How do soil temperature and moisture regulate N2O flux from an urban lawn?

Background/Question/Methods

Due to rapid urban population increase, the area of urban landscapes over the last several decades have expanded rapidly in the United States and other developed countries. On average, about one third of urban surface area in the United States is covered by lawn. Like agricultural fields, lawn is also an anthropogenic nitrous oxide (N₂O) source. However, the N₂O flux from urban lawns are less studied than fluxes from agricultural fields. N₂O flux is one of major components in the nitrogen cycle. N₂O is also a potent greenhouse gas with a global warming potential 300 times greater than CO₂. To better understand the seasonal dynamic of N₂O flux over urban ecosystems and how soil temperature and moisture regulate the flux, we have been measuring the flux continuously since the early summer of 2019 over a lawn in Lincoln, Nebraska. The measurement was done with an automated 4-chamber multiplexer system coupled with a N₂O gas analyzer. The minimum detectable N₂O flux of the multiplexer system is 0.05 nmol m^-2s^-1. The flux from four locations at our study site were sequentially measured every 30 min. Soil temperature and soil moisture at the depth of 5 cm were also measured. The grass type was a fescue with a mean vegetation height of approximately 5 cm and was not irrigated and had never been fertilized.

Results/Conclusions

Preliminary results from these long-term continuous N₂O flux measurements show the following. First, the flux ranged from 1.4 nmol m^-2s^-1 in the summer to almost zero in the winter, depending strongly on the soil moisture and soil temperature. The high flux values observed from the summertime were comparable to that of published values from agricultural fields. In early winter, N₂O flux almost stopped when the soil temperature at a 5 cm depth dropped below 5^oC. Second, soil temperature was the main driver for the diurnal variation in N₂O flux. Third, rain events enhanced the N₂O flux quickly, especially in the summer when the soil moisture was below 0.2 v/v. Fourth, our preliminary results seem to indicate that during the wintertime the N₂O flux increased rapidly once the ice and snow were melted. We observed the flux jumped from zero to up to 1.2 nmol m^-2s^-1 in early February 2020 when the soil temperature was still around zero. The implications of these preliminary results will be discussed.