CO2 Dynamics Research Articles

Differential effects of wetting and drying on soil CO2 concentration and flux in near-surface vs. deep soil layers

Soil stores over 2500 Pg carbon (C), with the majority of C stored in deep soil layers (> 30 cm). Soil C can be lost to the atmosphere when organic compounds are mineralized to carbon dioxide (CO2, via oxidative decay or respiration) and moved upward through the soil profile (via diffusion). Soil moisture status can influence the balance between respiration and diffusion, thereby altering the soil CO2 concentration and flux. However, it is unclear how wetting and drying influence soil CO2 dynamics in surface vs. deep soil layers. Thus, we irrigated three soil profiles in Mediterranean arable land and continuously monitored soil CO2 concentration at 15, 30, 50, 70 and 90 cm during wetting and drying phases under ambient temperature conditions. We estimated gas diffusivity, CO2 flux, and temperature responses of soil CO2 concentration during the experiment. Decreases in gas diffusivity during the wetting period coincided with increases in soil CO2 concentrations. However, the negative gas diffusivity-soil CO2 concentration relationship did not hold true all the time, implying that CO2 production was the driving factor for the apparent soil CO2 concentration. We observed hysteretic responses of soil CO2 concentration to temperature as soil moisture varied, with deeper soil CO2 concentration being more sensitive to temperature than surface soil CO2 concentration, especially during the drying phase. The movement of CO2 was upward at all depths during the ambient phase, but the direction and the magnitude of CO2 fluxes varied across the depth gradient during the wetting and drying phases. This study highlights that the relative contribution of gas diffusivity vs. CO2 production to soil CO2 concentration changes with wetting and drying, and that the responses of soil CO2 concentration to temperature are dependent on the antecedent environmental conditions. Also, the downward movement of CO2 during the wetting and drying phases suggests that quantifying surface soil CO2 efflux may underestimate dynamic C processes in deeper soils.

Environmental investments decreased partial pressure of CO2 in a small eutrophic urban lake: Evidence from long-term measurements

Inland waters emit large amounts of carbon dioxide (CO2) to the atmosphere, but emissions from urban lakes are poorly understood. This study investigated seasonal and interannual variations in the partial pressure of CO2 (pCO2) and CO2 flux from Lake Wuli, a small eutrophic urban lake in the heart of the Yangtze River Delta, China, based on a long-term (2000–2015) dataset. The results showed that the annual mean pCO2 was 1030 ± 281 μatm (mean ± standard deviation) with a mean CO2 flux of 1.1 ± 0.6 g m−2 d−1 during 2000–2015, suggesting that compared with other lakes globally, Lake Wuli was a significant source of atmospheric CO2. Substantial interannual variability was observed, and the annual pCO2 exhibited a decreasing trend due to improvements in water quality driven by environmental investment. Changes in ammonia nitrogen and total phosphorus concentrations together explained 90% of the observed interannual variability in pCO2 (R2 = 0.90, p < 0.01). The lake was dominated by cyanobacterial blooms and showed nonseasonal variation in pCO2. This finding was different from those of other eutrophic lakes with seasonal variation in pCO2, mostly because the uptake of CO2 by algal-derived primary production was counterbalanced by the production of CO2 by algal-derived organic carbon decomposition. Our results suggested that anthropogenic activities strongly affect lake CO2 dynamics and that environmental investments, such as ecological restoration and reducing nutrient discharge, can significantly reduce CO2 emissions from inland lakes. This study provides valuable information on the reduction in carbon emissions from artificially controlled eutrophic lakes and an assessment of the impact of inland water on the global carbon cycle.

