Gaseous fluxes in the nitrogen and carbon budgets of subsurface flow constructed wetlands

In 2001 and 2002, fluxes of N₂O, CH₄, CO₂ and N₂ were measured in two constructed wetlands (CW) for domestic wastewater treatment in Estonia. The difference between the median values of N₂O, CH₄, and N₂ fluxes in the horizontal subsurface flow (HSSF) CWs was non-significant, being 1.3-1.4 and 1.4-4.1 mg m⁻² d⁻¹ for N₂O-N and CH₄-C, and 0.16-0.17 g N m⁻² d⁻¹ for N₂-N respectively. The CO₂-C flux was significantly lower (0.6 g C m⁻² d⁻¹) in one of the HSSF filters of a hybrid CW, whereas the single HSSF and VSSF filters emitted 1.7 and 2.0 g C m⁻² d⁻¹. The median value of CH₄-C emission in CWs varied from 1.4 to 42.6 g C m⁻² d⁻¹, being significantly higher in the VSSF filter beds. We also estimated C and N budgets in one of the HSSF CWs (312.5 m²) for 2001 and 2002. The total C input into this system was similar in 2001 and 2002, 772 and 719 kg C year⁻¹, but was differently distributed between constituent fluxes. In 2001, the main input flux was soil and microbial accumulation (663 kg C year⁻¹ or 85.8% of total C input), followed by plant net primary production (NPP) (10.2%) and wastewater inflow (3.9%). In 2002, 55.7% of annual C input was bound in plant NPP, whereas the increase in soil C formed 28.5% and wastewater inflow 15.7%. The main C output flux was soil respiration, including microbial respiration from soil and litter, and the respiration of roots and rhizomes. It formed 120 (97.5%) and 230 kg C year⁻¹ (98.2%) in 2001 and 2002 respectively. The measured CH₄-C flux remained below 0.1% of total C output. The HSSF CW was generally found to be a strong C sink, and its annual C sequestration was 649 and 484 kg C year⁻¹ per wetland in 2001 and 2002 respectively. However, negative soil and microbial accumulation values in recent years indicate decreasing C sequestration. The average annual N removal from the system was 38-59 kg N year⁻¹ (46-48% of the initial total N loading). The most important flux of the N budget was N₂-N emission (22.7 kg in 2001 and 15.2 kg in 2002), followed by plant belowground assimilation (2.3 and 11.9 kg N year⁻¹ in 2001 and 2002), and above-ground assimilation (1.9 and 9.2 kg N year⁻¹, respectively). N₂O emission was low: 0.37-0.60 kg N year⁻¹ .