Impact of fertiliser nitrogen formulation, and N stabilisers on nitrous oxide emissions in spring barley
Introduction
Concentrations of atmospheric greenhouse gases (GHGs) have increased since the beginning of the industrial era, due to anthropogenic activities (U.S. EPA, 2015). Between 1990 and 2005, global non-carbon dioxide (CO2) GHG emissions grew by 10% to approximately 10,800 megatons CO2 equivalent (Mt CO2 eq) and are expected to increase by 43% by 2030 (U.S. EPA, 2012). Globally, the agriculture sector accounts for the largest proportion of non-CO2 GHG emissions, accounting for 54% in 2005 (U.S. EPA, 2012). Nitrous oxide comprises approximately 32% of agricultural emissions (U.S. EPA, 2012) and is a potent GHG, with a global warming potential 265 times that of CO2 over a 100 year time frame (Myhre et al., 2013). The atmospheric concentration of N2O has increased at an average rate of 0.75 ppb yr−1, rising 20% since 1750 to 324 ppb (IPCC, 2014). Emissions associated with nitrogen (N) application to agricultural soils comprise 60% of global N2O emissions and are projected to increase from 6.1 to over 7 Tg N2O-N yr−1 by 2030, due to increased global population and food demand (Reay et al., 2012). The use of mineral fertilisers has been one of the principal drivers of this increase in emissions (Davidson, 2009). Excess N application has resulted in enhanced reactive N losses to the environment (Bell et al., 2015). Furthermore N2O is the single most important ozone-depleting gas and is expected to remain so throughout the 21st century (Ravishankara et al., 2009).
In order to generate total N2O emissions for inputting into national inventories, the quantity of a given activity (e.g. tonnes of fertiliser applied) is multiplied by an emission factor (EF). This emission factor is defined as the percentage of N2O emitted as a proportion of the N applied. The IPCC default EF for direct N2O emissions, associated with the application of mineral or organic fertiliser to managed soils, (termed EF1) is 1% of the N applied (IPCC, 2006). This value is a crude estimate as it does not account for crop and soil type, climatic conditions or management practices, all of which affect N2O emissions (Dobbie and Smith, 2003a, Dobbie and Smith, 2003b, Dobbie et al., 1999, Lesschen et al., 2011). Country and cropping system specific data would allow temperate regions to use the Tier 2 emission inventory methodology, where these more detailed and accurate emission factors that are specific for soil and crop type are required (IPCC, 2006). Subsequently, these data could support the development of new N fertiliser recommendations in Ireland; therefore promoting continued reductions of GHG emissions in line with the 2030 targets to reduce GHG emissions by 40% (EC, 2014).
In Ireland the agricultural sector contributes 32% of national GHG emissions (Duffy et al., 2015). Nitrogen application to agricultural soils is one of the key categories, accounting for 22% of total emissions from agriculture and this is projected to increase by 12% by 2020 (EPA, 2013). The focus of this study is on arable land, specifically examining the N2O emissions resulting from the addition of N fertiliser to spring cereal crops, which is one of the largest contributors to GHGs from this land use type. Altering fertiliser formulation and/or rate as well as the incorporation of inhibitors may be a key abatement strategy for reducing N2O emissions from agriculture (Harty et al., 2016).
Calcium ammonium nitrate (CAN) is the dominant N fertiliser used by arable farmers in Ireland. CAN contains 27% N, of which 50% is in the nitrate-N form and immediately contributes to the soil nitrate pool. Nitrate is then available for N2O loss through the denitrification processes. Nitrification may also be an important source of N2O from the application of urea or ammonium based fertilisers (Bremner and Blackmer, 1978). Substituting CAN with urea as an alternative N fertiliser formulation has the potential to reduce direct N2O emissions, associated with denitrification, because urea or ammonium N forms are not immediately available for denitrification after application. However, there is potential for nitrifier denitrification to be a source of N2O (Kool et al., 2011) coupled with the potential for urea to favour N loss as ammonia during urea hydrolysis. The addition of a urease inhibitor has potential to reduce ammonia volatilisation which not only contributes to air pollution but which can also contribute to indirect N2O emissions (Watson et al., 2009, Forrestal et al., 2015). The addition of a nitrification inhibitor has potential to regulate the soil nitrate pool and further reduce direct N2O emissions by both nitrification and denitrification (Dobbie and Smith, 2003a). The rate of N fertiliser application is also important as generally the higher the N fertiliser rate, the higher the N2O emissions (Hinton et al., 2015). Using the IPCC default EF1 assumes a linear relationship between N2O emissions and N fertiliser rate which Hinton et al. (2015) observed. Other studies have observed nonlinear relationships between N2O emissions and N fertiliser rate (Hoben et al., 2011, McSwiney and Robertson, 2005).