Effects of phytoplankton community composition and productivity on sea surface pCO2 variations in the Southern Ocean

The Southern Ocean is a vast net sink for atmospheric carbon dioxide (CO2), with marine phytoplankton playing a crucial role in CO2 fixation. We assessed how changes in the dominant phytoplankton community and net primary productivity (NPP) affected variations in the partial pressure of CO2 in surface water (pCO2sw) in the Indian sector of the Southern Ocean during austral summer. pCO2sw was negatively correlated with total phytoplankton and diatom abundances, as estimated from pigment signatures, in the zone south of the Antarctic Circumpolar Current; however, pCO2sw was not correlated with haptophyte abundance. Additionally, a stronger correlation was found between pCO2sw and total phytoplankton NPP than between chlorophyll a concentration and pCO2sw. We reconstructed pCO2sw at inter-annual scale using satellite data and assessed the inter-annual variability of air-sea CO2 flux. Over the period from 1997 to 2007, the integrated CO2 fluxes over the study region showed very large variations from a small source to a strong sink. Variations in the integrated CO2 fluxes were also correlated with changes in satellite-derived phytoplankton community in the Indian sector of the Southern Ocean and changes in the dominant phytoplankton community may control CO2 dynamics in the marginal ice zone.

Use of Geospatial, Hydrologic, and Geochemical Modeling to Determine the Influence of Wetland-Derived Organic Matter in Coastal Waters in Response to Extreme Weather Events

Flooding from extreme weather events (EWE), such as hurricanes, exports large amounts of dissolved organic matter (DOM) to both estuaries and coastal waters globally. Hydrologic connectivity of wetlands to adjacent river channels during flood events is potentially a major control on the DOM exported to coastal waters after EWEs. In this study, a geographic information system based flood model was used to: 1) determine the volume of flooded wetlands in a river corridor following Hurricane Matthew in 2016; 2) compute the resulting volume fluxes of DOM to the Neuse River Estuary-Pamlico Sound (NRE-PS), in eastern North Carolina and 3) use the flood model to quantify the wetland contribution to DOM export. The flood model-derived contributions were validated with a Bayesian Monte Carlo mixing model combining measurements of DOM quality: specific UV Absorbance at 254 nm (SUVA254), spectral slope ratio (SR), and stable isotope ratios of dissolved organic carbon (13C-DOC). Results indicated that 1) hydrologic connectivity of the freshwater riparian wetlands caused the wetlands to become the primary source of organic matter (OM) that was exported into the NRE-PS after Matthew and 2) this source lingered in these coastal waters in the months after the storm. Thus, in consideration of the pulse-shunt concept, EWE such as Hurricane Matthew cause pulses of DOM from wetlands, which were the primary source of the OM shunted from the terrestrial environment to the estuary and sound. Wetlands constituted ca. 48% of the annual loading of DOC into the NRE and 16% of DOC loading into the PS. Results were consistent with prior studies in this system, and other coastal ecosystems, that attributed a high reactivity of DOM as the underlying reason for large CO2 releases following EWE. Adapting pulse-shunt concept to estuaries requires the addition of a “processing” step to account for the DOM to CO2 dynamics, thus a new pulse-shunt process is proposed to incorporate coastal waters. Our results suggest that with increasing frequency and intensity of EWE, strengthening of the lateral transfer of DOM from land to ocean will occur and has the potential to greatly impact coastal carbon cycling.

Spatial variations in CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; fluxes in the Saguenay Fjord (Quebec, Canada) and results of a water mixing model

Abstract. The Saguenay Fjord is a major tributary of the St. Lawrence Estuary and is strongly stratified. A 6–8 m wedge of brackish water typically overlies up to 270 m of seawater. Relative to the St. Lawrence River, the surface waters of the Saguenay Fjord are less alkaline and host higher dissolved organic carbon (DOC) concentrations. In view of the latter, surface waters of the fjord are expected to be a net source of CO2 to the atmosphere, as they partly originate from the flushing of organic-rich soil porewaters. Nonetheless, the CO2 dynamics in the fjord are modulated with the rising tide by the intrusion, at the surface, of brackish water from the Upper St. Lawrence Estuary, as well as an overflow of mixed seawater over the shallow sill from the Lower St. Lawrence Estuary. Using geochemical and isotopic tracers, in combination with an optimization multiparameter algorithm (OMP), we determined the relative contribution of known source waters to the water column in the Saguenay Fjord, including waters that originate from the Lower St. Lawrence Estuary and replenish the fjord's deep basins. These results, when included in a conservative mixing model and compared to field measurements, serve to identify the dominant factors, other than physical mixing, such as biological activity (photosynthesis, respiration) and gas exchange at the air–water interface, that impact the water properties (e.g., pH, pCO2) of the fjord. Results indicate that the fjord's surface waters are a net source of CO2 to the atmosphere during periods of high freshwater discharge (e.g., spring freshet), whereas they serve as a net sink of atmospheric CO2 when their practical salinity exceeds ∼5–10.

