An Interannual Comparative Study on Ecosystem Carbon Exchange Characteristics in the Dinghushan Biosphere Reserve, a Dominant Subtropical Evergreen Forest Ecosystem

Compared with other forest systems, research interest in the potential for a stronger ecosystem carbon sequestration of evergreen forests throughout subtropical China has greatly increased. The eddy covariance technique is widely employed to determine accurate forest-atmosphere carbon dioxide (CO2) flux, which is subsequently used to determine forest ecosystem carbon exchange characteristics. The Dinghushan Biosphere Reserve, a subtropical monsoon evergreen broad-leaved forest, is a suitable study area due to its warm and humid climate (compared with other regions within the same latitude), consequently playing a role in the carbon cycle in the region. For this study, we hypothesized that the forest land in this region generally acts as a carbon sink, and that its carbon sequestration capacity increases over time despite the influence of climatic factors. Here, we compared net CO2 flux data derived from the eddy covariance technique over an 8-year study window. Additionally, we ascertained the effects of various environmental factors on net CO2 flux, while also using the Michaelis–Menten model and a physiologically based process model to track and report on ecosystem carbon exchange characteristics. We observed seasonal trends in daily ecosystem flux, indicative of sensitivity to climatic factors, such as air temperature, precipitation, and sunlight. The carbon sequestration capacity of the region exhibited seasonal variability, increasing from October to March (−264 g C m−2 year−1, i.e., 48.4%) while weakening from April to September (−150 g C m−2 year−1, i.e., 40.4%) on average. The net ecosystem exchange (NEE) rate varied from −518 to −211 g C m−2 year−1; ecosystem respiration (Re) varied from 1,142 to 899 g C m−2 year−1; and gross primary production (GPP) varied from 1,552 to 1,254 g C m−2 year−1. This study found that even though the Dinghushan Biosphere Reserve generally acts as a carbon sink, its carbon sequestration capacity did not increase significantly throughout the study period. The techniques (models) used in this study are suitable for application in other ecosystems globally, which can aid in their management and conservation. Finally, the Dinghushan Biosphere Reserve is both an exemplary and a model forest system useful in exploring CO2 absorption and sequestration from the atmosphere.

Technical note: Novel triple O₂ sensor aquatic eddy covariance instrument with improved time shift correction reveals central role of microphytobenthos for carbon cycling in coral reef sands

The aquatic eddy covariance technique stands out as a powerful method for benthic O2 flux measurements in shelf environments because it integrates effects of naturally varying drivers of the flux such as current flow and light. In conventional eddy covariance instruments, the time shift caused by spatial separation of the measuring locations of flow and O2 concentration can produce substantial flux errors that are difficult to correct. We here introduce a triple O2 sensor eddy covariance instrument (3OEC) that by instrument design eliminates these errors. This is achieved by positioning three O2 sensors around the flow measuring volume, which allows the O2 concentration to be calculated at the point of the current flow measurements. The new instrument was tested in an energetic coastal environment with highly permeable coral reef sands colonised by microphytobenthos. Parallel deployments of the 3OEC and a conventional eddy covariance system (2OEC) demonstrate that the new instrument produces more consistent fluxes with lower error margin. 3OEC fluxes in general were lower than 2OEC fluxes, and the nighttime fluxes recorded by the two instruments were statistically different. We attribute this to the elimination of uncertainties associated with the time shift correction. The deployments at ∼ 10 m water depth revealed high day- and nighttime O2 fluxes despite the relatively low organic content of the coarse sediment and overlying water. High light utilisation efficiency of the microphytobenthos and bottom currents increasing pore water exchange facilitated the high benthic production and coupled respiration. 3OEC measurements after sunset documented a gradual transfer of negative flux signals from the small turbulence generated at the sediment–water interface to the larger wave-dominated eddies of the overlying water column that still carried a positive flux signal, suggesting concurrent fluxes in opposite directions depending on eddy size and a memory effect of large eddies. The results demonstrate that the 3OEC can improve the precision of benthic flux measurements, including measurements in environments considered challenging for the eddy covariance technique, and thereby produce novel insights into the mechanisms that control flux. We consider the fluxes produced by this instrument for the permeable reef sands the most realistic achievable with present-day technology.

