Closed Chamber Technique Research Articles

Variability of soil organic carbon stocks and soil CO2 efflux across urban land use and soil cover types

Urban soils are an important soil organic carbon (SOC) store, but soils are also a natural source of carbon dioxide (CO2) due to soil CO2 efflux derived from decomposition and root related respiration. Little is known about the temporal and spatial variability of SOC stocks and soil CO2 efflux in urban areas, in particular in the southern hemisphere. The objectives of our study were to quantify SOC stocks and soil CO2 efflux in tree (urban forests) and grass dominated (parklands) urban ecosystems and to investigate the underlying processes which determine urban SOC stocks and soil CO2 efflux. Soil organic C stocks were quantified to 30cm depth and soil CO2 efflux was determined using a closed chamber technique at 16 sites across Auckland, New Zealand. Vegetation and soil characteristics were measured to 30cm depth at each study plot. In addition, the diurnal and seasonal variability in soil CO2 efflux was assessed at six sites. Median soil organic C stocks (0–10cm) were significantly higher in parkland soils (4.8kgm−2, IQR: 0.9) compared to urban forest soils (2.7kgm−2, IQR: 0.1), which was largely due to higher bulk densities in parkland soils. In contrast, median soil CO2 efflux was not significantly different between parklands (5.2μmolm−2s−1, IQR: 0.8) and urban forests (4.5μmolm−2s−1, IQR: 1.9) sites. This suggests that increasing soil bulk densities without restricting root growth may enhance SOC stocks without increasing soil CO2 efflux. Soil temperature and soil water content explained 54% (urban parkland) and 71% (urban forests) of the temporal variation in soil CO2 efflux. Urban SOC stocks and soil CO2 efflux were similar to non-urban forests and grasslands in New Zealand, highlighting the importance of considering urban soils when assessing SOC stocks and soil CO2 efflux.

Calculations of automatic chamber flux measurements of methane and carbon dioxide using short time series of concentrations

Abstract. The closed chamber technique is widely used to measure the exchange of methane (CH4) and carbon dioxide (CO2) from terrestrial ecosystems. There is, however, large uncertainty about which model should be used to calculate the gas flux from the measured gas concentrations. Due to experimental uncertainties the simple linear regression model (first-order polynomial) is often applied, even though theoretical considerations of the technique suggest the application of other, curvilinear models. High-resolution automatic chamber systems which sample gas concentrations several hundred times per flux measurement make it possible to resolve the curvilinear behavior and study the information imposed by the natural variability of the temporal concentration changes. We used more than 50 000 such flux measurements of CH4 and CO2 from five field sites located in peat-forming wetlands ranging from 56 to 78° N to quantify the typical differences between flux estimates of different models. In addition, we aimed to assess the curvilinearity of the concentration time series and test the general applicability of curvilinear models. Despite significant episodic differences between the calculated flux estimates, the overall differences are generally found to be smaller than the local flux variability on the plot scale. The curvilinear behavior of the gas concentrations within the chamber is strongly influenced by wind-driven chamber leakage, and less so by changing gas concentration gradients in the soil during chamber closure. Such physical processes affect both gas species equally, which makes it possible to isolate biochemical processes affecting the gases differently, such as photosynthesis limitation by chamber headspace CO2 concentrations under high levels of incoming solar radiation. We assess the possibility to exploit this effect for a partitioning of the net CO2 flux into photosynthesis and ecosystem respiration as an example of how high-resolution automatic chamber measurements could be used for purposes beyond the estimation of the net gas flux. This shows that while linear and curvilinear calculation schemes can provide similar net fluxes, only curvilinear models open additional possibilities for high-resolution automatic chamber measurements.

