Intercomparison of national & IPCC methods for estimating N loss from agricultural land

Abstract

The Intergovernmental Panel on Climate Change (IPCC) estimate indirect N2O emissions from agriculture as 2.5% of nitrate leached, which is itself estimated as a proportion of manure/fertiliser inputs. However, this assumes leaching losses are linearly related to N inputs, over-simplifying a complex N loss function which depends on the interactions between over-winter rainfall, soil type, cropping, and the rate/timing of fertiliser/manure applications. Consideration of these factors would produce a more robust method for estimating N losses. Three alternatives were compared for estimating nitrate leaching and hence indirect N2O loss in England & Wales: (i) IPCC method, (ii) IPCC method using UK-specific manure inputs, (iii) NEAP-N, a model developed for modelling N losses at a national scale. Introducing UK-specific livestock manure data into the IPCC calculation reduced the mean indirect N2O loss from agricultural land from 19980 t N a−1 using default IPCC data (method (i)) to 14335 t N a−1 (method (ii)), indicating that IPCC default data are inappropriate for representing agricultural conditions in the UK. Adopting the NEAP-N method reduced the calculated indirect N2O flux further to 8890 t N a−1. The IPCC approach generated unrealistically low losses from peas/beans and ``zero'' loss estimates from fallow and set-aside land due to their nil fertiliser N additions, highlighting a limitation in this method which assumes all nitrate leaching is derived from fertiliser or manure additions. NEAP-N uses spatially distributed agricultural census information to relate land use and crop type to dominant soil type and hydrologically effective rainfall (HER), as it is these factors which strongly influence nutrient loss. In contrast, IPCC methods (i) and (ii) do not take account of soil type and HER factors, and simply compute nitrate leached as a function of nitrogen applied. Furthermore, the IPCC method, which is based solely on the amount of nitrogen applied, will not characterise reductions in nitrate loss arising from changes to the timing of fertiliser and/or manure applications. This is an important component of strategies to reduce agricultural nitrate loss (e.g. Nitrate Vulnerable Zone regulations). NEAP-N predictions of nitrate leaching losses compare favourably with measurements of stream nitrate fluxes, whereas IPCC methods repeatedly overestimate measured loss two or threefold. We conclude that, for the UK, country-specific estimates using NEAP-N should be used in national greenhouse gas inventories in preference to values using the default IPCC method.