Nitrous oxide emissions from multiple combined applications of fertiliser and cattle slurry to grassland

R. L. M. Schils 0 J. W. van Groenigen 0 G. L. Velthof 0 P. J. Kuikman 0 Responsible Editor: Ute Skiba. 0 J. W. van Groenigen Department of Soil Quality, Wageningen University and Research Centre , P.O. Box 47, 6700 AA Wageningen, The Netherlands Fertiliser and manure application are important sources of nitrous oxide (N2O) emissions from agricultural soils. The current default IPCC emission factor of 1.0% is independent of the type of fertiliser and manure, and application time, method and rate. However, in the IPCC Tiered system it is possible to use more specific emission factors that better reflect the actual fertiliser and manure management in a given country or region. The first and primary aim of this study was to determine whether the combination of cattle slurry injection with fertiliser application, which is common practice in intensively managed grasslands in the Netherlands and neighbouring countries, warrants an adjusted emission factor. A second aim was to evaluate whether alternative emission factors, based on N uptake and N surplus, respectively, give more insight in the N2O emission rates of various fertilisation strategies. In a 2-year field experiment on sandy soil in the Netherlands we measured the annual N2O emission from grasslands receiving repeated simultaneous applications of fertiliser and cattle slurry. The N2O fluxes and N uptake by grass were measured from plots receiving calcium ammonium nitrate (CAN) at four application rates, either with or without additional application of liquid cattle slurry, applied through shallow soil injection. The average emission factor for fertiliser-only treatments was 0.15%. The annual N2O emissions were similar for treatments receiving only fertiliser or only cattle slurry. In the first experimental year, application of cattle slurry increased the emission factor for fertiliser to 0.35%, but the second year showed no effect of cattle slurry on the emission from fertiliser. With regard to the first objective, we conclude that these results do not conclusively justify an adjusted emission factor for combined application of fertiliser and cattle slurry. To minimise risks however, it is sensible to avoid simultaneous application of fertiliser and cattle slurry. The N2O emission factor expressed as percentage of kg N uptake by grass was consistently higher after combined application of fertiliser and cattle slurry (0.29%), compared to fertiliser-only (0.17%). With regard to the second objective we conclude that an emission factor based on N uptake expresses the relatively inefficient N supply of cattle slurry to crop growth better than the traditional emission factor based on N application. - Soil applied fertiliser and animal wastes are the two most important sources of direct nitrous oxide (N2O) emissions from agricultural soils (Mosier et al. 1998). In the European Union (EU-15), 40% of the direct soil emission is attributed to fertiliser application, and another 21% to manure application (EEA 2006). In the year 2000, grasslands in the EU-15 received approximately 24% of the total amount of applied fertiliser N, and 32% of the applied manure N (Velthof et al. 2007). The default IPCC emission factor, i.e. the percentage of applied N emitted as N2O, is 1.00%, irrespective of the type of fertiliser or manure (IPCC 2006). Also application time, application method and application level are not considered. The default emission factor is based on an average of published measurements on a wide range of soil types, crops, fertiliser types, N sources, N levels and application times and methods (Bouwman 1996; Bouwman et al. 2002; Stehfest and Bouwman 2006). Individual countries can use a different emission factor that corresponds to country-specific soil types or farm management. The data used to derive the IPCC default emission factor originate predominantly from fertiliser experiments. For grassland, the dataset contains relatively few experiments with organic N sources or combinations of fertiliser and organic N. This is in contrast with the farming practice of intensively managed grasslands in Europe, where both fertiliser and cattle slurry are usually applied shortly after one another. In those cases, interactions between those different N sources should be considered. Therefore, following the Tiered system for country-specific emission factors, it is useful to measure N2O emissions of fertiliser N in combination with applied cattle slurry. In recent years, some experiments were carried out with combined application of fertiliser and cattle slurry to grassland (Clayton et al. 1997; Dittert et al. 2005; Stevens and Laughlin 2001, 2002). All experiments reported higher N2O emissions when fertiliser and cattle slurry where applied simultaneously than when they were applied separately or with a larger interval in-between. These higher N2O losses were attributed to enhanced denitrification through the simultaneous availability of fertiliser derived nitrate (NO3) and cattle slurry derived easy degradable carbon (C). However, in these experiments cattle slurry was surface-applied. Efforts in recent years to reduce ammonia losses have led to an increased use of animal slurry injection techniques, especially in the Netherlands. Studies that compared application techniques of cattle slurry have either shown no effect of application technique (e.g. Velthof et al. 1996) or a higher N2O emission following cattle slurry injection (e.g. Rodhe et al. 2006). However, these experiments had no treatments with combined fertiliser and cattle slurry application and therefore did not consider possible interactions. To our knowledge, there are no published N2O emission measurements following a combined application of fertiliser and injected cattle slurry on grassland. Intensively managed grasslands in North-West Europe are usually fertilised four to seven times a year, combined with two to four cattle slurry applications (Unwin and Vellinga 1994; Schrder et al. 2007). Therefore, the current practice might require a specific emission factor, other than the IPCC default. The primary objective of this paper was to assess the annual N2O emission from intensively managed grasslands fertilised according to common agricultural practice in the Netherlands, i.e. with repeated simultaneous applications of fertiliser and injected cattle slurry. More specific, we aim to determine the effect of cattle slurry application on the N2O emission from fertiliser. The IPCC emission factors imply a linear relationship between N input and N2O emissions. The principal advantage of this approach is that the activity data, at least for fertiliser use, are relatively easy to collect and use in national inventory systems. A drawback is that the fixed emission factors only stimulate mitigation through reduced inputs and are not an incentive to improve the N use efficiency by crops. Experiments with different application rates have shown increased emission fac (...truncated)