Early Snowmelt Enhances the Carbon Sequestration of Hummock-Forming <i>Sphagnum</i> Mosses on Boreal Wetlands

Sphagnum mosses are globally important owing to their considerable peat-forming ability and their potential impact on global climatic cycles acting as a long-term net carbon sink. However, changes in climatic conditions due to global warming may affect the relations between Sphagnum mosses and vascular plants but also the competition among Sphagnum, and thus alter the accumulation of carbon on boreal wetlands. Sphagnum mosses are a plant genus with a favorable ability to grow in low solar irradiance and temperature conditions compared to vascular plants. This may be increasingly beneficial in increased wintertime temperatures and predated snowmelt conditions. To understand particularly the importance of early spring photosynthetic activity and thus the role of the length of growing season on carbon balance, we analyzed the CO2 exchange of Sphagnum mosses with closed chamber technique in two categories of microtopographical habitats, hummocks and lawns, during four seasons 2010-2013 on a raised bog in Central Finland. During CO2 exchange measurements, instantaneous net ecosystem exchange (NEE) and ecosystem respiration (RE) were measured. Our results show that the mean measured seasonal NEE, i.e. the instantaneous net carbon sequestration, of hummocks was generally only slightly higher than the NEE of lawns, but the mean measured seasonal RE of hummocks was clearly and significantly higher than the RE of lawns in every study year. A reason for the observed still higher seasonal carbon sequestration of hummocks than that of lawns besides the slightly higher rate of carbon accumulation was the longer duration of physiologically active growing season. Therefore, hummock-forming Sphagnum mosses exposed firstly from snow cover showed to get the extra time for photosynthesis and thus extra benefit compared to other mire plants. This may be further enhanced by the expansion of hummock-forming Sphagnum moss dominated raised bogs towards northern aapa-mire region due to the global warming.

The greenhouse gas balance of a drained fen peatland is mainly controlled by land-use rather than soil organic carbon content

Abstract. Drained organic soils are considered to be hotspots for greenhouse gas (GHG) emissions. Arable lands and intensively used grasslands, in particular, have been regarded as the main producers of carbon dioxide (CO2) and nitrous oxide (N2O). However, GHG balances of former peatlands and associated organic soils not considered to be peatland according to the definition of the Intergovernmental Panel on Climate Change (IPCC) have not been investigated so far. Therefore, our study addressed the question to what extent the soil organic carbon (SOC) content affects the GHG release of drained organic soils under two different land-use types (arable land and intensively used grassland). Both land-use types were established on a Mollic Gleysol (labeled Cmedium) as well as on a Sapric Histosol (labeled Chigh). The two soil types differed significantly in their SOC contents in the topsoil (Cmedium: 9.4–10.9 % SOC; Chigh: 16.1–17.2 % SOC). We determined GHG fluxes over a period of 1 or 2 years in case of N2O or methane (CH4) and CO2, respectively. The daily and annual net ecosystem exchange (NEE) of CO2 was determined by measuring NEE and the ecosystem respiration (RECO) with the closed dynamic chamber technique and by modeling the RECO and the gross primary production (GPP). N2O and CH4 were measured with the static closed chamber technique. Estimated NEE of CO2 differed significantly between the two land-use types, with lower NEE values (−6 to 1707 g CO2-C m−2 yr−1) at the arable sites and higher values (1354 to 1823 g CO2-C m−2 yr−1) at the grassland sites. No effect on NEE was found regarding the SOC content. Significantly higher annual N2O exchange rates were observed at the arable sites (0.23–0.86 g N m−2 yr−1) than at the grassland sites (0.12–0.31 g N m−2 yr−1). Furthermore, N2O fluxes from the Chigh sites significantly exceeded those of the Cmedium sites. CH4 fluxes were found to be close to zero at all plots. Estimated global warming potential, calculated for a time horizon of 100 years (GWP100) revealed a very high release of GHGs from all plots ranging from 1837 to 7095 g CO2 eq. m−2 yr−1. Calculated global warming potential (GWP) values did not differ between soil types and partly exceeded the IPCC default emission factors of the Tier 1 approach by far. However, despite being subject to high uncertainties, the results clearly highlight the importance of adjusting the IPCC guidelines for organic soils not falling under the definition in order to avoid a significant underestimation of GHG emissions in the corresponding sectors of the national climate reporting. Furthermore, the present results revealed that mainly the type of land-use, including the management type, and not the SOC content is responsible for the height of GHG exchange from intensive farming on drained organic soils.