Effect of elevated CO2 on litter functional traits, mass loss and nutrient release of two subtropical species in free air carbon enrichment facility

Assessing the impact of increasing carbon dioxide (CO2) on nutrient cycling requires understanding of the relationship between the concentration of CO2 and litter dynamics. It is, therefore necessary to understand leaf litter traits in response to increased atmospheric CO2. Variations in plant litter chemistry under elevated carbon dioxide (e−CO2) are the critical factors in ecosystem feedback. Litter fall, litter substrate quality, nutrient flux and decay rate in two tropical tree of Tectona grandis (Teak) and Butea monosperma (Butea) were examined. Saplings were exposed to elevated CO2 treatment (e−CO2 -550 ppm) and ambient CO2 (a−CO2; - 395 ppm) in the FACE (Free air CO2 enrichment) facility in CSIR- National Botanical Research Institute in Indo-Gangetic plain region. Litter was analysed for concentrations of carbon (C), nitrogen (N), soluble sugars (SS), lipids, lignin, cellulose, hemi-cellulose (HC) and C-based defensive compounds; soluble phenolics (SP). Nutrient concentration in naturally senesced litter for Butea and Teak was significantly different in elevated and ambient rings. Elevated CO2 significantly increased C/N (+8.27 % and +21.94 %), lowered mean litter N concentration (–3.27 % and -12.15 %), and affected the concentrations of soluble sugars, soluble phenolic and lipids in leaf litter of Butea and Teak plantations, respectively. Increase in litter biomass production (+19.75 % and 22.84 %) for both the plants under e−CO2, resulted in significant increase in the flux of N, SS, SP and lipid in to the soil. Elevated CO2 increased cellulose (+18.21 %; 12.30 %), hemicellulose (+8.48 %; +10.18 %) and lignin inputs to soils (+35.22; +43.36 %) in Butea and Teak plantation respectively. The study indicated that changes in litter biomass production and litter substrate quality during exposure to e−CO2 could significantly alter the input of nitrogen, soluble phenolic, soluble sugars, lipids, cellulose, and lignin to soils, and so also the biogeochemical cycle of the prevailing ecosystem.

Spatial and temporal dynamics of pCO2 and CO2 flux in tropical Lake Malawi

AbstractNumerous studies have documented CO2 dynamics in temperate lakes, but few such studies have been conducted on tropical lakes. Spatial and seasonal variation of air and water pCO2, along with supporting limnological and meteorological variables, were measured aboard a vessel of opportunity along the north–south axis of Lake Malawi. These measurements were used to estimate annual net lake‐atmosphere CO2 flux and infer mechanisms regulating it. Surface pCO2 and CO2 flux varied significantly with season and location. Temporally, the lake was CO2‐undersaturated during the rainy season and the mixing season, and supersaturated at the onset of the mixing season and during the hot, stratified season. Concurrent measurements of lake temperature, weather conditions, phytoplankton biomass, and seston δ13C suggest that periods of net CO2 flux into the lake correspond with higher phytoplankton growth rates resulting from internal nutrient loading in the mixing season and allochthonous nutrient inputs in the rainy season. Unlike the rest of the lake, the southernmost region of the lake was usually CO2 supersaturated even though phytoplankton productivity is highest in this region. While the upwelling of hypolimnetic water at the southern end of the lake is a major source of nutrients that drive phytoplankton photosynthesis and CO2 uptake, the CO2 introduced in upwelled water appears to overwhelm photosynthetic capacity locally, especially at the onset of the mixing season. Lake Malawi appears to be a net CO2 sink with an annual whole‐lake CO2 flux of −2.17 ± 0.25 × 1010 mol C yr−1 and a mean daily CO2 flux of −2.05 ± 0.27 mmol C m−2 d−1. A comparison of deep‐water C : P ratios with epilimnetic seston C : P ratios suggests that P is recycled more efficiently than C in the lake's anoxic hypolimnion, and so P vertical mixing creates a carbon deficit that is met by flux from the atmosphere into the lake.

