This method can help answer key questions in the field of global greenhouse gas emission assessment, such as the contribution of paddy fields as a source or sink into the termination of climate change. The technique is suitable for plot experiments as it is manageable with limited resources and can identify relationships among ecosystem properties and gas fluxes. The application of this technique can contribute to the holistic evaluation of the agroecosystem and define appropriate mitigation strategies.
Building database of a greenhouse gas emission in temperate rice is of fundamental importance for national inventory calculation in order to drive future policy and regulations. Filled closed static chamber method was firstly conceived and implemented to upland soils. It can be applied in flooded conditions by including the plans within the chamber headspace and avoiding water disturbance.
The chambers are made of three main parts. The anchor, the lid, and extensions. Anchors isolate the soil column beneath the chamber, preventing lateral diffusion.
The effective seal between the parts is ensured by a water-filled channel. The anchor is a 75 centimeter by 36 centimeter by 25 centimeter rectangular box made of stainless steel. A water fillable channel is welded to the upper rectangular perimeter of the anchor.
Two holes on each of the four sides of the anchor, five centimeters from the upper water channel, ensure fastest charge of ponding water within the chamber during the field drainage events. The lid is a rectangular box of stainless steel sized 75 centimeters by 36 centimeters by 20 centimeters with an internal volume of 54 liters. The lid must perfectly fit the water fillable channel.
The lid is covered by a four centimeter thick closed cell foam, which is protected by a light reflective coating. Each lid is equipped with a vent valve made of a curved piece of plastic tubing sized for the chamber volume and wind conditions. The vent valve fits into a 1.5 centimeter hole in the center of one of the two 36 centimeter lateral faces of the lid.
The plastic tube is secured to the lid by a screw connector. A sampling port for withdrawal of gas samples is located in a seven centimeter by seven centimeter niche dug into the cell foam in the center of the top of the lid. Within the niche, a one centimeter hole is closed by a rubber stopper that fits a Teflon tube.
When the stopper is placed in its niche, the Teflon tube extrudes three centimeters and intrudes 17 centimeters. The outwards part of the tube is connected to a one way stopcock to manage the opening and closure of the sampling port. Each lid is equipped with a 12 volt PC fan powered by a rechargeable and portable battery, to ensure air mixing.
Extension attachment to the chamber may be necessary to include the plants inside the chamber, depending on their size. Extensions are rectangular boxes made of stainless steel and with an upper water fillable channel. Always run the measurement events at the same time each day to minimize diurnal variability.
Prior to each paddy field visit, evacuate at least three 12 milliliter glass vials closed with butyl rubber septa per field chamber in the laboratory. When arriving in the field, place wooden planks on the concrete blocks to reach the anchors. Afterwards, fill the channels placed on the upper perimeter of the anchors with water.
Use a folding ruler to measure the height of the rice plants. Add one extension to contain the rice plant when it is 20 to 40 centimeters above the soil surface. Use two extensions when the rice plant is 40 to 60 centimeters, and so on.
Interpose extensions between the anchor and lid, filling all water fillable channels. Close each chamber, placing the lid in the water filled channel of the upper extension. During the closing period, withdraw at least three gas samples at equal time intervals.
At samplings, connect a 50 milliliter syringe equipped with a one way stopcock to the sampling port. Then, open the two stopcocks and rinse the syringe by moving the plunger up and down three times before withdrawing 35 milliliters of chamber headspace. Finally, close the two stopcocks.
Disconnect the syringe from the sampling port and store it apart. Following the withdrawal of gas from the chamber headspace, transfer the samples to the evacuated vials quickly because plastic syringes may leak, even with a closed stopcock. Perform the transfer with a 25 gauge hypodermic needle.
First, fit the needle into the stopcock. Then, open it and flush the needle with five milliliters of sample. Insert the needle into the septum and push the remaining 30 milliliters of sample into a pre-evacuated vial before withdrawing the needle.
During the chamber closure, measure the headspace temperature every three to five minutes with a temperature data logger. Consider the sampling event complete after the closure period. Remove the lid and subsequently all used extensions.
After each sampling event, measure the headspace height of each chamber from the soil or from the ponding water using a folding ruler. At the end of each sampling event, transfer the vials to the laboratory for analysis. Determine gas concentration in the collected sample by gas chromatography and convert concentration into an absolute amount by Ideal Gas Law.
Depending on the emission pattern, estimate the flux choosing between a linear or non linear model. This is an example of seasonal variation of methane daily fluxes including both a cropping cycle and inter-cropping periods. As demonstrated with error bars, such results may vary greatly, mostly due to the spatial heterogeneity of microbial processes responsible for greenhouse gas production.
To address high variability that makes treatment differences impossible to detect, simply increase the number of replicates. In this example of seasonal variation of methane daily fluxes, there is an insufficient number of measuring events that do not cover all the pivotal moments for greenhouse gas emissions. As a result, it provides an undesired underestimation of yearly fluxes.
Shown here is an example of cumulative fluxes over a cropping season. Here, a sufficient number of measuring events were adopted. Conversely, in this case, the seasonal variation of fluxes were not sufficiently explored.
After watching this video, you should have a good understanding of how to apply a closed chamber technique for the evaluation of greenhouse gas emissions in the specific context of a paddy field. Once mastered, this technique can be applied in two to four hours depending on the number of monitored chambers. Several variations of the described technique are possible within the structure of the main principles.
For example, variations in chamber geometry, chambers'materials and type of greenhouse gas analysis may be explored. While attempting this procedure, it is important to remember that the trickiest critical point is probably the calculation of fluxes based on the greenhouse gas concentration variation during the chamber enclosure. Using the package HMR for calculation, remember to select the best model to apply on a case by case basis.
