Phương pháp phân tích khí hồng ngoại buồng tĩnh kín và ứng dụng trong việc xác định lượng carbon phát thải qua hô hấp đất tại rừng ngập mặn Cần Giờ

Nghiên cứu hô hấp đất là một hướng nghiên cứu có đóng góp quan trọng vào việc đánh giá khả năng trao đổi và tích trữ Carbon (C) của hệ sinh thái rừng. Sự phát thải C của hệ sinh thái thông qua quá trình hô hấp của đất rừng có thể được xác định bằng nhiều phương pháp và thiết bị khác nhau, nhưng phổ biến nhất là phương pháp buồng tĩnh kín. Trong nghiên cứu này, hô hấp đất của hệ sinh thái tự nhiên rừng ngập mặn Cần Giờ (Thành phố Hồ Chí Minh) được đo bằng phương pháp buồng kín di động (DC-Dynamic chamber method) tại 12 sinh cảnh rừng tự nhiên với kích thước ô mẫu 20 m x 20 m. Thông lượng CO2 phát thải từ đất vào khí quyển thu được qua buồng kín được lưu chuyển đến thiết bị phân tích khí hồng ngoại IRGA xách tay và quay trở lại buồng phục vụ việc đánh giá các thông số đo đạc. Kết quả cho thấy rừng ngập mặn Cần Giờ phát thải C qua hô hấp đất với thông lượng trung bình 4,39 µmolCO2.m-².s-¹. Lượng CO2 phát thải qua đất thay đổi theo không gian, thời gian và có mối tương quan với nhiệt độ và độ ẩm buồng đo. Nhiệt độ và độ ẩm buồng đo cùng chế độ thủy triều có tác động đến sự phát thải CO2 qua hô hấp đất tại mỗi vị trí ô mẫu.

The Multiscale Monitoring of Peatland Ecosystem Carbon Cycling in the Middle Taiga Zone of Western Siberia: The Mukhrino Bog Case Study

The peatlands of the West Siberian Lowlands, comprising the largest pristine peatland area of the world, have not previously been covered by continuous measurement and monitoring programs. The response of peatlands to climate change occurs over several decades. This paper summarizes the results of peatland carbon balance studies collected over ten years at the Mukhrino field station (Mukhrino FS, MFS) operating in the Middle Taiga Zone of Western Siberia. A multiscale approach was applied for the investigations of peatland carbon cycling. Carbon dioxide fluxes at the local scale studied using the chamber method showed net accumulation with rates from 110, to 57.8 gC m−2 at the Sphagnum hollow site. Net CO2 fluxes at the pine-dwarf shrubs-Sphagnum ridge varied from negative (−32.1 gC m−2 in 2019) to positive (13.4 gC m−2 in 2017). The cumulative May-August net ecosystem exchange (NEE) from eddy-covariance (EC) measurements at the ecosystem scale was −202 gC m−2 in 2015, due to the impact of photosynthesis of pine trees which was not registered by the chamber method. The net annual accumulation of carbon in the live part of mosses was estimated at 24–190 gC m−2 depending on the Sphagnum moss species. Long-term carbon accumulation rates obtained by radiocarbon analysis ranged from 28.5 to 57.2 gC m−2 yr−1, with local extremes of up to 176.2 gC m−2 yr−1. The obtained estimates of various carbon fluxes using EC and chamber methods, the accounting for Sphagnum growth and decomposition, and long-term peat accumulation provided information about the functioning of the peatland ecosystems at different spatial and temporal scales. Multiscale carbon flux monitoring reveals useful new information for forecasting the response of northern peatland carbon cycles to climatic changes.