Temperature Effect Investigation toward Peat Surface CO2 Emissions by Planting Leguminous Cover Crops in Oil Palm Plantations in West Kalimantan

The aim of this research was to know the impact of planting leguminous cover crops (LCCs) of Mucuna bracteata and Calopogonium mucunoides in oil palm plantation on peatland on reducing CO2 emissions. Atmosphere temperature, peat surface temperature, in-closed chamber temperature and peat surface CO2 fluxes were monitored on two adjacent experimental plots. The first experimental plot was on the newly opened peat surface (NOPS) and another was on the eight years planted oil palm land (EPOL). The closed chamber techniques adopted from International Atomic Energy Agency (IAEA) (1993) were implemented to trap CO2 emissions emitted from 24 treatment plots at the 1st, 3rd and 6th months observations. Average CO2 fluxes observed on no LCCs plots in the NOPS site were 61.25 ± 8.98, 33.76 ± 2.92 and 33.75 ± 3.45 g/mh , while in the EPOL site were 55.38 ± 15.95, 29.90 ± 5.32 and 27.70 ± 4.62 g/mh at the 1st, 3rd and 6th months monitoring, respectively. Average CO2 fluxes observed on the planted M. bracteata plots in the NOPS site were 68.2 ± 24.5, 12.88 ± 3.70 and 10.40 ± 1.28 g/mh, whereas in the EPOL site were 54.04 ± 6.70, 11.45 ± 2.00 and 9.33 ± 3.49 g/mh at the 1st, 3rd and 6th months monitoring, respectively. Average CO2 flux observed on the planted C. mucunoides plots in the NOPS site were 66.5 ± 23.7, 15.41 ± 1.51 and 9.74 ± 2.55 g/mh, while in the EPOL site were 47.00 ± 5.00, 9.34 ± 1.23 and 10.52 ± 4.80 g/mh at the 1st, 3rd and 6th months, respectively. P-value for the experimental sites was 0.008 (< 0.05), indicating the significant difference in the level of CO2 fluxes between the sites. P-value for the treatments in the experimental plots was 0.000 (< 0.05), indicating markedly different level of CO2 fluxes among treatments. P-value for the age of M. bracteata and C. mucunoides planted on the experimental plots was 0.000 (< 0.05), indicating the significant difference in the level of CO2 fluxes due to the enhanced LCCs age performing at the increase of shading effects. The comparison of CO2 fluxes among experimental plots shows that planting M. bracteata and C. mucunoides on the peatland could reduce CO2 emission.

Fertiliser and seasonal urine effects on N2O emissions from the urine-fertiliser interface of a grazed pasture

Significant areas of ruminant-grazed pastures are simultaneously covered by excreted urine and fertiliser nitrogen (N). However, the effect of overlapping N inputs on nitrous oxide (N2O) emission factors has not been studied. Three rates of 15N-labelled urea fertiliser were applied with either no urine, an autumn-urine or a spring-urine application. These treatments were applied to perennial ryegrass pasture (Lolium perenne L.) and N2O fluxes were determined over 373 days using standard static closed chamber techniques. Cumulative N2O-N fluxes ranged from 766 to 4332 g N2O-N ha–1 (0.36%–0.74% of total N applied) and were lowest in the absence of urine; however, no fertiliser rate effect occurred regardless of urine presence or season of application. Urine-elevated N2O-N fluxes followed urine applications for up to 40 days, resulting in lower fertiliser contributions to the N2O-N fluxes at these times. Total 15N recoveries as N2O-N were ≤0.04% and did not differ with fertiliser rate.