In this study, N2O emissions were measured from spring barley after fertiliser applications of CAN and urea with and without N stabilisers. Nitrogen stabilisers are fertiliser additives that reduce environmental N losses thereby stabilising the N in the soil. These can either a reduce urea N loss via volatilisation and are termed urease inhibitors or reduce N loss via denitrification of nitrate and are termed nitrification inhibitors. These stabilisers can thus increase fertiliser use efficiencies by increasing plant N uptake and crop yields. The N stabilisers evaluated in this study were the urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT (trade name Agrotain™) and also referred to as n-BTPT in other studies), the nitrification inhibitor dicyandiamide (DCD), and the Maleic-Itaconic acid Co-polymer (MICO (trade name NutriSphere-N®)) which is a urease and nitrification inhibitor. The aims of this study were to quantify the effect of N fertiliser rate and formulation on direct N2O emissions from spring barley in a temperate maritime climate and to develop crop specific emission factors for use in national N2O emissions inventories. The hypothesis of this study is that changing N fertiliser source from CAN to stabilised urea reduces N2O emissions.
Section snippets
Site description
Field plot trials were conducted on spring malting barley on a free-draining loam soil located in Marshalstown, Co. Wexford (Table 1). This field site was located within the main malting barley growing region in Ireland (Duffy et al., 2015) and was representative of the typical soil type used for arable cropping. The site history was long term arable production for at least 20 years.
Crop husbandry
The spring barley (Hordeum vulgare L.) cultivar used was ‘Sebastian’. The site was ploughed to approximately 20 cm
Soil and climatic conditions
The weather during the experiment was typical of the weather for this region with most of the rainfall occurring during the autumn and winter months and the highest temperatures occurring during the summer months (Figs. 1 a and 2 a ). In both years, the highest average daily temperature was 17 °C in July and the highest total monthly rainfall was in October with 189 mm in 2013 and 173 mm in 2014. Total monthly rainfall and average temperature were higher in April (68.4 mm) and May (74.8 mm) in 2014
Effect of environmental factors on N2O emissions
The application of N resulted in a peak in soil mineral N concentrations with CAN producing significantly higher NO3− peaks compared to other N forms and all fertiliser formulations producing NH4+ peaks. This study showed that using a urea based fertiliser reduced the soil NO3− pool compared to CAN. Thus, there is less TON for denitrification and leaching from the urea based fertilisers. The soil NH4+ pool was similar regardless of the N formulation used.
Whilst rainfall and temperature at the
Conclusions
Overall, N2O emissions from the fertilisers tested in this study were less than half the IPPC default value of 1%. The lack of a clear relationship between fertiliser rate and direct N2O emissions questions the appropriateness of the IPCC default values on soils with low emissions in temperate conditions. This site is representative of the soil type for the majority of spring barley in Ireland and so, based on this study, it is likely that N2O emissions from the majority of spring barley in
Acknowledgements
We thank the Department of Agriculture, Food and the Marine (Grant No. 11/S/138), the Agricultural Greenhouse Gas Research Initiative for Ireland (Grant No. 10/RD/SC/716) and the Walsh Fellowship Scheme for funding this work. We thank the technical and farm staff at Teagasc Johnstown Castle and Oak Park for their help with sampling and analysis and thanks to Jim Grant for help with statistical analysis. We thank the agricultural catchments program for weather data and farmer James Masterson for
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