For welfare reasons, fattening pigs on organic farms are raised on deep litter in the open with simple, flexible climate tents for shelter. Within this system, the nitrogen (N) flow both during fattening of the pigs from 17 to 100 kg liveweight in winter and during composting of the deep litter from the enclosures in summer was studied. During the fattening period, the pigs were provided with barley straw at four daily rates (0·67, 1·21, 1·32 and 1·87 kg) per kg increase in liveweight. During the summer period, N balances from four enclosures receiving straw at around the 1·32 kg rate were examined. Temperatures in the deep litter mat and in the compost heap were determined, as well as concentrations of ammonia (NH3) and nitrous oxide (N2O) in the compost heaps. The temperature increased after 4 weeks to between 40 and 70 °C in the deep litter mats receiving > 1·21 kg straw; the highest temperatures were observed in the mats with the highest straw rate. Temperatures exceeded 50 °C in all compost heaps. In the deep litter mats and compost heaps the highest temperatures were recorded for the longest periods in the material containing the most straw. The initial concentration of NH3 in the gas phase of the compost heaps was > 10 mg NH3-N/m3, which dropped to zero within 1 month. After the NH3 concentration had fallen, the amount of N2O increased significantly for a period in the two heaps with 1·21 and 1·32 kg straw. Over 120 days in summer, an average of 16·3±6% N was lost from the four enclosures strawed at a rate of 1·32 kg (±0·2 kg) straw per kg increase in liveweight. During the winter 12–24% of the N excreted was lost during a fattening period of 119–134 days, and 15–42% of the N stored was lost during the following composting period of 143 days. In total, 29–54% of the N added to the deep litter mat in excretion and straw was lost during the fattening period (119–134 days) and subsequent storage (143 days). The lowest loss was recorded from deep litter strawed daily with 1·32 kg straw per kg increase in pig liveweight.