Greenhouse Gas Fluxes from Sugarcane and Pigeon Pea Cultivated Soils

Quantification of greenhouse gas (GHG) emissions from agricultural lands is essential for strategic planning towards GHG efficient development in India. We measured the fluxes of CH4, N2O and CO2 during cultivation of two important crops; sugarcane and pigeon pea following the closed chamber technique. Both the soils acted as net CH4 sinks, but sources of N2O and CO2. Pigeon pea soil acted as a weak sink, removing 0.054 ± 0.002 kg CH4 ha−1 from the atmosphere, while sugarcane soil removed 11.061 ± 0.093 kg CH4 ha−1. Urea application in sugarcane field increased the fluxes of N2O, but the total N2O emission over growth period of sugarcane (355 days) was similar to the total emissions during pigeon pea cultivation (245 days); 2.69 ± 0.09 and 2.07 ± 0.17 kg N2O ha−1, respectively. CO2 fluxes from pigeon pea cultivation were higher than sugarcane cultivation. Pigeon pea cultivation was a low input farming, but its global warming potential was higher than that of sugarcane cultivation. This study presents the GHG estimates from cultivation of the two important crops in India for which GHG estimates are lacking.

Release of nitrous oxide and dinitrogen from a transition bog under drained and rewetted conditions due to denitrification: results from a [15N]nitrate–bromide double-tracer study

Denitrification is well known being the most important nitrate-consuming process in water-logged peat soils, whereby the intermediate compound nitrous oxide (N2O) and the end product dinitrogen (N2) are ultimately released. The present study was aimed at evaluating the release of these gases (due to denitrification) from a nutrient-poor transition bog ecosystem under drained and three differently rewetted conditions at the field scale using a 15N-tracer approach ([15N]nitrate application, 30 kg N ha−1) and a common closed-chamber technique. The drained site is characterized by a constant water table (WT) of –30 cm (here referred to as D30), while rewetted sites represent a constant WT of –15 cm, a constant WT of 0 cm (i.e. waterlogged), and an initial WT of 0 cm (which decreased slightly during the experiment), respectively, (here referred to as R15, R0, and R0d, respectively). The highest N2O emissions were observed at D30 (291 µg N2O–N m−2 h−1) as well as at R0d (665 µg N2O–N m−2 h−1). At the rewetted peat sites with a constant WT (i.e. R15 and R0), considerably lower N2O emissions were observed (maximal 37 µg N2O–N m−2 h−1). Concerning N2 only at the initially water-logged peat site R0d considerable release rates (up to 3110 µg N2–N m−2 h−1) were observed, while under drained conditions (D30) no N2 emission and under rewetted conditions with a constant WT (R15 and R0) significantly lower N2 release rates (maximal 668 µg N2–N m–2 h−1) could be detected. In addition, it has been found that natural WT fluctuations at rewetted peat sites, in particular a rapid drop down of the WT, can induce high emission rates for both N2O and N2.

Greenhouse gas emissions from coastal freshwater wetlands in Veracruz Mexico: Effect of plant community and seasonal dynamics

Wetlands play an important role in modulating atmospheric concentrations of Greenhouse Gases (GHGs), such as methane (CH4), nitrous oxide (N2O) and carbon dioxide (CO2). Despite the fact that ∼30% of wetlands in the world occur in tropical latitudes, little is known about GHG emissions from these ecosystems and the variables that control such emissions. We investigated the CH4, N2O and CO2 emissions in tropical freshwater marshes (FM) and swamps (FS) on the coastal plain of Veracruz, Mexico. GHGs were measured every two months using the closed chamber technique from April 2010 to February 2012 (CH4 and N2O) and during the last year for CO2. The ranges of the emissions were 20–2000 mg C–CH4 m−2 d−1, −2–16 mg N–N2O m−2 d−1 and 0.5–18 g C–CO2 m−2d−1. There were not significant differences in the emissions between FM and FS (P=0.314, 0.528 and 0.213 for CH4, N2O, and CO2, respectively). However, significant differences (P < 0.001) in CH4 and CO2 emissions were found in the different seasons. During the rainy and windy seasons, the water level was higher than during dry season, this enhanced reduced conditions in the soils and caused significantly higher (P < 0.05) CH4 emissions (>364 mg m−2 d−1) than during dry season (<150 mg m−2 d−1). The opposite was observed for CO2 emissions, during dry season, wetland soils were more aerobic and CO2 fluxes increased (FM: 11 ± 2, FS: 12 ± 1 g m−2 d−1). N–N2O emissions (FM: 1.5 ± 0.96, FS: 2.4 ± 0.9 mg m−2 d−1) were similar during different climatic season (P=0.056). Based on the global warming potential, CH4 was the main contributor to total GHGs emission during the rainy and windy seasons (75–93%), while CO2 was predominant during the dry season (79.6%) and N2O contributed with less than 7% in the three seasons. Water level and redox potential were found to be main factors influencing GHG emissions in these wetlands.

