Plant species effects on methane emissions from freshwater marshes

Wetland plants not only stimulate CH4 emissions from wetlands to the atmosphere by providing the gas conduit and releasing organic compounds through root exudation and debris to increase CH4 production, but also reduce CH4 emissions by delivering O2 into the underground to accentuate CH4 oxidation in the rhizosphere. The capacity of the plants Carex lasiocarpa, Carex meyeriana and Deyeuxia angustifolia to transport CH4 from freshwater marshes in Sanjiang plain, China to the atmosphere was measured. Their integrated effects on dissolved porewater CH4 concentration and CH4 emission were studied by clipping plants just above the water surface. The amount and fraction of CH4 emissions via intact plants C. lasiocarpa, C. meyeriana and D. angustifolia was 16.0, 20.8 and 8.0 μg CH4 stem−1 h−1, and 73–82%, 75–86% and 28–31%, respectively, indicating that when CH4 is released by the diffusive rather than pressurized transport through the aerenchyma system, cyperaceous plants have a significantly higher gas transport capacity than gramineous plants. After plants were clipped 3 cm above the water surface, CH4 emissions and CH4 concentrations in the C. lasiocarpa marsh with a standing water depth of ∼20 cm increased significantly; only slight increases were measured in the C. meyeriana marsh with a standing water layer of ∼15 cm. The redox potentials (Eh) in the vertical profiles of both marshes were further lowered after plants were clipped. These results suggest that apart from being a conduit for gas transport, C. lasiocarpa made a greater contribution to CH4 oxidation than CH4 production, whereas C. meyeriana made nearly the same contribution to CH4 oxidation as to CH4 production. In contrast, a decrease in dissolved porewater CH4 concentration and a very limited increase in CH4 emission in the D. angustifolia marsh with a standing water depth of ∼5 cm indicated that D. angustifolia might make a greater contribution to CH4 production than to CH4 oxidation. Thereafter, as the standing water depth and the capacity of plants to transport CH4 from wetlands increased, plant integrated effect of stimulating CH4 production by releasing root exudates and debris minus accentuating CH4 oxidation by excreting O2 from roots not only decreased but also varied from increasing CH4 concentration for D. angustifolia to reducing CH4 concentration for C. lasiocarpa. The higher CH4 emission in the C. lasiocarpa marsh than in the D. angustifolia marsh was due to the high CH4 transport capacity of C. lasiocarpa rather than the stimulating effect of C. lasiocarpa on CH4 production. This finding suggests that when models are used for estimation of CH4 emissions from natural wetlands, difference in plant species effect on CH4 production and oxidation should be involved in.