Nitrogen addition turns a temperate peatland from a near-zero source into a strong sink of nitrous oxide

Peatlands, as important global nitrogen (N) pools, are potential sources of nitrous oxide (N₂O) emissions. We measured N₂O flux dynamics in Hani peatland in a growing season with simulating warming and N addition for 12 years in the Changbai Mountains, Northeastern China, by using static chamber-gas chromatography. We hypothesised that warming and N addition would accelerate N₂O emissions from the peatland. In a growing season, the peatland under natural conditions showed near-zero N₂O fluxes and warming increased N₂O emissions but N addition greatly increased N₂O absorption compared with control. There was no interaction between warming and N addition on N₂O fluxes. Pearson correlation analysis showed that water table depth was one of the main environmental factors affecting N₂O fluxes and a positive relationship between them was observed. Our study suggests that the N₂O source function in natural temperate peatlands maybe not be so significant as we expected before; warming can increase N₂O emissions, but a high dose of N input may turn temperate peatlands to be strong sinks of N₂O, and global change including warming and nitrogen deposition can alter N₂O fluxes via its indirect effect on hydrology and vegetation in peatlands.

Water level variation at a beaver pond significantly impacts net CO₂ uptake of a continental bog

The carbon (C) dynamics of northern peatlands are sensitive to hydrological changes owing to ecohydrological feedback. We quantified and evaluated the impact of water level variations in a beaver pond (BP) on the CO2 flux dynamics of an adjacent, raised Sphagnum – shrub-dominated bog in southern Canada. We applied the CoupModel to the Mer Bleue bog, where the hydrological, energy and CO2 fluxes have been measured continuously for over 20 years. The lateral flow from the bog to the BP was estimated by the hydraulic gradient between the peatland and the BP's water level and the vertical profile of peat hydraulic conductivity. The model outputs were compared with the measured hydrological components, CO2 flux and energy flux data (1998–2019). CoupModel was able to reproduce the measured data well. The simulation shows that variation in the BP water level (naturally occurring or due to management) influenced the bog net ecosystem exchange of CO2 (NEE). Over 1998–2004, the BP water level was 0.75 to 1.0 m lower than during 2017–2019. Simulated net CO2 uptake was 55 g C m−2 yr−1 lower during 1998–2004 compared to 2017–2019 when there was no BP disturbance, which was similar to the differences in measured NEE between those periods. Peatland annual NEE was well correlated with water table depth within the bog, and NEE also shows a linear relation with the water level at the BP, with a slope of −120 g CO2-C m−2 yr−1 m−1. The current modelling predicts the bog may switch from CO2 sink to source when the BP water levels drop lower than ~ 1.7 m below the peat surface at the eddy covariance tower, 250 m from the BP. This study highlights the importance of natural and human disturbances to adjacent water bodies in regulating net CO2 uptake function of northern peatlands.

Seasonal flux patterns and carbon transport from low-oxygen eddies at the Cape Verde Ocean Observatory: lessons learned from a time series sediment trap study (2009–2016)

Mesoscale eddies are abundant in the eastern tropical North Atlantic and act as oases for phytoplankton growth due to local enrichment of nutrients in otherwise oligotrophic waters. It is not clear whether these eddies can efficiently transfer organic carbon and other flux components to depth and if they are important for the marine carbon budget. Due to their transient and regionally restricted nature, measurements of eddies' contribution to bathypelagic particle flux are difficult to obtain. Rare observations of export flux associated with low-oxygen eddies have suggested efficient export from the surface to the deep ocean, indicating that organic carbon flux attenuation might be low. Here we report on particle flux dynamics north of the Cabo Verde islands at the oligotrophic Cape Verde Ocean Observatory (CVOO; approx. 17∘35′ N, 24∘15′ W). The CVOO site is located in the preferred pathways of highly productive eddies that ultimately originate from the Mauritanian upwelling region. Between 2009 and 2016, we collected biogenic and lithogenic particle fluxes with sediment traps moored at ca. 1 and 3 km water depths at the CVOO site. From concurrent hydrography and oxygen observations, we confirm earlier findings that highly productive eddies are characterized by colder and less saline waters and a low-oxygen signal as well. Overall, we observed quite consistent seasonal flux patterns during the passage of highly productive eddies in the winters of 2010, 2012 and 2016. We found flux increases at 3 km depth during October–November when the eddies approached CVOO and distinct flux peaks during February–March, clearly exceeding low oligotrophic background fluxes during winter 2011 and showing an enhanced particle flux seasonality. During spring, we observed a stepwise flux decrease leading to summer flux minima. The flux pattern of biogenic silicate (BSi) showed a stronger seasonality compared to organic carbon. Additionally, the deep fluxes of total mass showed an unusually higher seasonality compared to the 1 km traps. We assume that BSi and organic carbon/lithogenic material had different sources within the eddies. BSi-rich particles may originate at the eddy boundaries where large diatom aggregates are formed due to strong shear and turbulence, resulting in gravitational settling and, additionally, in an active local downward transport. Organic carbon associated with lithogenic material is assumed to originate from the interior of eddies or from mixed sources, both constituting smaller, dust-ballasted particles. Our findings suggest that the regularly passing highly productive eddies at CVOO repeatedly release characteristic flux signals to the bathypelagic zone during winter–spring seasons that are far above the oligotrophic background fluxes and sequester higher organic carbon than during oligotrophic settings. However, the reasons for a lower carbon flux attenuation below eddies remain elusive.