Measurement and mitigation of nitrous oxide emissions from a high nitrogen input vegetable system.

The emission and mitigation of nitrous oxide (N2O) from high nitrogen (N) vegetable systems is not well understood. Nitrification inhibitors are widely used to decrease N2O emissions in many cropping systems. However, most N2O flux measurements and inhibitor impacts have been made with small chambers and have not been investigated at a paddock-scale using micrometeorological techniques. We quantified N2O fluxes over a four ha celery paddock using open-path Fourier Transform Infrared spectroscopy in conjunction with a backward Lagrangian stochastic model, in addition to using a closed chamber technique. The celery crop was grown on a sandy soil in southern Victoria, Australia. The emission of N2O was measured following the application of chicken manure and N fertilizer with and without the application of a nitrification inhibitor 3, 4-dimethyl pyrazole phosphate (DMPP). The two techniques consistently demonstrated that DMPP application reduced N2O emission by 37–44%, even though the N2O fluxes measured by a micrometeorological technique were more than 10 times higher than the small chamber measurements. The results suggest that nitrification inhibitors have the potential to mitigate N2O emission from intensive vegetable production systems, and that the national soil N2O emission inventory assessments and modelling predictions may vary with gas measurement techniques.

Measurements of selected greenhouse gases exhalation by using the closed-chamber technique and calculation of hour expiration with regard to CO₂ emissions

The article presents the use of a closed-chamber method used for measuring the size of exhalation gases from the soil. The paper presents the assumptions of the method, the measuring kit and information regarding the conversion of the gas concentration in the measuring chamber into the amount of gases emitted from the soil. The authors presented a way of converting concentration of emission gases (for example CO₂) in [ppm] units into the exhalation values [mg (CO₂) x m⁻² x h⁻¹], which are more convenient for comparing the results of environmental tests. The work also includes guidelines for conducting the field measurements and allows one to estimate the size of the exchange of gases between soil and atmosphere.

Physiological and anatomical variations in three rice (Oryza sativaL.) genotypes for transport and emission of methane

Plant-mediated transport is the major pathway of methane (CH4) emission from wetland paddy fields. The present experiment was conducted to understand the mechanism of CH4 transport through rice paddies. A field measurement of CH4 emission was done with three rice genotypes Ranjit, Piolee and Swarna Mahsuri to identify plant physiological and anatomical characteristics influencing gas emission, which could provide insight to the mechanism of CH4 transport. CH4 emission estimation by a closed chamber technique revealed significant variations in emission of CH4 among the varieties. Variety Ranjit was identified as a low emitting variety compared to the others. The leaf area index, tiller number, leaf and root dry weight were closely associated with CH4 emission. The rate of leaf transpiration, stomatal conductance, leaf stomatal frequency and size of the xylem vessels were identified to be strong determinants of CH4 emission suggesting a transport pathway of CH4 along with the transpirational xylem water flow and subsequently through open stomata of leaf blade. Low emitting variety Ranjit exhibited superior yield and yield attributing characteristics in terms of panicle dry weight and high density grain (%) highlighting the role of source–sink carbohydrate partitioning in regulating CH4 emissions. Cultivation of low emitting varieties with higher grain productivity can be an effective environmentally-friendly biological mitigation strategy for CH4 emission from rice paddies.

