A short rotation coppice (SRC) with poplar was established in a randomised fertilisation experiment on sandy loam soil in Potsdam (Northeast Germany). The main objective of this study was to assess if negative environmental effects as nitrogen leaching and greenhouse gas emissions are enhanced by mineral nitrogen (N) fertiliser applied to poplar at rates of 0, 50 and 75 kg N ha−1 year−1 and how these effects are influenced by tree age with increasing number of rotation periods and cycles of organic matter decomposition and tree growth after each harvesting event. Between 2008 and 2012, the leaching of nitrate (NO3 −) was monitored with self-integrating accumulators over 6-month periods and the emissions of the greenhouse gases (GHG) nitrous oxide (N2O) and carbon dioxide (CO2) were determined in closed gas chambers. During the first 4 years of the poplar SRC, most nitrogen was lost through NO3 − leaching from the main root zone; however, there was no significant relationship to the rate of N fertilisation. On average, 5.8 kg N ha−1 year−1 (13.0 kg CO2equ) was leached from the root zone. Nitrogen leaching rates decreased in the course of the 4-year study parallel to an increase of the fine root biomass and the degree of mycorrhization. In contrast to N leaching, the loss of nitrogen by N2O emissions from the soil was very low with an average of 0.61 kg N ha−1 year−1 (182 kg CO2equ) and were also not affected by N fertilisation over the whole study period. Real CO2 emissions from the poplar soil were two orders of magnitude higher ranging between 15,122 and 19,091 kg CO2 ha−1 year−1 and followed the rotation period with enhanced emission rates in the years of harvest. As key-factors for NO3 − leaching and N2O emissions, the time after planting and after harvest and the rotation period have been identified by a mixed effects model.
