The chamber methods were compared on low emission sub-surface soil and high emission surface soil in this study. The experiment was conducted in a closed room without any exhaust fan to maintain a constant temperature.
3.2.1 Description of soils
The soil used in this study was a Manawatu fine sandy loam (Typic Fluvial Recent Soil) under a permanent ryegrass pasture and had micaceous clay mineralogy (Saggar et al. 1999). Soils for the high and low emission studies were sampled from 0-6 cm and 2.5-7.5 cm depth. The reason for this was based on the root density differences reported by Kusumo (2009), that the surface soils would give 13.2 mg dry root g-1 dry soil to decompose compared with 6.1 mg dry root g-1dry soil at 2.5-7.5 cm soil depth.
For the low emission study, a sub-surface soil sample (2.5-7.5 cm depth; total C= 30 g kg-1, total N= 3.0 g kg-1, pH= 5.4, Olsen P= 24.6 mg kg-1 soil, exchangeable K= 249.6 mg kg-1 soil, CEC= 16 cmolc kg-1) was collected from one spot at 2.5-7.5cm depth avoiding the
dense pasture root mass contribution to readily decomposable C in autumn (March 2009). The sub-surface soil sample was passed through a 5mm sieve and stored at 20ºC for 2 weeks, allowing some decomposition of the readily decomposable C. This was a strategy to produce a low emission soil. It was assumed that two weeks storage would be sufficient to decompose the readily decomposable C. Moisture content of the soil was determined on the day when soil was put into each of the six closed-base replicate PVC chambers (23.5 cm diameter; 16 cm high). Two kg of field moist sub-surface soil (22.1 % gravimetric moisture content) was transferred into each of the six replicate chambers and firmly packed to obtain a final bulk density of 0.98 g cm-3. Moisture content in the replicate chambers was maintained throughout the experiment by weighing the chambers and spraying the required amount of deionised water onto the surface.
For the high emission study, surface soil samples (0-6.0 cm depth; total C= 32 g kg-1, total N= 3.2 g kg-1, pH= 5.3, Olsen P= 64.9 mg kg-1 soil, exchangeable K= 292.5 mg kg-1 soil, CEC= 17 cmolc kg-1) were collected from eight randomly selected areas in winter (June
2009). Three (6 cm depth and 10.5 cm diameter) cores were taken from each replicate area, sprayed with glyphosate and left in the glasshouse for 7 days to kill the surface grass. Each core was then cut into pieces and the soils from the three cores of each replicate were combined, weighed and then put in the chambers. Moisture content of the soil was determined at this time. Each closed-base chamber was filled with 2276 g of field moist surface soil (34.0 % gravimetric moisture content) and firmly packed to obtain a final bulk density of 0.86 g cm-3. Similar amounts of field moist soil were filled in the eight chambers representing eight replicates. Moisture content did not vary much between the eight replicate chambers but differences in root masses were not accounted for. The total C content given for the surface soil is of a 2 mm sieved ring ground sample and does not take roots present in the soil into account. Moisture content in the replicate chambers was maintained throughout the experiment by weighing the chambers and spraying the required amount of deionised water onto the surface.
3.2.2 Measurement of CO2 flux
Low emission study: Static alkali trap chamber method and infrared gas analyser-
EGM-1were compared on the low emission sub-surface soil.
3.2.2.1 Static alkali trap chamber method: Carbon dioxide flux was determined by absorbing CO2 using 30 ml of NaOH solution of two concentrations (0.5M and 1.0M) each in a sealed
chamber headspace for a specific period of time. A plastic petri dish covering 13.1% of a chamber’s area was used for storing 30 ml of NaOH solution. The petri dish was elevated approximately 7 cm above the soil surface in the centre of the chamber. Two horizontal sticks were placed across the chamber to elevate the petri dish. For the six replicated chambers, 0.5M and 1.0M NaOH solutions were placed respectively in three chambers each. The enclosure times were 4, 8, 12, 16, 20 and 24 hours for absorbing CO2 from each
chamber. After every absorption interval the chambers were opened, NaOH solution from each chamber was taken out and stored in a plastic container for titration to calculate the amount of CO2 emitted during that particular absorption interval. The amount of CO2 emitted
was further divided by the interval duration to calculate the hourly rate. The chambers were left open for 45 minutes to remove the existing CO2 concentration built up in the chambers.