Managing fertiliser nitrogen to reduce nitrous oxide emissions and emission intensities from a cultivated Cambisol in Scotland

Emissions of nitrous oxide (N2O) were measured from an arable site in south east Scotland for twelve months during 2011–2012 using an intensive sampling strategy. This fully replicated and blocked field experiment aimed to provide accurate measurements of N2O emissions from one of the UK's principle geoclimatic zones supporting agricultural production and to produce robust N2O emission factors (EFs). Calculated EFs were compared to the IPCC's default Tier 1 EF of 1.25%, and the new value of 1%, to assess their suitability for use in locations throughout the UK. Emissions from ten treatments fertilised with either ammonium nitrate or urea at rates of 0kgNha−1 to 200kgNha−1, and sown with spring barley, were measured using the static closed chamber technique. Potential N2O mitigation options were investigated; these included the use of a nitrification inhibitor (NI), split fertiliser applications and variations in the form and quantity of fertiliser applied. Crop yields were measured to enable calculation of N2O emission intensities for each treatment; this is an important factor to consider when assessing N2O mitigation options due to the need to maintain crop yields. Cumulative N2O emissions varied between 1.32kg N2O-Nha−1 and 3.82kg N2O-Nha−1 with a mean 42% decrease in emissions associated with the use of the NI. Increases in crop yield were associated with increases in N fertiliser application, and the amendment of treatments with a NI and the use of a split fertiliser application significantly decreased crop yields by approximately 10% and 5% respectively. Annual EFs ranged between −0.28% to 1.35%. Emission intensities decreased with increasing fertiliser application at low N application rates, and the optimum fertiliser application rate to obtain minimum emissions but maximum crop yield was 160kgNha−1.

Grazing effects on carbon fluxes in a Northern China grassland

Grazing management is critical and can play a significant role in driving C sink and source activity in grassland ecosystems. In this study, CO2 exchange patterns during the growing season were evaluated in the grasslands of northern China using a closed-chamber technique at three stocking rates sites. The results showed that heavy grazing markedly reduced green biomass, plant standing dead, and litter mass as well as reduced net ecosystem exchange (NEE) (P < 0.05). Principal component analysis (PCA) indicated that spatial and temporal patterns varied in ungrazed, moderate grazing, and heavy grazing sites. High NEE was associated with high biomass, high temperature, and high soil water content (SWC), ecosystem respiration (Re), and soil respiration (Rs). Rs was higher at the moderate and heavy grazing sites than the ungrazed site. In contrast, Re and canopy respiration (Rc) were higher at the ungrazed than the other grazing sites. These results indicated that grazing influenced vegetation and soil characteristics, which altered the spatial and temporal patterns of CO2 fluxes in grasslands of northern China. Therefore, reducing stocking rates on heavily grazed grasslands of northern China to moderate grazing levels would enhance NEE, and benefit biomass and animal production.

Greenhouse gas emissions from storage and field application of anaerobically digested and non-digested cattle slurry

