (r = 0.55) seasons. The negative correlation between soil temperature and RS in the summer and autumn seasons evolved positively in winter. The results of the study suggest that the maintenance practices applied in the hazelnut garden could increase RS in the spring when soil moisture and temperature are optimal but have no effect in other seasons or on an annual basis.

Temporal variability of air-water gas exchange of carbon dioxide in clam and fish aquaculture ponds.

The growing trend of land-based aquaculture has heightened the significance of comprehensively assessing air-water carbon dioxide (CO2) gas exchange in these inland waters, given their potential impact on carbon neutral strategies. However, temporal variations of partial pressure of CO2 (pCO2) and CO2 flux in clam and fish aquaculture ponds were barely investigated. We assessed the water surface pCO2 in one to five months intervals by deploying a lab-made buoy in three clam ponds and three fishponds located in tropical and subtropical climates. Measurements were conducted over a 24 h period each time, spanning from April 2021 to June 2022, covering the stocking, middle, and harvesting stages of the culture cycle. Diurnal pCO2 variations were dominantly controlled by biologically driven changes in dissolved inorganic carbon and total alkalinity (~97 %), while temperature and salinity effects were minor (~3 %). Clam ponds acted as a sink of atmospheric CO2 during stocking stages and transitioned to a source during middle to harvesting stages. In contrast, fishponds acted as a source of atmospheric CO2 throughout culture cycles and CO2 flux strengthened when reaching harvesting stages. Overall, clam ponds acted as a weak sink for atmospheric CO2 (-2.8 ± 17.3 mmol m-2 d-1), whereas fishponds acted as a source (16.8 ± 21.7 mmol m-2 d-1). CO2 emission was stronger during daytime coinciding with higher windspeeds compared to nighttime in fishponds. We suggest incorporating high temporal resolution measurements to account for diurnal and culture-stage variations, enabling more accurate estimates of air-water CO2 flux in aquaculture ponds. Moreover, the findings of this study highlight the importance of feeding, aeration, and biological activities (photosynthesis, remineralization, and calcification) in controlling the air-water CO2 flux in aquaculture ponds and such information can be used in implementing better strategies to achieve carbon neutral goals.

Ecological water diversion activity changes the fate of carbon in a eutrophic lake.

Lake eutrophication mitigation measures have been implemented by ecological water diversion, however, the responses of carbon cycle to the human-derived hydrologic process still remains unclear. With a famous river-to-lake water diversion activity at eutrophic Lake Taihu, we attempted to fill the knowledge gap with integrative field measurements (2011-2017) of gas carbon (CO2 and CH4) flux, including CO2-equivalent, and dissolved carbon (DOC and DIC) at water-receiving zone and reference zone. Overall, results showed the artificial water diversion activity increased gas carbon emissions. At water-receiving zone, total gas carbon (expressed as CO2-equivalent) emissions increased significantly due to the occurring of water diversion, with CO2 flux increasing from 9.31 ± 16.28 to 18.16 ± 12.96 mmol C m-2 d-1. Meanwhile, CH4 emissions at water-receiving zone (0.06 ± 0.05 mmol C m-2 d-1) was double of that at reference zone. Water diversion decreased DOC but increased DIC especially at inflowing river mouth. Temporal variability of carbon emissions and dissolved carbon were linked to water temperature, chlorophyll a, and nutrient, but less or negligible dependency on these environment variables were found with diversion occurring. Water diversion may increase gas carbon production via stimulating DOC mineralization with nutrient enrichment, which potentially contribute to increasing carbon emissions and decreasing DOC at the same time and the significant correlation between CO2 flux and CH4 flux. Our study provided new insights into carbon biogeochemical processes, which may help to predict carbon fate under hydrologic changes of lakes.

Research ReportDiurnal global ocean surface pCO2 and air-sea CO2 flux reconstructed from spaceborne LiDAR data.

The ocean absorbs a significant amount of carbon dioxide (CO2) from the atmosphere, helping regulate Earth's climate. However, our knowledge of ocean CO2 sink levels remains limited. This research focused on assessing daily changes in ocean CO2 sink levels and air-sea CO2 exchange, using a new technique. We used LiDAR technology, which provides continuous measurements during day and night, to estimate global ocean CO2 absorption over 23 years. Our model successfully reproduced sea surface partial pressure of CO2 data. The results suggest the total amount of CO2 absorbed by oceans is higher at night than during the day. This difference arises from a combination of factors like temperatures, winds, photosynthesis, and respiration. Understanding these daily fluctuations can improve predictions of ocean CO2 uptake. It may also help explain why current carbon budget calculations are not fully balanced-an issue scientists have grappled with. Overall, this pioneering study highlights the value of LiDAR's unique day-night ocean data coverage. The findings advance knowledge of ocean carbon cycles and their role in climate regulation. They underscore the need to incorporate day-night variability when assessing the ocean's carbon sink capacity.