A self-deigned pond with different height steps was used to simulate the real flood current and ebb current by controlling water influx and efflux, and improved static closed chambers were placed on two steps of different heights to study methane facilitated by P. australis from different submergence condition, long-term submergence and short-term submergence, respectively. The results showed that mean methane emissions from the site of long-term submergence with and without P. australis were 1.60 mgCH₄ m^-2h^-1 and 0.50 mgCH₄ m^-2h^-1, respectively, while mean methane emissions from the site of short-term submergence with and without P. australis were 0.94 mgCH₄ m^-2h^-1 and 0.55 mgCH₄ m^-2h^-1, respectively. These sites at which P. australis were clipped had lower emission rates indicating an overall stimulating effect of plants on methane emission. The P. australis contributed 41.5%-69% of the total methane emission. In the site of long-term submergence, methane emission reduced obviously when the P. australis was clipped. During a cycle of flood current and ebb current, from pre-submergence, during the period of submergence to post-submergence, methane emissions from the site of long-term submergence and the site of short-term submergence reached the minimum (1.21 mgCH₄ m^-2h^-1) and maximum (1.18 mgCH₄ m^-2h^-1) when sites were submerged, and methane emission during the period of submergence differed significantly from pre-submergence and post-submergence, indicating that the transport effect of P. australis was affected directly by tidal submergence.
Incubation experiment was further conducted to study other possible reasons of different methane emissions besides transport effect. The study mainly focused on aerobic methane emission from aboveground tissues of P. australis. The stems and leaves were cut into small sections and sealed for measuring methane emission from these detached tissues. The results showed that methane emissions from detached stems were 0.2-0.7μL/L. Physically cutting the stems hastened elimination of methane from the tissues in the incubation and then the stems ceased to emit methane, suggesting a microbial origin. P. australis transported microbially produced methane from wetland soil to the atmosphere. Methane concentration in the tissues of P. australis existed gradient change. Higher methane emission in the lower parts of stems than that in the higher parts of stems and leaves confirmed the transport effect of P. australis. Considering about the lowest methane emission from the site of long-term submergence happened in the period of submergence, our results may imply transport effect mainly exists in the lower parts of stems but not in leaves.
In summary, our results highlight the transport effect of P. australis on methane emission and the influence of tidal submergence. P. australis increased methane emission from estuarine wetland by facilitating methane from soil to the atmosphere, but not by producing and emitting methane under aerobic conditions. The tidal submergence increased methane emission from Phragmites wetland by affecting methane production and reduced methane emission probably by blocking the primary sites of methane release in the lower part of the plant stems. When the lower part of the plant stems was under water, methane emission to the atmosphere may decreased.