APPLICATION OF THE AUTOMATED CHAMBER METHOD FOR LONG-TERM GAS FLOW MEASUREMENTS IN SWAMP ECOSYSTEMS OF WESTERN SIBERIA

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Appropriate chamber deployment time for separate quantification of CH4 emissions via plant and ebullition from rice paddies using a modified closed-chamber method

A modified closed-chamber method for estimating total, plant-mediated, and bubbling (ebullition) emissions of CH4 from rice paddies has been developed to use high-time-resolution CH4 concentration data (~ 1 Hz) obtained by a spectroscopic mobile gas analyzer. Here we aimed at determining an appropriate minimum time length of chamber closure for accurate flux measurement by investigating 3255 datasets obtained from a 2-year field survey. To investigate the minimum time length for each chamber measurement, we generated a series of datasets from each measurement: by setting the hypothetical termination time of the chamber closure ahead in 1-min intervals, we obtained various chamber CH4 concentration time series with different durations of chamber closure, and separately estimated CH4 emissions via rice plants and bubbling from each. The estimated flux was sensitive to time length with short closure times, but became less sensitive with longer closure. We defined the minimum time length at which the difference in estimated flux between adjacent time windows was small enough (< 10% of plant-mediated emission). The estimated minimum time length differed from one measurement to another, but 10 min was sufficient for > 99% of cases. Detailed analysis showed a positive correlation between minimum time length and frequency of bubbling events; the time length needed to be longer as bubbling events became more frequent. From this relationship, we computed the appropriate chamber-duration time as a function of bubbling frequency. In the absence of ebullition, 4–5 min was sufficient, but as the bubbling frequency increased to 2.5 times per minute 15–20 min was necessary for accurate pathway-dependent flux measurements.

Features of the diffusive methane emission measurements on lakes by chamber method

&lt;p&gt;Methane is one of the main greenhouse gases in the atmosphere. Lakes are the third-largest natural source of methane on a global scale [Kirschke et al., 2013]. Currently, the chamber method is quite often applied in the measurements of diffusive GHG emissions from natural ecosystems, especially in remote areas, due to low cost and mobility. In lakes, methane can be transported to the atmosphere not only by diffusion but also by bubbling, so during measurements, it is important to divide these two pathways. We have complemented customary floating chambers with plastic shields located underside not blocking diffusive transfer but preventing gas bubbles from entering into the chamber.&lt;/p&gt;&lt;p&gt;The study was conducted on August 19&amp;#8211;20, 2019 in the vicinity of Vaskiny Dachi field station (68.8663&amp;#176; N, 70.3040&amp;#176; E, Central Yamal, Western Siberia, Russia). Measurements were carried out in the central part of the thermokarst lake with a depth of 1.6 m. To compare results of customary and modified chambers, samples were taken in parallel from chambers with and without shields (two chambers of each type) every two hours during the day. Sampling and flux calculations were conducted according to [Bastviken et al., 2010]. The methane concentrations in samples were determined in the laboratory by a Crystal 5000.2 gas chromatograph with a flame ionization detector.&lt;/p&gt;&lt;p&gt;According to the sign test for the 0.05 p-level, methane fluxes measured using chambers with and without shields differ statistically significant considering their diurnal dynamics. At the same time, within the group of fluxes measured by the same type of chambers, no statistically significant differences were found, and mean and median flux values are higher for chambers without shields. It appears that observed differences are not due to natural variability, but due to the contribution of bubble component to the fluxes measured by chambers without shields.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;References&lt;/p&gt;&lt;p&gt;Bastviken D., Santoro A.L., Marotta H. et al. 2010. Methane emissions from Pantanal, South America, during the low water season: toward more comprehensive sampling. &amp;#8211; Environmental Science &amp; Technology, vol. 44, No. 14, pp. 5450&amp;#8211;5455.&lt;/p&gt;&lt;p&gt;Kirschke S., Bousquet P., Ciais P. et al. 2013. Three decades of global methane sources and sinks. &amp;#8211; Nature Geoscience, vol. 6, No. 10, pp. 813&amp;#8211;823.&lt;/p&gt;

Quantifying soil denitrification in situ from depth profiles of 15N labelled denitrification products by diffusion-reaction modelling

