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  • 1. 1Application of the DNDC model to predict emissions of N 2O from Irish 2agriculture. 3 4M. Abdalla1, M. Wattenbach2, P. Smith2, P. Ambus3, M. Jones1 and M. Williams1 5 61Department of Botany, School of Natural Sciences, Trinity College Dublin, Dublin 2, Ireland 72School of Biological Sciences, University of Aberdeen, Cruickshank Building, St. Machar Drive, 8Aberdeen, AB24 3UU, UK. 93Riso Research Centre, Technical University of Denmark, Frederikborgvej 399, DK-4000,10Roskilde1112Key words: Nitrous oxide, DNDC model, arable, pasture1314ABSTRACT1516A mechanistic model that describes N fluxes from the soil, DeNitrification17DeComposition (DNDC), was tested against seasonal and annual data sets of nitrous18oxide flux from a spring barley field and a cut and grazed pasture at the Teagasc Oak19Park Research Centre, Co. Carlow, Ireland. In the case of the arable field, predicted20fluxes of N2O agreed well with measured fluxes for medium to high fertilizer input values21(70 to 160 kg N ha-1) but described poorly measured fluxes from zero fertilizer22treatments. In terms of cumulative flux values, the relative deviation of the predicted23fluxes from the measured values was a maximum of 6% for the highest N fertilizer inputs24but increased to 30% for the medium N and more than 100% for the zero N fertilizer25treatments. A linear correlation of predicted against measured flux values for all fertilizer26treatments (r2 = 0.85) was produced, the equation of which underestimated the seasonal27flux by 24%. Incorporation of literature values from a range of different studies on arable28and pasture land did not significantly affect the regression slope. DNDC describe poorly29measured fluxes of N2O from reduced tillage plots of spring barley. Predicted cumulative30fluxes of N2O on plots disc ploughed to 10cm, underestimated measured values by up to3155%.32 11

2. 1For the cut and grazed pasture the relative deviations of predicted to measured fluxes 2were 150 and 360% for fertilized and unfertilized plots. This poor model fit is considered 3due to DNDC overestimating the effect of initial soil organic carbon (SOC) on N 2O flux, 4as confirmed by a sensitivity analysis of the model. As the arable and grassland soils 5differed only in SOC content, reducing SOC to the arable field value significantly 6improved the fit of the model to measured data such that the relative deviations decreased 7to 9 and 5% respectively. Sensitivity analysis highlighted air temperature as the main 8determinant of N2O flux, an increase in mean daily air temperature of 1.5oC resulting in 9almost 90% increase in the annual cumulative flux. Using the Hadley Centre Global10Climate Model data (HCM3) and the IPCC emission scenarios A2 and B2, DNDC11predicted increases in N2O fluxes of approximately 30% (B2) and 60% (A2) from the12spring barley field and approximately 20% (A2 and B2) from the cut and grazed pasture13by the end of this centaury (2061-2090).141516171819202122232425262728293031 12 3. 1INTRODUCTION 2 3National inventories of N2O fluxes from agricultural soils, as required by signatory 4countries to the United Nations Framework Convention of Climate Change (UNFCC), 5are in the main derived from the use of the default IPCC Tier 1 method, where 1.25% of 6applied inorganic nitrogen to agricultural soils is assumed to be released to the 7atmosphere as nitrous oxide-N (Bouwman, 1996; IPCC, 1997; 2000). This standard 8reporting procedure has advantages in collating annual inventories but may mask 9significant variations in emission factors (EFs) on a regional scale (Schmid et al., 2001;10Laegreid and Aastveit, 2002). For instance in Ireland, published EFs derived from field11measurements of N2O using either eddy covariance or static chamber methods vary from123.4% for Cork grassland and 0.7 to 4.9% of the applied N fertilizer for the Wexford13grassland depending on soil type, land management, climate and year (Hsieh et al., 2005;14Hyde et al., 2005; Flechard et al., 2007).1516Given the considerable expense of establishing and maintaining relevant flux17measurement sites, the use of simulation models to estimate N 2O fluxes from agricultural18soils using soil and climate data has obvious benefits. Modelling also allows easy19interpretation of the complex links between soil physical, chemical and microbial20processes that underpin nitrification, denitrification and decomposition. Models can21simulate the processes responsible for production, consumption and transport of N 2O in22both the long and short term, and also on a spatial scale (Williams et al., 1992).2324Simulation models range from simple empirical relationships based on statistical analyses25to complex mechanistic models that consider all factors affecting N 2O production in the26soil (Li et al., 1992; Frolking et al., 1998; Stenger et al., 1999; Freibauer 2003; Roelandt27et al., 2005; Jinguo et al., 2006). Variations in soil moisture, soil temperature, carbon and28nitrogen substrate for microbial nitrification and denitrification are critical to the29determination of N2O emissions (Leffelaar and Wessel, 1988; Tanji, 1982; Frissel and30Van Veen, 1981; Batlach and Tiedje, 1981; Cho et al., 1979). One widely used31mechanistic model is DeNitrification DeComposition (DNDC) developed to assess N 2O, 13 4. 