2.2. Gas sampling and flux measurement
Gas samplings were carried out on the fourth day after the largest
spring tide in January, April, July and October of 2013, respectively,
each of which corresponds to local climate of winter, spring, summer
and autumn. Gas samples were collected simultaneously from the four
sites following the enclosed chamber technique described by Chen
et al. (2010) and all samplings were done 2 h before the lowest ebb
tide (Chen et al., 2010). To minimize the disturbance by human activity
while sampling, a wooden plank was deployed within each site one day
before sampling. A total of three vented static chambers were deployed
within each site with one in each transect. The static chambers were
made of clear Perspex with a sharpened base to aid insertion into coastal
sediments. The chambers were cylindrical with a bottom radius of 5 cm
and a height of 17 cm. A metal vent tube with a diameter of 1 mm was
fitted on the side surface of the cylinder (about 8 cm above the ground)
to allow equalization of pressure perturbations between the chamber and
ambient atmospheres, and silicone putty was used to ensure a gas tight
seal around the vent tube. The gas sampling hole was on top of the chamber
with a diameter of 3 cm. After the chamber was inserted into the soil,
the hole was gently plugged with a rubber stopper and sealed by silicone
putty. Gas samples were collected via 10 ml glass syringe by penetrating
the rubber with the attached hypodermic needle into the chamber.