No abstract

Nitrous oxide (N 2 O) fluxes from soil under mown grassland were monitored using static chambers over three growing seasons in intensively and extensively managed systems in Central Switzerland. Emissions were largest following the application of mineral (NH 4 NO 3 ) fertilizer, but there were also substantial emissions following cattle slurry application, after grass cuts and during the thawing of frozen soil. Continuous flux sampling, using automatic chambers, showed marked diurnal patterns in N 2 O fluxes during emission peaks, with highest values in the afternoon. Net uptake fluxes of N 2 O and subambient N 2 O concentrations in soil open pore space were frequently measured on both fields. Flux integration over 2.5 years yields a cumulated emission of 1 4.7 kg N 2 O-N ha À1 for the intensively managed field, equivalent to an average emission factor of 1.1%, and a small net sink activity of À0.4 kg N 2 O-N ha À1 for the unfertilized system. The data suggest the existence of a consumption mechanism for N 2 O in dry, areated soil conditions, which cannot be explained by conventional anaerobic denitrification. The effect of fertilization on greenhouse gas budgets of grassland at the ecosystem level is discussed.

N 2 O is a potent greenhouse gas involved in the destruction of the protective ozone layer in the stratosphere and contributing to global warming. The ecological processes regulating its emissions from soil are still poorly understood. Here, we show that the presence of arbuscular mycorrhizal fungi (AMF), a dominant group of soil fungi, which form symbiotic associations with the majority of land plants and which influence a range of important ecosystem functions, can induce a reduction in N 2 O emissions from soil. To test for a functional relationship between AMF and N 2 O emissions, we manipulated the abundance of AMF in two independent greenhouse experiments using two different approaches (sterilized and re-inoculated soil and non-mycorrhizal tomato mutants) and two different soils. N 2 O emissions were increased by 42 and 33% in microcosms with reduced AMF abundance compared to microcosms with a well-established AMF community, suggesting that AMF regulate N 2 O emissions. This could partly be explained by increased N immobilization into microbial or plant biomass, reduced concentrations of mineral soil N as a substrate for N 2 O emission and altered water relations. Moreover, the abundance of key genes responsible for N 2 O production (nirK) was negatively and for N 2 O consumption (nosZ) positively correlated to AMF abundance, indicating that the regulation of N 2 O emissions is transmitted by AMF-induced changes in the soil microbial community. Our results suggest that the disruption of the AMF symbiosis through intensification of agricultural practices may further contribute to increased N 2 O emissions.

No abstract