Carbon and nitrogen recycling from microbial necromass to cope with C:N stoichiometric imbalance by priming

The impact of increasing amounts of labile C input on priming effects (PE) on soil organic matter (SOM) mineralization remains unclear, particularly under anoxic conditions and under high C input common in microbial hotspots. PE and their mechanisms were investigated by a 60-day incubation of three flooded paddy soils amended with13C-labeled glucose equivalent to 50–500% of microbial biomass C (MBC). PE (14–55% of unamended soil) peaked at moderate glucose addition rates (i.e., 50–300% of MBC). Glucose addition above 300% of MBC suppressed SOM mineralization but intensified microbial N acquisition, which contradicted the common PE mechanism of accelerating SOM decomposition for N-supply (frequently termed as “N mining”). Particularly at glucose input rate higher than 3 g kg−1 (i.e., 300–500% of MBC), mineral N content dropped on day 2 close to zero (1.1–2.5 mg N kg−1) because of microbial N immobilization. To cope with the N limitation, microorganisms greatly increased N-acetyl glucosaminidase and leucine aminopeptidase activities, while SOM decomposition decreased. Several discrete peaks of glucose-derived CO2 (contributing >80% to total CO2) were observed between days 13–30 under high glucose input (300–500% of MBC), concurrently with CH4 peaks. Such CO2 dynamics was distinct from the common exponential decay pattern, implicating the recycling and mineralization of 13C-enriched microbial necromass driven by glucose addition. Therefore, N recycling from necromass was hypothesized as a major mechanism to alleviate microbial N deficiency without SOM priming under excess labile C input. Compound-specific 13C-PLFA confirmed the redistribution of glucose-derived C among microbial groups, i.e., necromass recycling. Following glucose input, more than 4/5 of total 13C-PLFA was in the gram-negative and some non-specific bacteria, suggesting these microorganisms as r-strategists capable of rapidly utilizing the most labile C. However, their 13C-PLFA content decreased by 70% after 60 days, probably as a result of death of these r-strategists. On the contrary, the 13C-PLFA in gram-positive bacteria, actinomycetes and fungi (K-strategists) was initially minimal but increased by 0.5–5 folds between days 2 and 60. Consequently, the necromass of dead r-strategists provided a high-quality C–N source to the K-strategists. We conclude that under severe C excess, N recycling from necromass is a much more efficient microbial strategy to cover the acute N demand than N acquisition from the recalcitrant SOM.

Diversity and temperature indirectly reduce CO2 concentrations in experimental freshwater communities

Biodiversity loss and climate warming are occurring in concert, with potentially profound impacts on ecosystem functioning. We currently know very little about the combined effects of these changes on the links between the community structure, dynamics and the resulting in situ CO2 concentrations in freshwater ecosystems. Here we aimed to determine both individual and combined effects of temperature and non-resource diversity (species inedible for a given consumer) on CO2 concentration. Our analysis further aimed to establish both direct effects on CO2 concentrations and potential indirect effects that occur via changes to the phytoplankton and zooplankton biomasses. Our results showed that there were no interactive effects of changes in temperature and diversity on CO2 concentration in the water. Instead, independent increases in either temperature or non-resource diversity resulted in a substantial reduction in CO2 concentrations, particularly at the highest non-resource diversity. The effects of non-resource diversity and warming on CO2 were indirect, resulting largely from the positive impacts on total biomass of primary producers. Our study is the first to experimentally partition the impacts of temperature and diversity on the consumer–resource dynamics and associated changes to CO2 concentrations. It provides new mechanistic insights into the role of diverse plankton communities for ecosystem functioning and their importance in regulating CO2 dynamics under ongoing climate warming.