Estimation of ecosystem evapotranspiration in a Robinia pseudoacacia L. plantation with the use of the eddy covariance technique and modeling approaches

Τhe eddy covariance technique provides reliable ecosystem-level ET measurements. These measurements, when combined with models and satellite products, could offer high spatiotemporal coverage and valuable mechanistic interpretation of the underlying processes. This study address one-year eddy covariance measurements from a Robinia pseudoacacia site in Northern Greece and remote sensing products, we (a) provide a medium-term description of daily ET fluxes for a R. pseudoacacia plantation in a degraded land, (b) assess the contribution of environmental drivers (e.g., net radiation, temperature etc) on ET and (c) evaluate a simple satellite and meteorological driven model for larger-scale applications, based on the land surface water index (LSWI) and the FAO approach. R. pseudoacacia was found to have quite high water consumption, especially during leaf expansion. Net radiation and soil water content had the greatest effect on ecosystem evapotranspiration. LSWI was found to be correlated with both soil water content and evapotranspiration. Its use as an index for water limitation in models lead to high accuracy when compared to ET measurements. Our results (a) provide a significant contribution to the assessment of R. pseudoacacia ecophysiology and (b) highlight the potential of accurate ecosystem ET estimation with simple modeling approaches.

Ecosystem-scale measurements of CO2 and CH4 fluxes from a tropical peatland in Sarawak, Malaysia

<p>Tropical peatlands of Southeast Asia are a globally important carbon reservoir, storing an enormous amount of soil organic carbon as peat. These ecosystems are complex and poorly understood with large unknown biogeochemical processes. Despite the huge carbon stocks in these ecosystems, data on ecosystem-scale carbon dioxide (CO<sub>2</sub>) and methane (CH<sub>4</sub>) fluxes are still limited in comparison with mid- and high-latitude peatland ecosystems. The recent increase in the intensity of climate anomaly such as El Niño may alter the hydrological regime of this ecosystem, thus affects its carbon cycling. It is crucial to quantify the CO<sub>2</sub> and CH<sub>4</sub> fluxes of the ecosystem and understand their responses to environmental changes to predict the role of peat swamp forest in global carbon cycles. To date, the application of the eddy covariance technique to measure the ecosystem-scale CO<sub>2</sub> and CH<sub>4</sub> fluxes in tropical peatlands is still limited to few studies in Malaysia and Indonesia.</p><p>In 2010, we established a long-term greenhouse gas fluxes monitoring using the eddy covariance technique over a peat swamp forest in Sarawak, Malaysia. Here, we present the net ecosystem exchange of CO<sub>2</sub> (NEE) and CH<sub>4</sub> (F<sub>CH4</sub>) from February 2014 to January 2017 (3 years). We had quantified the NEE and F<sub>CH</sub><sub>4</sub>, the diurnal and seasonal variations of NEE and F<sub>CH</sub><sub>4</sub>, and the response of NEE and F<sub>CH</sub><sub>4</sub> to GWL. The F<sub>CH4</sub> was determined half-hourly as the sum of eddy CH<sub>4</sub> flux and CH<sub>4</sub> storage change in an air column below the flux measurement height. We had determined the global warming potential of this ecosystem from annual NEE and F<sub>CH</sub><sub>4</sub> using sustained-flux global warming potential (SGWP).  The annual F<sub>CH4</sub> was converted into a CO<sub>2</sub> equivalent unit using an SGWP factor of 45 which represents the SGWP for CH<sub>4</sub> over a timescale of 100 years. Our preliminary result showed that the CH<sub>4</sub> emission potentially offset the CO<sub>2</sub> sequestration, which was higher than those reported in other regions in the world.</p>

ICOS eddy covariance flux-station site setup: a review

The Integrated Carbon Observation System Research Infrastructure aims to provide long-term, continuous observations of sources and sinks of greenhouse gases such as carbon dioxide, methane, nitrous oxide, and water vapour. At ICOS ecosystem stations, the principal technique for measurements of ecosystem-atmosphere exchange of GHGs is the eddy-covariance technique. The establishment and setup of an eddy-covariance tower have to be carefully reasoned to ensure high quality flux measurements being representative of the investigated ecosystem and comparable to measurements at other stations. To fulfill the requirements needed for flux determination with the eddy-covariance technique, variations in GHG concentrations have to be measured at high frequency, simultaneously with the wind velocity, in order to fully capture turbulent fluctuations. This requires the use of high-frequency gas analysers and ultrasonic anemometers. In addition, to analyse flux data with respect to environmental conditions but also to enable corrections in the post-processing procedures, it is necessary to measure additional abiotic variables in close vicinity to the flux measurements. Here we describe the standards the ICOS ecosystem station network has adopted for GHG flux measurements with respect to the setup of instrumentation on towers to maximize measurement precision and accuracy while allowing for flexibility in order to observe specific ecosystem features.