Emission of N2O, N2, CH4, and CO2 from constructed wetlands for wastewater treatment and from riparian buffer zones

We measured nitrous oxide (N2O), dinitrogen (N-2) methane (CH4), and carbon dioxide (CO2) fluxes in horizontal and vertical flow constructed wetlands (CW) and in a riparian alder stand in southern Estonia using the closed chamber method in the period from October 2001 to November 2003. The replicates' average values of N2O, N-2, CH4 and CO2 fluxes from the riparian gray alder stand varied from -0.4 to 58 mu g N2O-N m(-2) h(-1), 0.02-17.4 mg N-2-N m(-2) h(-1), 0.1-265 mu g CH4-C m(-2) h(-1) and 55-61 mg CO2-C m(-2) h(-1), respectively. In horizontal subsurface flow (HSSF) beds of CWs, the average N-2 emission varied from 0.17 to 130 and from 0.33 to 119 mg N-2-N m(-2) h(-1) in the vertical subsurface flow (VSSF) beds. The average N2O-N emission from the microsites above the inflow pipes of the HSSF CWs was 6.4-31 mu g N2O-N m(-2) h(-1), whereas the outflow microsites emitted 2.4-8 mu g N2O-N m(-2) h(-1). In VSSF beds, the same value was 35.6-44.7 mu g N2O-N M-2 h(-1). The average CH4 emission from the inflow and outflow microsites in the HSSF CWs differed significantly, ranging from 640 to 9715 and from 30 to 770 mu g CH4-C m(-2) h(-1), respectively. The average CO2 emission was somewhat higher in VSSF beds (140-291 Mg CO2-C m(-2) h(-1)) and at the inflow microsites of HSSF beds (61-140 mg CO2-C m(-2) h(-1)). The global warming potential (GWP) from N2O and CH4 was comparatively high in both types of CWs (4.8 +/- 9.8 and 6.8 +/- 16.2 +/- CO2 eq ha(-1) a(-1) in the HSSF CW 6.5 +/- 13.0 and 5.3 +/- 24.7 t CO2 eq ha(-1) a(-1) in the hybrid CW, respectively). The GWP of the riparian alder forest from both N2O and CH4 was relatively low (0.4 +/- 1.0 and 0.1 +/- 0.30 t CO2 eq ha(-1) a(-1), respectively), whereas the CO2-C flux was remarkable (3.5 +/- 3.7 t ha(-1) a(-1)). The global influence of CWs is not significant. Even if all global domestic wastewater were treated by wetlands, their share of the trace gas emission budget would be less than 1%. (c) 2005 Elsevier B.V. All rights reserved.