1NO, N2 and CO2 emissions from agricultural soils (Li et al., 1992a, 1994; Li 2000). The 2rainfall driven process-based model DNDC (Li et al., 1992) was originally written for 3USA conditions. It has been used for simulation at a regional scale for the United States 4(Li et al., 1996) and China (Li et al., 2001). Advantages of DNDC are that it has been 5extensively tested and has shown reasonable agreement between measured and modelled 6results for many different ecosystems such as grassland (Brown et al., 2001; Hsieh et al., 72005; Saggar et al., 2007), cropland (Li, 2003; Cai et al., 2003, Yeluripati et al., 2006; 8Pathak et al., 2006; Tang et al., 2006) and forest (Li, 2000; Stange et al., 2000; Kesik et 9al., 2006). The model has reasonable data requirement and is suitable for simulation at10appropriate temporal and spatial scales.11The DNDC model contains 4 main sub-models (Li et al., 1992; Li, 2000); the soil climate12sub-model calculates hourly and daily soil temperature and moisture fluxes in one13dimension, the crop growth sub-model simulates crop biomass accumulation and14partitioning, the decomposition sub-model calculates decomposition, nitrification, NH 315volatilization and CO2 production whilst the denitrification sub-model tracks the16sequential biochemical reduction from nitrate (NO3) to NO2-, NO, N2O and N2 based on17soil redox potential and dissolved organic carbon.1819This paper presents a field evaluation of DNDC for an Irish sandy loam soil under both20arable and grassland crops with different fertilizer and tillage regimes. Results are21discussed in terms of the suitability of this model for estimating annual and seasonal22fluxes of N2O from Irish agriculture. In addition, DNDC is used to estimate future N2O23fluxes from Irish agriculture due to climate change using climate data generated by the24Hadley Centre Global Climate models (HadCM3; Sweeney and Fealy, 2003).252627282930 1 4 5. 1MATERIALS AND METHODS 2 3Experiments 4Measurements of N2O flux were carried out for a spring barley field from AprilAugust for 5two consecutive seasons (2004/05), and for a cut and grazed pasture from October 2003 to 6November 2004. Both fields were located at the Oak Park Research Centre, Carlow, 7Ireland (52o86 N, 6o54 W). The arable field was seeded with spring barley (cv. Tavern) at 8a density of 140 kg ha-1 and managed under two different tillage regimes; conventional 9tillage where inversion ploughing to a depth of 22 cm was carried out in March, five weeks10prior to planting, and reduced tillage to a depth of 15 cm which was carried out in11September of the year before. The field was sprayed with weed killer (Roundup Sting) at124.0L ha-1, three times per season, once pre- and twice post-planting.13The cut and grazed pasture has been permanent grassland for at least the past eighty years14and was ploughed and reseeded in October 2001 with perennial ryegrass (Lolium perenne15L., cv Cashel) at a density of 13.5 kg ha -1 and white clover (Trifolium repens L., cv Aran)16at a density of 3.4 kg ha-1. Daily minimum and maximum air temperature (oC) and rainfall17in (mm) were recorded at the Teagasc Research Centre Weather Station (Met Eireann).18Initial soil properties and climate factors of both sites are summarized in Table 1.1920For the arable field in 2004, three rates of N-fertilization 140 (N1), 70 (N2) and 0 (N3) kg21N ha-1, were applied once on the 27th of April, whereas in 2005, two fertilizer applications22took place on the 12th of April 106 (N1), 53 (N2) and 0 (N3) kg N ha-1, and on the10th of23May 53 (N1), 26 (N2) and 0 (N3) kg N ha-1. The total amount of N-fertilization applied in242005 was therefore 159 (N1), 79 (N2) and 0 (N3) kg N ha-1. For the cut and grazed pasture,25nitrogen fertilizer was applied at a total rate of 200 kg N ha -1 y-1 divided in to two26applications of 128 and 72 kg N ha -1 on the 2nd of April and the 27th of May respectively.27Separate areas of the field were kept unfertilized as control plots. Fertilizer was applied in28the form of Calcium Ammonium Nitrate (CAN). Animal grazing was from July to29November 2003 and from July to November 2004 with a stocking rate of 2 cattle ha-1.30Field N2O fluxes 15 6. 1Nitrous oxide fluxes were measured from 24 replicated chambers at the arable field and 7 2replicated chambers at the cut and grazed pasture, using the methodology of Smith et al., 3(1995). Measurements were taken every week except for times of fertilizer application 4where sampling was increased to 2 times per week. Samples were taken using a 60 ml 5gas-tight syringe after flushing of the syringe to ensure adequate mixing