Emissions of the greenhouse gases (GHG), methane (CH4) and nitrous oxide (N2O) from non-digested and digested cattle slurry were measured during storage in a pilot-scale facility and during subsequent field application. In three treatments, non-digested cattle slurry (CS), digested cattle slurry (DCS) and digested cattle slurry covered with a roof (DCS-R), GHG emissions were measured during more than three months of storage in summer and in winter. After each storage season, CS and DCS were applied in the field before sowing, either in late summer or in spring, and compared with an unfertilised control (Control). GHG measurements were conducted using a closed chamber technique on both storage tanks and in the field, and the experiments were organised according to a randomised complete block design with three blocks. In the field, three closed chambers were placed randomly in each small plot. For every experimental unit, 7–9 measurements were made over time. Mean daily CH4 emissions during summer storage were 2.37, 7.79 and 6.78g CH4-Cm−3d−1 slurry for CS, DCS and DCS-R, respectively, and were significantly higher for DCS and DCS-R compared with CS (p<0.001). DCS-R gave significantly (p<0.05) lower CH4 emissions than DCS. The limited number of gas samples analysed during the study period may have resulted in increased uncertainty in the estimates. Mean daily CH4 emissions during winter storage were very low, with no significant differences (p>0.05) between treatments. Non-negligible N2O emissions were only detected from DCS-R in summer (cumulative mean emissions 5.98gN2O-Nm−2), corresponding to an emissions factor for N2O-N (EFN2O) of 0.24%.In the field, cumulative emissions of N2O were very limited for CS and DCS (EFN2O 0.59% and 0.44%, respectively, in autumn and 0.20% and 0.10%, respectively, in spring).A combination of summer storage and autumn spreading of DCS had the largest impact on global warming potential (GWP100) in terms of CO2-equivalents (CO2e) (28.7kg CO2em−3 slurry), with the impact from storage dominating. Presence of a roof reduced CH4 emissions, but also stimulated formation of N2O during summer and therefore had no net effect on GWP100. With winter storage and spring spreading, CS gave the lowest impact (2.51kg CO2em−3 slurry).

Modeling gross primary production and ecosystem respiration in a semiarid grassland of Mongolia

We constructed an empirical model in which the rates of gross primary production (GPP) and ecosystem respiration (Reco) were calculated using meteorological, soil and plant variables, based on our previous measurements of carbon dioxide (CO2) fluxes using a closed-chamber technique in Mongolian grasslands. In the previous studies, we determined GPP and Reco using transparent and opaque chambers and simultaneously measured the environmental variables inside and beside the chambers. These measurements allowed us to quantify direct and accurate relationships between the rates of GPP and Reco and their controlling factors. The GPP, Reco and their difference net ecosystem CO2 exchange (NEE) were computed in a semiarid grassland site of Mongolia for the growing seasons of 2010 and 2011 using observed values of photosynthetically active radiation, air and soil temperatures, vapor pressure deficit, soil water content and estimated values of aboveground biomass from a remotely sensed vegetation index. Model performance was validated by comparing the modeled and observed NEE values using an eddy covariance technique. Results showed that the model successfully reproduced the magnitude and seasonal variations of the observed NEE, and regression analysis showed reasonable agreement in both years. Cumulative rates of GPP and Reco during the plant-growing season (May − September) were 200.3 and 210.5 g carbon (C) m−2 in the dry year (2010) and 342.3 and 300.1 g C m−2 in the wet year (2011), respectively. This result indicates that drought was more effective in reducing CO2 uptake by the plant than in reducing ecosystem respiration. Consequently, the grassland ecosystem was a net carbon source during the growing season in 2010 and a net carbon sink in 2011.

Gross photosynthesis explains the ‘artificial bias’ of methane fluxes by static chamber (opaque versus transparent) at the hummocks in a boreal peatland

The closed chamber technique has been widely employed to detect methane emissions, despite little being known about whether the absence or presence of light will impact the flux estimation. Here, we employed a laser greenhouse gas analyzer with an opaque—transparent chamber pair to measure the methane emission rate in a boreal peatland complex. Microtopography (i.e., hummocks and hollows) in natural and drained peatlands, and plant communities (i.e., grasses and shrubs) in a pasture converted from natural peatlands, were considered to cover the local heterogeneity. Our results indicated that opaque chambers (0.58–0.78 g CH4 m−2 during the growing season) measured a significantly higher (∼2–3 times) methane emission at the hummocks than transparent chambers (∼0.24 g CH4 m−2); however, a similar phenomenon was not found at the hollows or at other measurement plots. Gross photosynthesis explained 44%–47% of the temporal variation of the ‘artificial bias’ (the difference in methane flux obtained by the opaque versus transparent chambers) at the hummocks. Additionally, both water table depth and surface soil moisture significantly explained spatial variations of methane emissions. Our study suggests that microtopography has a significant influence on the artificial bias in methane emission estimation and the artificial properties of a chamber (transparency/opacity) method can be vitally important in some cases (i.e., hummocks), and negligible in others (i.e., hollows). The observed connection between the photosynthesis process and the ‘artificial bias’ of closed chambers (opaque versus transparent) can be used to improve methane flux modeling. Separate parameterization schemes are needed for methane transportation under the presence or absence of light.

