Soil N cycling processes in a pasture after the cessation of grazing and CO2 enrichment

The effect of elevated CO2 (eCO2) on nitrous oxide (N2O) emissions is important as it could negate some of the potential benefits of eCO2 on carbon sink activity and thus affect the concentration of greenhouse gases in the atmosphere. Grazed grassland is a significant source of N2O but no studies have so far measured eCO2 effects on processes related to N2O production in grassland grazed by animals. Here we examine microbial functional genes related to N2O production in grassland that had received 14 years of CO2 enrichment. The measurements were made 18 months after enrichment was stopped and in areas that allowed us to distinguish between grazing with and without the return of animal excreta. Our results showed that grazing with excreta return compared to grazing with no excreta return had no effect on soil pH, but had a positive effect on soil total nitrogen (TN), soil NO3- concentration, nosZ (nitrous oxide reductase) gene abundance, and NEA/DEA (nitrifying enzyme activity/denitrifying enzyme activity). Elevated CO2 significantly increased soil NH4+ concentration and nosZ abundance (in the area where excreta were returned) but reduced the activity of NEA and ANEA (autotrophic nitrifying enzymes) and DEAN2 (the potential flux of N2 from denitrification). In addition, HNEA (heterotrophic nitrifying enzyme activity) was only evident under eCO2. These responses indicate long-term effects of eCO2 on processes promoting nitrous oxide production.