The photosynthate costs of processes (amino acid and protein synthesis and turnover, and pH regulation) associated with the utilization of nitrate (NO3−), ammonium (NH4+) or glutamine (Gln) for plant growth were estimated. Based on these estimates, the effects of these forms of nitrogen (N) on the carbon balance of plants and on shoot–root biomass allocation were evaluated. The results indicated that NO3− as an N source for plant growth is not substantially more expensive to utilize than either NH4+ or Gln, particularly in the long term when costs due to protein turnover dominate the total costs of N utilization. It is also suggested that the photosynthate use in processes associated with N assimilation has little impact on the carbon balance of plants, and hence on shoot–root biomass allocation.