Knowledge of the microbiology of dinitrogen (N-2)-fixing bacteria in compost rich in de-inking paper sludge (DPS) is limited. Dinitrogen (N-2)-fixing bacteria from DPS composts were isolated and studied for their N-2-fixing activity in vitro and in vivo. Two Gram-negative N2-fixing isolates were identified as Pseudomonas. At 20 degrees C, both isolates revealed that N-2-fixing activity was higher than that of three arctic Pseudomonas strains. Their N-2-fixing activity was found to occur between 18 and 25 degrees C, a pattern that was similar to the reference isolate Azotobacter ATCC 7486. Composts successfully showed N-2-fixing activity after carbohydrate amendments both with and without inoculation of a N-2-fixing isolate. These results suggest that DPS composts support N-2-fixing bacteria and that N-2-fixing activity is dependent on a usable carbohydrate source. (c) 2005 Elsevier Ltd. All rights reserved.