Enchytraeids are involved both directly and indirectly in decomposition processes and nitrogen mineralization in soil. Their influence is especially important in nitrogen poor ecosystems such as heathland where the enchytraeid species, Cognettia sphagnetorum, is often abundant and playing a significant role in the N-cycling. The objective of this study was to quantify NH4+–N excretion of C. sphagnetorum at different temperatures. The results were combined with investigations of population dynamics during one year to estimate annual NH4+–N excretion of the population of C. sphagnetorum in a dry Danish heath soil. C. sphagnetorum significantly increased its NH4+–N excretion rate with increasing temperature. At 5 °C about 0.5 μg NH4+–N mg dry weight−1 day−1 was excreted increasing to about 3.3 μg NH4+–N mg dry weight−1 day−1 at 20 °C. Average enchytraeid biomass in the field was 2.5–3.5 g dry weight m−2 during cool and wet periods. Dry and warm conditions in May and June, 2008, had a drastic and long-term negative impact on the enchytraeid community. The excretion of NH4+–N by enchytraeids was therefore highest during the cool and moist months despite low temperatures (October 2007–May 2008) and amounted to about 2 mg NH4+–N m−2 day−1 during this period. The estimated annual NH4+–N excretion of the enchytraeid community was approximately 0.3 g N m−2 year−1. The results of the present study and the method described for estimation of N-excretion can increase our understanding of enchytraeids’ role in nitrogen mineralization.