GHG mitigation from sewage effluents: C sequestration after reuse in different urban cropping systems

Large volumes of greenhouse gases (GHGs) are emitted during conventional treatment of sewage. On the other hand, sewage reuse may enhance biomass production, improve soil carbon sequestration and carbon balance. Therefore, the carbon balance in plant biomass and soils was assessed in 8-year long experiment where food grain (FGPS), fodder (FPS), vegetable (VPS) and agro-forestry (AFS) production systems were irrigated either with sewage (SW) or groundwater (GW), along with the application of varying doses of N and P (25-100% of the recommended). The build-up in soil organic carbon (SOC) was 41-69% of that added through SW. The maximum annual increment in SOC occurred with AFS (1.59 Mg ha(-1)) followed by FPS (1.13 Mg ha(-1)), VPS (0.89 Mg ha(-1)) and FGPS (0.77 Mg ha(-1)). The total carbon stocks accumulated in soils and plant biomass, in terms of CO2 equivalence, were 33.4-39.8, 41.4-45.7, 35.6-41.6 and 14.5-18.4 Mg ha(-1) year(-1) for FGPS, AGFS, FPS and VPS, respectively, while those accumulated with 50-75% NP under SW irrigation were similar to 100% NP fertilizers under GW. These were equal to the GHGs released daily during conventional treatment of 0.42-1.06, 0.50-1.24, 0.44-1.09 and 0.20-0.48 million liters of sewage.