Emission of N 2O, N 2, CH 4, and CO 2 from constructed wetlands for wastewater treatment and from riparian buffer zones

Abstract

We measured nitrous oxide (N 2O), dinitrogen (N 2), methane (CH 4), and carbon dioxide (CO 2) 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 N 2O, N 2, CH 4 and CO 2 fluxes from the riparian gray alder stand varied from −0.4 to 58 μg N 2O-N m −2 h −1, 0.02–17.4 mg N 2-N m −2 h −1, 0.1–265 μg CH 4-C m −2 h −1 and 55–61 mg CO 2-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 N 2O-N emission from the microsites above the inflow pipes of the HSSF CWs was 6.4–31 μg N 2O-N m −2 h −1, whereas the outflow microsites emitted 2.4–8 μg N 2O-N m −2 h −1. In VSSF beds, the same value was 35.6–44.7 μg N 2O-N m −2 h −1. The average CH 4 emission from the inflow and outflow microsites in the HSSF CWs differed significantly, ranging from 640 to 9715 and from 30 to 770 μg CH 4-C m −2 h −1, respectively. The average CO 2 emission was somewhat higher in VSSF beds (140–291 mg CO 2-C m −2 h −1) and at the inflow microsites of HSSF beds (61–140 mg CO 2-C m −2 h −1). The global warming potential (GWP) from N 2O and CH 4 was comparatively high in both types of CWs (4.8 ± 9.8 and 6.8 ± 16.2 t CO 2 eq ha −1 a −1 in the HSSF CW 6.5 ± 13.0 and 5.3 ± 24.7 t CO 2 eq ha −1 a −1 in the hybrid CW, respectively). The GWP of the riparian alder forest from both N 2O and CH 4 was relatively low (0.4 ± 1.0 and 0.1 ± 0.30 t CO 2 eq ha −1 a −1, respectively), whereas the CO 2-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%.