Limiting CO&lt;SUB align="right"&gt;2 fleet emissions in the automotive industry - a portfolio planning approach

In this article, a novel portfolio planning approach as a means to limit CO2 fleet emissions in the automotive industry is presented. This research is motivated by several factors challenging product portfolio planning in times of structural changes. Besides ever stricter CO2 legislation, dynamics and uncertainties related to the transformation towards electric mobility need to be considered when deciding on the future range of vehicles to be offered on the market. These portfolio decisions are of particular importance due to long life cycles, long development times and high product-specific investments of vehicle projects. This approach supports the definition of vehicle projects and cycle plans on an aggregation level in line with industry-specific planning requirements and allows for the consideration of temporal dynamics and uncertainties to handle structural changes. To this end, illustrative cycle plans are generated and evaluated based on financial and environmental criteria considering different scenarios of market development.

Effect of nutrient alteration on pCO2(water) and chlorophyll-a dynamics in a tropical aquaculture pond situated within a Ramsar site: a microcosm approach.

Tropical sewage-fed aquaculture ponds of East Kolkata Wetlands (EKW), a Ramsar site, act as deposition center of organic matter and nutrient-rich sewage from the Kolkata metropolis, which in turn is utilized as fish feed in aquaculture ponds. Increasing nutrient load due to multifarious anthropogenic activities usually alters the chlorophyll-a (chl-a) and the partial pressure of carbon dioxide in water [pCO2(water)] dynamics in such aquatic systems. In this regard, the effect of nutrient level alteration [8 times and 24 times, dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) in addition, respectively] on chl-a and pCO2(water) was tested upon waters of EKW aquaculture ponds in three different seasons by means of microcosm for 8 consecutive days. In both DIN 8× and DIP 8× treatments, an overall increase in chl-a concentrations was observed by the end of experiment; however, in DIN 24× and DIP 24× treatments, chl-a in most of the seasons was found to diminish after a steep rise in its concentration during 4th or 5th day. In all the seasons, the surface waters were supersaturated with CO2 under in situ conditions. The extent of supersaturation enhanced with nutrient addition during monsoon; however, in post-monsoon and pre-monsoon, nutrient addition led to undersaturation of CO2. Though chl-a concentration exhibited substantial variability during the microcosm experiments, the variation in trophic state index was not that much remarkable. In DIN-treated microcosms, uptake rate of DIP was significantly high compared to the reverse scenario, which implied that DIP has a more limiting character than DIN. In P-enriched treatments, the lack of N supplies probably limit production of autotrophic biomass. Thus it can be concluded that the nature of sewage and its DIN/DIP content can significantly alter the primary productivity the CO2 dynamics of such aquaculture ponds in future.

Vacuum ultraviolet photodissociation dynamics of CO2 near 133 nm: The spin-forbidden O(3Pj=2,1,0) + CO(X1Σ+) channel.

Understanding vacuum ultraviolet (VUV) photodissociation dynamics of CO2 is of considerable importance in the study of atmospheric chemistry and planetary chemistry. Yet, photodissociation dynamics of the spin-forbidden O(3Pj=2,1,0) + CO(X1Σ+) channel has not been clearly understood so far. Here, we study the O(3Pj) + CO(X1Σ+) dissociation processes in the VUV photodissociation of CO2 at the photolysis wavelengths between 129.02 and 134.67 nm by using the time-sliced velocity-mapped ion imaging technique. From the vibrational-resolved images of the O(3Pj=2,1,0) photofragment, the total kinetic energy releases, the CO(X1Σ+) cofragment vibrational state distributions, and the product angular distributions have been derived, respectively. The experimental observations show that the total kinetic energy releases for the three 3Pj spin-orbit states (j = 2, 1, 0) exhibit a broad CO(X1Σ+) vibrational energy distribution with significant inverted characteristics, especially at short photoexcitation wavelengths, indicating that the VUV photodissociation could take place in a relatively linear geometry of the triplet state, with one C-O bond extended and the other compressed. Furthermore, a notable photolysis wavelength dependent feature has also been found in the product angular distributions of all three spin-orbit channels (j = 2, 1, 0): Only the vibrational-state specific anisotropy parameter β values at 130.18 nm behave more anisotropic, while all those at other photolysis wavelengths are near the value β = 0.5 for O(3Pj=2,1) channels or β = 0.25 for the O(3Pj=0) channel, with small fluctuations. This anomalous phenomenon suggests that the different nonadiabatic interactions, such as singlet-triplet coupling, may play a key role in the formation of O(3Pj=2,1,0) + CO(X1Σ+) products, with strong photolysis wavelength dependence.

