The partitioning of N2O emissions between denitrification and other sources in natural and semi-natural land use types in the UK Fotis Sgouridis1 and Sami Ullah2 1University of Bristol, 2University of Birmingham, UK. ABSTRACT INTRODUCTION RESULTS DISCUSSION The partitioning of N2O emission to its different sources can further improve our understanding of the controls on the different microbial processes responsible for N2O production and consumption and ultimately inform GHG mitigation strategies. We have measured in situ N2O fluxes from natural and semi-natural ecosystems in two replicated UK catchments using an adapted 15N-Gas Flux method. The mean contribution of denitrification to the total N2O flux (DN2O/TN2O) ranged between 9 and 59 %. Soil moisture was the key environmental driver regulating the partitioning of N2O, which should be taken into account when developing Tier 2 emission factors for GHG mitigation. Natural and semi-natural terrestrial ecosystems (Fig. 1) have been under-represented in the UK greenhouse gas (GHG) inventory, thus increasing the uncertainty of annual GHG emission estimates. This uncertainty is further exacerbated by the high spatio-temporal variability of the processes responsible for nitrous oxide (N2O) emission. Total N2O fluxes were 40 times higher in the intensive grasslands than in the peatlands and ranged between 0.05 and 1.98 kg N ha-1 y-1 (Table 1). The mean contribution of denitrification to the total N2O flux (DN2O/TN2O) ranged between 9 and 59 % and was lowest in a well-drained forest and highest in a poorly-drained forest soil, while in peatlands and grassland soils it was 48% and 41% on average (Fig. 4). The 15N-N2O emitted from a labelled soil NO3- pool can be attributed to the process of denitrification (DN2O), while the unlabelled N2O flux could be due to autotrophic or heterotrophic nitrification and/or nitrifier denitrification. In a well-drained forest soil, nitrification was quantitatively more important than denitrification for N2O emission, counteracting the methane consumption (Table 1). At intermediate soil moistures (e.g. grasslands) both processes contribute equally to the N2O emission. In peatlands despite the high soil moisture only 48% of DN2O was measured, which warrants further investigation3. The emission factor of N deposition-induced N2O emissions from poorly drained forest, well drained forest and organic soils was 0.5, 1.6 and 0.3%, respectively (Table 1), which varied from the assumed 1% EF by the IPCC. a b c d Table 1: Total N2O fluxes, Global Warming Potential contribution of N2O and N2O emission factors across the land use types. Figure 4: Mean total N2O fluxes and the ratio DN2O/TN2O. OS= Peatlands; MW=Mixed woodland, DW=Deciduous woodland, SIG & IG= Semi and improved grasslands. Figure 1: Natural and semi-natural land use types in the UK: a) peat bog, b) unimproved grassland, c) mixed woodland and d) improved grassland. N2O (kg N ha-1 y-1) % GWPN2O+CH4 % EF Peatlands 0.05 2.5 0.3 Mixed woodland 0.26 148.9 1.6 Deciduous Woodland 0.10 6.2 0.5 Semi-improved grassland 0.65 85.4 0.4 Improved grassland 1.98 100 0.9 The partitioning of N2O sources under field conditions and across a wide range of land use types using 15N tracers, which has not previously been done, can further improve our understanding of the controls on the different processes and ultimately inform mitigation strategies. Total dissolved nitrogen availability was the key driver of N2O emissions across land use types (Fig. 5a). Other important controlling factors were carbon availability and soil moisture. Soil moisture was the key environmental driver regulating the partitioning of N2O between denitrification and other sources (Fig. 5b). STUDY SITES AND METHODS The Conwy and the Ribble-Wyre River catchments (Figs. 2&3) were selected as representative UK catchments. In situ N2O fluxes were measured monthly between April 2013 and October 2014 using static chambers1. An adapted 15N-Gas Flux method2 for low level additions of 15N tracer (0.03 - 0.5 kg 15N ha-1), appropriate for natural (unfertilised) ecosystems, was used to elucidate the relative contribution of denitrification to net N2O production (DN2O/TN2O). CONCLUSIONS Up to 60 % of N2O emission was attributed to denitrification, while the quantitative importance of both nitrification and denitrification as N2O sources varied across land use types primarily driven by differences in soil moisture content. The assumption that 1% of the deposited N on natural ecosystems is emitted as N2O, may overestimate this source in the case of natural organic soils or underestimate it in the case of N-rich well drained forest soils. C-MW C-AG C-PB C-IG R-IG R-SIG R-HL R-DW CONTACT Fotis Sgouridis School of Geographical Sciences University of Bristol Email: f.sgouridis@bristol.ac.uk Phone: +44 (0) 1179 288078 REFERENCES Sgouridis F & Ullah S, 2015. Env. Sci. & Technology, 49(24), 14110-14119. Sgouridis F, Stott A & Ullah S, 2016. Biogeosciences, 13, 1821-1835. Sgouridis F & Ullah S, 2017. JGR-Biogiosciences (in review). Figure 2: Conwy sites: C-PB =peat bog; C-AG =acid grassland; C-IG = improved grassland; C-MW = mixed woodland. Figure 3: Ribble sites: R-SIG = unimproved grassland; R-IG = improved grassland; R-HL = heathland; R-DW = deciduous w. Figure 5: Linear Regressions between: (a) log10-transformed total nitrous oxide fluxes and total dissolved nitrogen and (b) the ratio DN2O/TN2O and volumetric water content per plot averaged over 17 months (n = 40).