Urbanisation-related land use change from forest and pasture into turf grass modifies soil nitrogen cycling and increases N₂O emissions

Abstract. Urbanisation is becoming increasingly important in terms of climate change and ecosystem functionality worldwide. We are only beginning to understand how the processes of urbanisation influence ecosystem dynamics, making peri-urban environments more vulnerable to nutrient losses. Brisbane in South East Queensland has the most extensive urban sprawl of all Australian cities. This research estimated the environmental impact of land use change associated with urbanisation by examining soil nitrogen (N) turnover and subsequent nitrous oxide (N₂O) emissions using a fully automated system that measured emissions on a sub-daily basis. There was no significant difference in soil N₂O emissions between the native dry sclerophyll eucalypt forest and an extensively grazed pasture, wherefrom only low annual emissions were observed amounting to 0.1 and 0.2 kg N₂O ha⁻¹ yr⁻¹, respectively. The establishment of a fertilised turf grass lawn increased soil N₂O emissions 18-fold (1.8 kg N₂O ha⁻¹ yr⁻¹), with highest emissions occurring in the first 2 months after establishment. Once established, the turf grass lawn presented relatively low N₂O emissions for the rest of the year, even after fertilisation and rain events. Soil moisture was significantly higher, and mineralised N accumulated in the fallow plots, resulting in the highest N₂O emissions (2.8 kg N₂O ha⁻¹ yr⁻¹) and significant nitrate (NO₃⁻) losses, with up to 63 kg N ha⁻¹ lost from a single rain event due to reduced plant cover removal. The study concludes that urbanisation processes creating peri-urban ecosystems can greatly modify N cycling and increase the potential for losses in the form of N₂O and NO₃⁻.