Molecular Refraction and Nonlinear Refractive Index of Supercritical Carbon Dioxide under Clustering Conditions

Abstract—A model of a clustered binary medium that describes the anomalous behavior of the linear (Lorentz–Lorentz function) and nonlinear (nonlinear refractive index) optical properties of supercritical CO2 is proposed. The validity of this model describing the anomalous behavior of the nonlinear refractive index of supercritical CO2 in the vicinity of the Widom line is verified using a numerical simulation of the molecular dynamics of CO2.

Dynamics and potential drivers of CO2 concentration and evasion across temporal scales in high-alpine streams

Carbon dioxide (CO2) evasion from streams greatly contributes to global carbon fluxes. Despite this, the temporal dynamics of CO2 and its drivers remain poorly understood to date. This is particularly true for high-altitude streams. Using high-resolution time series of CO2 concentration and specific discharge from sensors in twelve streams in the Swiss Alps, we studied over three years the responsiveness of both CO2 concentration and evasion fluxes to specific discharge at annual scales and at the scale of the spring freshet. On an annual basis, our results show dilution responses of the streamwater CO2 likely attributable to limited supply from sources within the catchment. Combining our sensor data with stable isotope analyses, we identify the spring freshet as a window where source limitation of the CO2 evasion fluxes becomes relieved. CO2 from soil respiration enters the streams during the freshet thereby facilitating CO2 evasion fluxes that are potentially relevant for the carbon fluxes at catchment scale. Our study highlights the need for long-term measurements of CO2 concentrations and fluxes to better understand and predict the role of streams for global carbon cycling.

Dynamics of atmospheric carbon dioxide over different land cover types in India.

This study presents an analysis of high-resolution space borne retrievals of the column-averaged dry-air mole fraction of carbon dioxide [Formula: see text] and the role of vegetation in controlling atmospheric CO2 dynamics over the Indian region. Nadir and glint mode [Formula: see text] retrievals from the Orbiting Carbon Observatory-2 (OCO-2) spectrometer for the period September 2014-July 2017 are studied with satellite-derived normalized difference vegetation index (NDVI) and rainfall over different land cover types. The atmospheric [Formula: see text] variability shows a strong negative correlation with satellite-derived NDVI. Higher rainfall favours the vegetative growth and photosynthetic activity, thus lowers atmospheric [Formula: see text] concentration. The mean monthly [Formula: see text] over terrestrial region of India is observed as 400.18 ± 3.85 ppm with seasonal variations over different land cover types. The correlation of [Formula: see text] with NDVI over mixed forest and deciduous broadleaf forest types was estimated as - 0.86, - 0.76 (p < 0.01), respectively. The study could also highlight strong seasonal [Formula: see text] variability at higher latitudinal zones of India. It is significant to note that space-based observations indicate that atmospheric carbon dioxide levels have surpassed a significant milestone of 400 ppm in recent times. The study provides significant inputs towards improving our understanding of terrestrial biogeochemical carbon cycle over India.

