Earthworms play an important role in the nitrogen cycle in the soil. Through their activities they affect the mineralization of organic matter directly and indirectly. However, the presence of organic matter also affects earthworm abundances. For this study, we selected 2 grasslands differing in organic matter content at nine dairy farms on sandy soils in the Noordelijke Friese Wouden (NFW) in the Netherlands. We expected a larger number of earthworms and a higher mineralization rate in grasslands with a higher organic matter content. We also expected a positive relationship between earthworm abundance and nitrogen mineralization. At each farm the grassland with the highest organic matter content contained the largest number of earthworms (up to 858 worms m(-2)), (r = 0.286 (p = 0.036)). These grasslands also had the highest root biomass (r = 0.504 (p = 0.0001). With an increase in organic matter in the soil (from 5 to 10.2%), potential nitrogen mineralization increased from 138 to 310 kg N-1 ha 6 months(-1). No relationships between the calculated amount of nitrogen mineralized by earthworms and the potential and actual nitrogen mineralization were found. Nitrogen mineralization due to earthworm activities, calculated using production ecological formulas, ranged from 4 to 24 ka N ha(-1) month(-1). (c) 2007 Elsevier Masson SAS. All rights reserved.