Treated wastewater disposal is a mounting problem faced by treatment facilities across California and US. This issue is particularly acute in rural, mountainous, and rapidly urbanizing regions like the Sierra Foothills and other rural communities where improvements in infrastructure are difficult to maintain. Treated municipal wastewater has been shown to be effective to grow high biomass used to provide electricity for wastewater treatment facility operations. The application of treated wastewater creates a potential for significant carbon (C) storage in woody biomass, but also, annual surface litter deposition and root turnover provides additional benefits of enhanced soil C sequestration. Furthermore, potential C and N losses to greenhouse gas emissions (GHG) may be potentially reduced through the formation of stable soil organic matter (SOM). The objective of this study was to investigate the effects of long-term wastewater irrigation on SOM and GHG emissions from pasture and poplars stands aged between two and seven years old. We hypothesized that treated wastewater irrigation would optimize poplar biomass production and lead to SOM formation reducing losses of C and N to greenhouse gases. Our results suggest that converting wastewater-irrigated pasture to poplar stand resulted in changes in belowground C and N pools. We observed a decline in DON, DOC, microbial biomass and potentially mineralizable N (PMN) concentrations in the upper 5 cm of soil. Older poplar stands accumulated more DOC with time in the subsurface soil horizons (5-15 and 15-30 cm) compared to younger poplar stands and pastures. Moreover, wastewater-irrigated pastures have the greatest CO2 flux. Converting pasture to young poplar stand significantly reduces CO2 emissions. Finally, soil beneath poplar stands became gradually N saturated as stands got older, resulting in higher litter N content, litter decomposability, and greater N2O flux to the atmosphere compared to pasturelands.