Decomposition of mixtures of cover crop residues increases microbial functional diversity

To improve sustainability in agricultural systems, winter cover crops are increasingly replacing fallow to stimulate soil functions that reduce nutrient losses and greenhouse gas production, reduce pests for the next cash crops, increase soil organic matter pools and reduce erosion. Several of these functions are highly dependent on soil microbes decomposing cover crop residues. Since cover crop species differ in their traits it is hypothesized that plant species residue mixtures with complementary characteristics perform better by stimulating soil microbial functional diversity. To test this, residues of cover crop monocultures and mixtures were mixed with agricultural soil in a microcosm experiment, and fungal and bacterial biomass, microbial metabolic potential, greenhouse gas emissions and soil nutrients were measured during 50 days. Fungal biomass increased for all treatments compared to the control (no additions). However, there were no significant differences between cover crop mixtures and monocultures. Biolog ECO plates were used as a proxy for the metabolic potential of the microbial community. The number of substrates used was significantly higher in soil amended with residues of cover crop mixtures indicating an increased number of substrate niches for microbes. C:N ratio of cover crop residues was shown to be an important variable in explaining dynamics of CO2 and N2O emissions. Mixtures of cover crops showed reduced N2O and CO2 emissions compared to monocultures at the start of the experiment, but did not reduce greenhouse gas emissions over the whole incubation period. Adding nitrogen to the cover crop treatment with the highest C:N ratio (oat) did increase N2O emissions, but not CO2 emissions suggesting that decomposition rate of oat residues is not limited by nitrogen availability. Overall, mixtures of cover crops stimulated microbial functional diversity in soil incubations. Although this may have positive implications for soil quality and functioning in agricultural fields, further studies are needed to verify if these results hold under field conditions.

Eutrophic Lake Taihu as a significant CO2 source during 2000–2015

Inland lakes receive growing attentions on eutrophication and their roles in global carbon cycle. However, understanding how inland lakes contribute to global carbon cycle is seriously hampered due to a shortage of long-term records. This study investigated the carbon dioxide (CO2) flux from the Lake Taihu, a large (2400 km2) and shallow (mean depth 1.9 m) eutrophic lake in subtropical region, based on a long-term (2000–2015) measurement of the partial pressure of carbon dioxide (pCO2) at high spatiotemporal resolution. We found that the Lake Taihu was a significant source of atmospheric CO2 with an average CO2 emission flux at 18.2 ± 8.4 mmol m−2 d−1 (mean±1standard deviation) and a mean annual pCO2 value of 778 ± 169 μatm. The highest pCO2 and CO2 flux were observed in eutrophic zone with a high external input of carbon and nutrient, and the lowest in non-eutrophic zones with no direct external input of nutrient and carbon. A substantial seasonal pattern in pCO2 was observed, particularly in eutrophic pelagic area, and was significantly negatively correlated with chlorophyll a. Long-term measurement showed the interannual variation in annual lake CO2 dynamics, which was highly sensitive to human-induced nutrient input. Watershed input of carbon and nutrient leads to the high CO2 level, counterbalancing the in-lake primary production. All lines of evidence suggest that human activities may have predominate contribution to CO2 source in the Lake Taihu, and this mechanism might be widespread in global freshwater lakes.

Increased high‐latitude photosynthetic carbon gain offset by respiration carbon loss during an anomalous warm winter to spring transition

Arctic and boreal ecosystems play an important role in the global carbon (C) budget, and whether they act as a future net C sink or source depends on climate and environmental change. Here, we used complementary in situ measurements, model simulations, and satellite observations to investigate the net carbon dioxide (CO2 ) seasonal cycle and its climatic and environmental controls across Alaska and northwestern Canada during the anomalously warm winter to spring conditions of 2015 and 2016 (relative to 2010-2014). In the warm spring, we found that photosynthesis was enhanced more than respiration, leading to greater CO2 uptake. However, photosynthetic enhancement from spring warming was partially offset by greater ecosystem respiration during the preceding anomalously warm winter, resulting in nearly neutral effects on the annual net CO2 balance. Eddy covariance CO2 flux measurements showed that air temperature has a primary influence on net CO2 exchange in winter and spring, while soil moisture has a primary control on net CO2 exchange in the fall. The net CO2 exchange was generally more moisture limited in the boreal region than in the Arctic tundra. Our analysis indicates complex seasonal interactions of underlying C cycle processes in response to changing climate and hydrology that may not manifest in changes in net annual CO2 exchange. Therefore, a better understanding of the seasonal response of C cycle processes may provide important insights for predicting future carbon-climate feedbacks and their consequences on atmospheric CO2 dynamics in the northern high latitudes.