&lt;p&gt;Common field methods for measuring soil denitrification in situ include monitoring the accumulation of &lt;sup&gt;15&lt;/sup&gt;N labelled N&lt;sub&gt;2&lt;/sub&gt; and N&lt;sub&gt;2&lt;/sub&gt;O evolved from &lt;sup&gt;15&lt;/sup&gt;N labelled soil nitrate pool in soil surface chambers. Bias of denitrification rates derived from chamber measurements results from subsoil diffusion of &lt;sup&gt;15&lt;/sup&gt;N labelled denitrification products, but this can be corrected by diffusion modeling (Well et al., 2019). Moreover, precision of the conventional &lt;sup&gt;15&lt;/sup&gt;N gas flux method is low due to the high N&lt;sub&gt;2&lt;/sub&gt; background of the atmosphere. An alternative to the closed chamber method is to use concentration gradients of soil gas to quantify their fluxes (Maier &amp;&amp;#160; Schack-Kirchner, 2014). Advantages compared to the closed &amp;#160;chamber method include the facts that (i) time consuming work with closed chambers is replaced by easier sampling of soil gas probes, (ii) depth profiles yield not only the surface flux but also information on the depth distribution of gas production and (iii) soil gas concentrations are higher than chamber gas concentration, resulting in better detectability of &lt;sup&gt;15&lt;/sup&gt;N-labelled denitrification products. Here we use this approach for the first time to evaluate denitrification rates derived from depth profiles of &lt;sup&gt;15&lt;/sup&gt;N labelled N&lt;sub&gt;2&lt;/sub&gt; and N&lt;sub&gt;2&lt;/sub&gt;O in the field by closed chamber measurements published previously (Lewicka-Szczebak et al., 2020).&lt;/p&gt;&lt;p&gt;We compared surface fluxes of N&lt;sub&gt;2&lt;/sub&gt; and N&lt;sub&gt;2&lt;/sub&gt;O from &lt;sup&gt;15&lt;/sup&gt;N labelled microplots using the closed chamber method. Diffusion&amp;#8211;based soil gas probes (Schack-Kirchner et al., 1993) were used to sample soil air at the end of each closed chamber measurement. A diffusion-reaction model (Maier et al., 2017) will be &amp;#160;used to fit measured and modelled concentrations of &lt;sup&gt;15&lt;/sup&gt;N labelled N&lt;sub&gt;2&lt;/sub&gt; and N&lt;sub&gt;2&lt;/sub&gt;O. Depth-specific values of denitrification rates and the denitrification product ratio will be obtained from best fits of depth profiles and chamber accumulation, taking into account diffusion of labelled denitrification products to the subsoil (Well et al., 2019).&lt;/p&gt;&lt;p&gt;Depending on the outcome of this evaluation, the gradient method could be used for continuous monitoring of denitrification in the field based on soil gas probe sampling. This could replace or enhance current approaches by improving the detection limit, facilitating sampling and delivering information on depth-specific denitrification. &amp;#160;&lt;/p&gt;&lt;p&gt;References:&lt;/p&gt;&lt;p&gt;Lewicka-Szczebak D, Lewicki MP, Well R (2020) N2O isotope approaches for source partitioning of N2O production and estimation of N2O reduction &amp;#8211; validation with the 15N gas-flux method in laboratory and field studies. Biogeosciences, &lt;strong&gt;17&lt;/strong&gt;, 5513-5537.&lt;/p&gt;&lt;p&gt;Maier M, Longdoz B, Laemmel T, Schack-Kirchner H, Lang F (2017) 2D profiles of CO2, CH4, N2O and gas diffusivity in a well aerated soil: measurement and Finite Element Modeling. Agricultural and Forest Meteorology, &lt;strong&gt;247&lt;/strong&gt;, 21-33.&lt;/p&gt;&lt;p&gt;Maier M, Schack-Kirchner H (2014) Using the gradient method to determine soil gas flux: A review. Agricultural and Forest Meteorology, &lt;strong&gt;192&lt;/strong&gt;, 78-95.&lt;/p&gt;&lt;p&gt;Schack-Kirchner H, Hildebrand EE, Wilpert KV (1993) Ein konvektionsfreies Sammelsystem f&amp;#252;r Bodenluft. Zeitschrift Fur Pflanzenernahrung Und Bodenkunde, &lt;strong&gt;156&lt;/strong&gt;, 307-310.&lt;/p&gt;&lt;p&gt;Well R, Maier M, Lewicka-Szczebak D, Koster JR, Ruoss N (2019) Underestimation of denitrification rates from field application of the N-15 gas flux method and its correction by gas diffusion modelling. Biogeosciences, &lt;strong&gt;16&lt;/strong&gt;, 2233-2246.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;