Evaluation of a plot-scale methane emission model using eddy covariance observations and footprint modelling

Abstract. Most plot-scale methane emission models – of which many have been developed in the recent past – are validated using data collected with the closed-chamber technique. This method, however, suffers from a low spatial representativeness and a poor temporal resolution. Also, during a chamber-flux measurement the air within a chamber is separated from the ambient atmosphere, which negates the influence of wind on emissions. Additionally, some methane models are validated by upscaling fluxes based on the area-weighted averages of modelled fluxes, and by comparing those to the eddy covariance (EC) flux. This technique is rather inaccurate, as the area of upscaling might be different from the EC tower footprint, therefore introducing significant mismatch. In this study, we present an approach to validate plot-scale methane models with EC observations using the footprint-weighted average method. Our results show that the fluxes obtained by the footprint-weighted average method are of the same magnitude as the EC flux. More importantly, the temporal dynamics of the EC flux on a daily timescale are also captured (r2 = 0.7). In contrast, using the area-weighted average method yielded a low (r2 = 0.14) correlation with the EC measurements. This shows that the footprint-weighted average method is preferable when validating methane emission models with EC fluxes for areas with a heterogeneous and irregular vegetation pattern.

Rapid emission of nitrous oxide from fallow over summer following wetting in a Mediterranean-type environment

Field trials were conducted to study the emission of nitrous oxide during the summers of 2012 and 2013 from fields with harvested field pea, harvested wheat and winter fallow at Merredin and Cunderdin respectively, two wheat growing regions of Western Australia. The nitrous oxide emission fluxes from these treatments were regularly monitored during the postharvest summer period using a closed chamber technique, before and after wetting the soil. Very low nitrous oxide flux occurred in the dry soil conditions prior to wetting. However, significantly increased nitrous oxide flux was observed at both the sites after wetting, with emissions from a harvested field pea plots being 55–86 fold higher than prior to wetting at the two sites. After wetting, the field pea plots also had significantly higher emissions (3.47–3.87gN2O–Nha−1h−1) than those following winter fallow (1.17–1.95gN2O–Nha−1h−1), although no nitrogen fertiliser was applied to either during the crop growing period. The harvested wheat plots had emissions that were similar (1.18–3.15gN2O–Nha−1h−1) or higher than the winter fallow treatment. The sudden increase in nitrous oxide was observed 3h after wetting, with a significant reduction in nitrous oxide flux occurring approximately 20h after wetting. This indicates the need for regular monitoring of nitrous oxide flux to capture the emission pulses that occur under favourable conditions. Nitrous oxide flux was positively correlated with soil nitrate-N, water filled pore space and soil temperature at both the sites. The study shows that the use of winter fallow does not result in high soil nitrous oxide emissions over the following summer, compared with harvested wheat or field peas. The studies are important because more frequent summer rains are predicted in the region, and as shown in this study, high summer temperature together with rainfall could lead to nitrous oxide emission peaks.

Sewing Machine Technique For Iridodialysis Repair

Aim: To report a novel, safe, effective, easy, economical technique of iridodialysis repair with very short learning curve. Methods: The technique is demonstrated in 3 patients of post traumatic iridodialysis and one patient of post surgical iridodialysis. All patients underwent iridodialysis repair based on sewing machine principle using 26 G one and half inch needle, 10/0 prolene suture in a closed chamber technique And additional procedures required. Results: All patients did well post operatively with good visual recovery along from good cosmoses and minimal morbidity Conclusions: This technique shows novel, easy, simple, economical way of minimally invasive iridodialysis repair which even an average cataract surgeon can perform safely when require and is also repeatable in needed.