Aquatic Eddy Covariance: The Method and Its Contributions to Defining Oxygen and Carbon Fluxes in Marine Environments

Aquatic eddy covariance (AEC) is increasingly being used to study benthic oxygen (O2) flux dynamics, organic carbon cycling, and ecosystem health in marine and freshwater environments. Because it is a noninvasive technique, has a high temporal resolution (∼15 min), and integrates over a large area of the seafloor (typically 10–100 m2), it has provided new insights on the functioning of aquatic ecosystems under naturally varying in situ conditions and has given us more accurate assessments of their metabolism. In this review, we summarize biogeochemical, ecological, and biological insights gained from AEC studies of marine ecosystems. A general finding for all substrates is that benthic O2 exchange is far more dynamic than earlier recognized, and thus accurate mean values can only be obtained from measurements that integrate over all timescales that affect the local O2 exchange. Finally, we highlight new developments of the technique, including measurements of air–water gas exchange and long-term deployments. Expected final online publication date for the Annual Review of Marine Science, Volume 14 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Observed Methane Uptake and Emissions at the Ecosystem Scale and Environmental Controls in a Subtropical Forest

Methane (CH4) is one of the three most important greenhouse gases. To date, observations of ecosystem-scale methane (CH4) fluxes in forests are currently lacking in the global CH4 budget. The environmental factors controlling CH4 flux dynamics remain poorly understood at the ecosystem scale. In this study, we used a state-of-the-art eddy covariance technique to continuously measure the CH4 flux from 2016 to 2018 in a subtropical forest of Zhejiang Province in China, quantify the annual CH4 budget and investigate its control factors. We found that the total annual CH4 budget was 1.15 ± 0.28~4.79 ± 0.49 g CH4 m−2 year−1 for 2017–2018. The daily CH4 flux reached an emission peak of 0.145 g m−2 d−1 during winter and an uptake peak of −0.142 g m−2 d−1 in summer. During the whole study period, the studied forest region acted as a CH4 source (78.65%) during winter and a sink (21.35%) in summer. Soil temperature had a negative relationship (p < 0.01; R2 = 0.344) with CH4 flux but had a positive relationship with soil moisture (p < 0.01; R2 = 0.348). Our results showed that soil temperature and moisture were the most important factors controlling the ecosystem-scale CH4 flux dynamics of subtropical forests in the Tianmu Mountain Nature Reserve in Zhejiang Province, China. Subtropical forest ecosystems in China acted as a net source of methane emissions from 2016 to 2018, providing positive feedback to global climate warming.

Diurnal Tree Stem CH4 and N2O Flux Dynamics from a Riparian Alder Forest

Tree stems play an important role in forest methane (CH4) and nitrous oxide (N2O) flux dynamics. Our paper aimed to determine the unknown diurnal variability of CH4 and N2O exchange in grey alder tree stems. The gas fluxes in tree stems and adjacent soil were measured using manual static and dynamic chamber systems with gas chromatographic and laser-spectroscopic analysis, respectively. The alder trees were predominant emitters of CH4 and N2O; however, N2O emission from stems was negligible. The soil mainly emitted N2O into the atmosphere and was both a source and sink of CH4, depending on environmental conditions. Neither the tree stems nor the riparian forest soil showed significant differences in their CH4 and N2O fluxes between the daytime and nighttime, independently of the exchange rates. In contrast to several previous studies revealing a diurnal variability of greenhouse gas fluxes from tree stems, our investigation did not show any clear daytime–nighttime differences. On the other hand, we found quite clear seasonal dynamics initiated by changing environmental conditions, such as temperature and soil water conditions and tree physiological activity. Our results imply a transport role of tree stems for soil-produced CH4 and N2O rather than the production of these gases in tree tissues, even though this cannot be excluded.