Water repellency reduces soil CO2 efflux upon rewetting

Carbon dioxide (CO2) efflux from soil represents one of the biggest ecosystem carbon (C) fluxes and high-magnitude pulses caused by rainfall make a substantial contribution to the overall C emissions. It is widely accepted that the drier the soil, the larger the CO2 pulses will be, but this notion has never been tested for water-repellent soils. Soil water repellency (SWR) is a common feature of many soils and is especially prominent after dry periods or fires. An important unanswered question is to what degree SWR affects common assumptions about soil CO2 dynamics. To address this, our study investigates, for the first time, the effect of SWR on the CO2 pulse upon wetting for water-repellent soils from recently burned forest sites. CO2 efflux measurements in response to simulated wetting were conducted both under laboratory and in situ conditions. Experiments were conducted on severely and extremely water-repellent soils, with a wettable scenario simulated by adding a wetting agent to the water. CO2 efflux upon rewetting was significantly lower in the water-repellent scenarios. Under laboratory conditions, CO2 pulse was up to four times lower under the water-repellent scenario as a result of limited wetting, with 70% of applied water draining rapidly via preferential flow paths, leaving much of the soil dry. We suggest that the predominant cause of the lower CO2 pulse in water-repellent soils was the smaller volume of pores in which the CO2 was replaced by infiltrating water, compared to wettable soil. This study shows that SWR should be considered as an important factor when measuring or predicting the CO2 flush upon rewetting of dry soils. Although this study focused mainly on short-term effects of rewetting on CO2 fluxes, the overall implications of SWR on physical changes in soil conditions can be long lasting, with overall larger consequences for C dynamics.

Shallow Salt Marsh Tidal Ponds–An Environment With Extreme Oxygen Dynamics

In marshes, tidal ponds are increasing in number and areal coverage. Getting a better understanding of their unique biogeochemistry is a prerequisite for foreseeing their future role in salt marsh ecosystems. Using in situ microprofiling, this study investigated the spatiotemporal dynamics of O2, pH and CO2 in shallow salt marsh tidal ponds in the summer time. High benthic photosynthetic activity, fueled by CO2 from the sediment, resulted in steep vertical O2 gradients at the sediment-water interface, increasing from anoxia to extremely supersaturated peak concentrations up to 886±139 µmol L-1 (391% atmospheric O2 saturation) over a short distance of 6mm. These characteristic peaks developed even at low light conditions down to 150 µmol m-2 sec-1 photosynthetically active radiation (PAR). The oxygen gradients were restricted to the layer of benthic microalgae on the sediment surface and did not extend into the water column, which was well-mixed throughout the day showing no vertical variation. The benthic photosynthesis and respiration controlled the oxygen concentration in the water column, creating net supersaturated conditions during the day and hypoxic conditions at night. The tidal ponds were generally well-buffered showing only attenuated pH fluctuations ranging from 6.2 to 7.3, and no persistent gradients built up, despite the high photosynthetic activity at the sediment water interface. CO2 accumulated in the sediment and was present in the water column during the morning hours, but depleted in the afternoon due to the high photosynthetic uptake. Tidal ponds also experienced event-driven changes in their biogeochemistry. Sea foam developed on the water surface during the day and accumulated on one side of the pond blocking light penetration and lowering oxygen concentrations under the foam. Inundation at high tide caused a short-lived temporal variation in O2 and pH, which was restricted to the time of the flood. As the flooding water receded, the preceding O2 and pH conditions were immediately restored. Altogether shallow tidal ponds comprise a marsh habitat with distinctive spatiotemporal oxygen dynamics driven by benthic photosynthesis and respiration, which differ from the surrounding vegetated marsh, and could drive changes in salt marsh biogeochemistry in response to increased pond coverage.