Thereafter, the measurement of CO2 flux from each chamber was taken using EGM-1. After
taking the measurements with EGM-1, fresh alkali solutions were placed in each chamber for the next interval and the procedure was repeated after every absorption interval. Different absorption intervals were not compared simultaneously as it was assumed that low emission soil will give constant CO2 flux throughout the study period. The total amount of CO2
absorbed in NaOH solution was determined by back-titrating the excess NaOH with 0.2M HCl after precipitation of carbonates with BaCl2. All the chemicals used were of analytical
grade and solutions were prepared using deionised water. The solution of 0.2M HCl was standardised against Na2CO3 as per the method outlined by Lambert et al. (1949). The
chamber headspace volume was 5282 ml. The seal between the chamber top and its permanently installed base was perfect. Each chamber had an internal half-turn locking system and a greased O-ring which formed a gas-tight seal when closed with a lid.
3.2.2.2 Infrared (IR) gas analyser-Dynamic chamber method: Carbon dioxide flux was measured by a dynamic chamber (10cm diameter; 15cm high) coupled to a portable infrared gas analyzer (IRGA) in a closed circuit (EGM-1 equipped with SRC-1, PP systems). Mixing of the air in the closed soil respiration chamber (SRC) during measurement was ensured by a small fan running inside the chamber. The fan speed was 0.5 m s-1 and there was a mesh
screen between the fan and the soil surface to slow the air velocity at the soil surface. The mixed air was circulated (flow rate 200-400 ml min-1) from the chamber into the IRGA sensor cell and back to the chamber by a pump in the EGM-1. The IRGA contained software to calculate CO2 flux rates and each measurement took less than 2 minutes (PP systems
2010). Before each flux measurement, the SRC was moved away from the closed base chamber filled with the soil and the SRC mixing fan ran for 10s to restore the IRGA sensor cell to the ambient CO2 concentration in the room. Three CO2 flux measurements with
EGM-1 were made from three different locations within each chamber at the end of every absorption interval i.e. 4, 8, 12, 16, 20, 24 hours of the alkali trap method. The mean of three flux values are stated as the flux value from each replicate chamber. In this study a soil collar for the placement and formation of a seal for the dynamic chamber was not used.
High emission study: Static alkali trap chamber, infrared gas analyser-EGM-1 and static
chamber flux gradient methods were compared on high emission surface soils. Carbon dioxide measurements were made 2 hours after filling the chambers with soil.
3.2.2.3 Static chamber flux gradient method: Carbon dioxide gas samples (25 ml) were taken for a period of one hour with 60 ml polypropylene syringes fitted with 3-way stopcocks after sealing the chambers with a lid having one port. Five gas samples (25 ml each) were taken from each chamber at times t0, t10, t20, t30 and t60 (i.e. 0 min, 10 min, 20 min, 30 min and 60 min respectively, after closing the chamber). The gas samples were transferred to 12 ml evacuated vials and then analyzed using a Shimadzu GC- 17A gas chromatograph; CO2 flux
(mg m-2 hr-1) was estimated from the measurements made at different time intervals. The sample of the ambient air taken just after closing the chamber (t0) was used as a reference for calculating CO2 fluxes. Carbon dioxide gas samples (25 ml) were taken by syringe from
all the eight replicate chambers. After completion of a one hour measurement period, chambers were left open for 45 minutes to remove the existing CO2 concentration built up in
the chambers. Thereafter, 1.0 M NaOH traps were placed in the chambers to absorb CO2 for
4 hours. The seal between the chamber top and its permanently installed base was perfect. Each chamber had an internal half-turn locking system and a greased O-ring which formed a gas-tight seal when closed with a lid. The chamber headspace volume was 4937 ml.
3.2.2.4 Static chamber alkali trap method: Carbon dioxide flux rates were determined by absorbing CO2 in 1.0M NaOH placed in a plastic petri dish (13.1% of the chamber area) in
3.2.2.5 Infrared (IR) gas analyser-Dynamic chamber method: Three measurements from different locations within each chamber were made at the end of a 4 hour absorption interval, as for the alkali trap method.
3.2.3 Statistical analysis
An analysis of variance using SAS software (9.1) was performed on carbon dioxide fluxes measured by the three methods using the General Linear Model (GLM) procedure. Mean comparisons between CO2 fluxes were done using Fisher’s least significant difference
(LSD) at 5% level of significance.