Characteristics of annual N2O and NO fluxes from Chinese urban turfgrasses

Urban turfgrass ecosystems are expected to increase at unprecedented rates in upcoming decades, due to the increasing population density and urban sprawl worldwide. However, so far urban turfgrasses are among the least understood of all terrestrial ecosystems concerning their impact on biogeochemical N cycling and associated nitrous oxide (N2O) and nitric oxide (NO) fluxes. In this study, we aimed to characterize and quantify annual N2O and NO fluxes from urban turfgrasses dominated by either C4, warm-season species or C3, cool-season and shadeenduring species, based on year-round field measurements in Beijing, China. Our results showed that soil N2O and NO fluxes varied substantially within the studied year, characterizing by higher emissions during the growing season and lower fluxes during the non-growing season. The regression model fitted by soil temperature and soil water content explained approximately 50%-70% and 31%-38% of the variance in N2O and NO fluxes, respectively. Annual cumulative emissions for all urban turfgrasses ranged from 0.75 to 1.27 kg N ha(-1) yr(-1) for N2O and from 0.30 to 0.46 kg N ha(-1) yr(-1) for NO, both are generally higher than those of Chinese natural grasslands. Non-growing season fluxes contributed 17%-37% and 23%-30% to the annual budgets of N2O and NO, respectively. Our results also showed that compared to the cool-season turfgrass, annual N2O and NO emissions were greatly reduced by the warm-season turfgrass, with the high root system limiting the availability of inorganic N substrates to soil microbial processes of nitrification and denitrification. This study indicates the importance of enhanced N retention of urban turfgrasses through the management of effective species for alleviating the potential environmental impacts of these rapidly expanding ecosystems.