The objective of the present study was to examine whether we could improve the aeration of clayey soils that had been degraded by long-term irrigation with treated wastewater (TWW) through drip irrigation with TWW aerated with oxygen (O 2) nanobubbles (ONB). A lysimeter setup was irrigated using surface and subsurface drip systems, and the effects of those systems on soil oxygen concentration, nitrogen transformations, nitrous oxide (N 2 O) emissions, and crop yield were investigated. In the surface drip system, irrigation with ONB-aerated TWW increased soil oxygen concentrations from 15.6% to 19.7% (p ¼ 0.0001). In the subsurface drip system, soil oxygen concentrations increased from 18.2% to 19.2% (p ¼ 0.0266). In all treatments, nitrate was the dominant N form in the root zone porewater (soil solution) and leachates. Nitrite concentrations were low in all treatments (<4 mg L À1), yet a clear daily accumulation pattern (from~0.05 to~1.0 mg L À1) was observed in the ONB-aerated treatments. Irrigation with ONB-aerated TWW reduced cumulative N 2 O emissions by 37% in the surface irrigation system and 14% in the subsurface irrigation system. Our results imply that irrigation with ONB-aerated TWW may be an effective way to improve soil aeration, especially in clayey soils that have been degraded by prolonged irrigation with TWW. Such practices may reduce N 2 O production and the overall N leakiness of agricultural activity.