The efficiency of municipal biosolids (MB) and liquid swine manure (LSM) as fertilizers for old grass fields used for biomass production remains to be determined in northern areas. We determined the response of a 7 yr old grass field to organic and mineral N fertilization in two management zones. Soil and crop spatial variability was characterized, and two management zones were defined using soil electrical conductivity (EC). Nitrogen was applied at 160 kg total N ha^-1 for 3 yr as MB, LSM, or mineral fertilizer (M) along with an unfertilized control. Seasonal dry matter (DM) yields were 61% with no N applied, 87% with MB, and 95% with LSM of that with M. The apparent N recovery with LSM (38%) and MB (27%) was less than with M (51%). Management zones did not differ in responses of DM yield and apparent N recovery to fertilization treatments. Fertilization treatments affected the number of species and the contribution of the main species to DM yield. We concluded that MB and LSM are valuable sources of N for biomass production from old grass fields in northern areas and EC-defined management zones are unlikely to improve N management in similar situations.