Assessment of nitrogen fertilization for the CO2 balance during the production of poplar and rye

This study was designed to consider all nitrogen fertilizer-related effects on crop production and emission of greenhouse gases on loamy sandy soils in Germany over a period of nine years (1999–2007). In order to set up a CO2 balance for the production of energy crops, different nitrogen pathways were investigated, such as direct N2O emissions from the soil and indirect emissions related to NO3 leaching and fertilizer production. Fluxes of N2O were measured in an experimental field using closed chambers. Poplar (Populus maximowiczii × P. nigra) and rye (Secale cereale L.) as one perennial and one annual crop were fertilized at rates of 0 kg N ha−1 yr−1, 75 kg N ha−1 yr−1 and 150 kg N ha−1 yr−1. The mean N2O emissions from the soil ranged between 0.5 kg N ha−1 yr−1 and 2.5 kg N ha−1 yr−1 depending on fertilization rate, crop variety and year. The CO2 fixed in the biomass of energy crops is reduced by up to 16% if direct N2O emissions from soil and indirect N2O emissions from NO3 leaching and fertilizer production are included. Taking into account the main greenhouse gas emissions, which derive from the production and the use of N fertilizer, the growth of poplar and rye may replace the global warming potential of fossil fuels by up to 17.7 t CO2 ha−1 yr−1 and 12.1 t CO2 ha−1 yr−1, respectively.