Seasonal flux patterns and carbon transport from low oxygen eddies at the Cape Verde Ocean Observatory: lessons learned from a time series sediment trap study (2009–2016)

Mesoscale eddies are abundant in the eastern tropical North Atlantic and can form an oasis for phytoplankton growth due to local enrichment of nutrients in an otherwise oligotrophic ocean. It is not clear, whether these eddies can efficiently transfer organic carbon and other flux components to depth and if they are important for the marine carbon budget. Due to their transient and regionally restricted nature, measurements of eddies’ contribution to bathypelagic particle flux are difficult to obtain. The rare observations of export flux associated with low oxygen eddies have suggested efficient export from the surface to the deep ocean, indicating that organic carbon flux attenuation might be low. Here we report on particle flux dynamics north of the Cape Verde Islands at the oligotrophic Cape Verde Ocean Observatory (CVOO, approx. 17°35’ N/ 24°15’W). This region is a corridor for eddies and low-oxygen eddies regularly passing the position of CVOO between 2009 and 2016, while we collected biogenic and lithogenic particle flux with sediment traps moored at ca. 1 and 3 km water depth. Overall, we observed quite consistent sine-wave flux patterns during the passages of low oxygen eddies in the winters of 2010, 2012 and 2016. We found flux increases in 3 km depth in October-November when the eddies approached CVOO and distinct flux peaks in February–March, clearly exceeding low oligotrophic background fluxes in winter 2011 and showing an enhanced particle flux seasonality. During spring, we observed a stepwise flux decrease leading to summer flux minima. The flux pattern of biogenic silicate (BSi) shows a stronger seasonality compared to organic carbon. Additionally, the deep fluxes of total mass show an unusually higher seasonality compared to the 1 km traps. We assume that BSi and organic carbon/lithogenics had different sources within the eddies. BSi-rich particles may originate at the eddy boundaries where large diatom aggregates are formed due to strong shear, resulting in gravitational settling and, additionally, in an active local downward transport. Organic carbon associated with the lithogenic material is assumed to originate from the interior of eddies or from mixed sources both constituting smaller, dust-ballasted particles. Our findings suggest that the regularly passing low-oxygen Anticyclonic Modewater Eddies (ACME) at CVOO repeatedly release characteristic flux signals to the bathypelagic in the winter-spring season far above the oligotrophic background fluxes and sequester higher organic carbon than expected for oligotrophic settings. However, the reasons for a lower carbon flux attenuation below ACMEs remain elusive.

P047 The ATG16L1 T300A polymorphism in Crohn’s Disease patients leads to abnormal autophagy flux in dendritic cells

Background Crohn’s disease is a chronic inflammatory bowel disease, whose pathogenesis is largely determined by an inappropriate immune response towards luminal microbiota in genetically susceptible hosts, affecting in particular genes involved in the autophagy process. The autophagy-related ATG16L1 T300A variant is one of the most frequently associated polymorphisms with susceptibility to Crohn’s disease, but the functional consequences of this polymorphism on the dynamic properties of the autophagy flux remains unknown. Methods Using dynamic monitoring of LC3 lipidation, a key protein marker of autophagy, we have quantitatively analysed the autophagic flux of human primary dendritic cells, which are very potent antigen-presenting cells whose function is critical in Crohn’s disease. Results The detailed analysis of the dynamic properties of the autophagic flux reveal that the maturation process in dendritic cells from healthy subjects is associated with a decrease of the velocity and the intensity on the autophagy flux although the turnover of autophagosome formation was maintained, demonstrating the capacity of resilience of the autophagy flux for a maintenance of functional autophagy during maturation process. Strikingly, the expression of the ATG16L1 T300A variant, but not NOD2 R702W variant, in dendritic cells from patients with Crohn’s disease is associated with a disorder of the autophagic flux dynamics in immature dendritic cells, that was not modulated upon maturation. The lack of adaptation of the autophagy flux in ATG16L1 T300A expressing dendritic cells of Crohn’s disease patients, during their maturation was not associated with altered phenotypic acquisition of maturation markers but highlighted a very specific autophagic flux signature. Conclusion Dendritic cells of Crohn’s patients expressing the autophagy-related ATG16L1 T300A have an altered autophagy flux, that could affect their function and contribute to the pathogenesis of Crohn’s disease.