Constructed wetlands

Constructed wetlands can be used for reducing N discharge from wastewater or leaching from agricultural soils. Hernandez and Mitsch (2006) investigated

N2O fluxes induced by N retention in constructed riverbank marshes. Higher N2O fluxes occurred in higher marsh areas with periodical flooding than in the lower areas with permanent flooding. Wetland plants in the lower marshes favoured N2O fluxes due to aerenchyma transport. N2O emission in relation to denitrification, estimated using the static-core acetylene-inhibition method, was 0.005-0.03. This is about one order lower than the CEF3 of riparian wetlands receiving high agricultural N loads (Hefting et al, 2003, 2006). However, care must be taken when comparing these data since different methods were used to estimate denitrification. N2O fluxes from constructed wetlands for sewage treatment (Mander et al, 2003; Teiter and Mander, 2005) exhibited high N2O emissions per unit area (up to 590g N ha-1 day-1) but due to their relatively small surface area, the impact of these systems at larger scales is small. Because N2 fluxes had been determined in sediment cores incubated under N2-free atmospheres, CEF3 could be derived from the N2O to (N2+N2O) ratio, which ranged approximately from 0.001 to 0.01. Similar to the river bank marshes, this ratio and the total N2O fluxes were inversely related to the water level.

Until now, studies in riparian areas and constructed wetlands have focused on surface emissions, with few reports of N2O in the saturated zone of these systems (Blicher-Mathiesen and Hoffmann, 1999). It is thus not clear to what extent emissions originate from the saturated or unsaturated zone, respectively. Moreover the convective flux from riparian areas to streams is poorly known. Elevated N2O levels in river water draining arable land with intensive upland rice production in the Yangtze River delta in China are interpreted as resulting from convective N2O fluxes from riparian areas (Xiong et al, 2006). The significance of this path needs to be further investigated.

0 0

Post a comment