1. In grass species that occur in pastures or hay meadows, life spans of roots determine much of the carbon and nutrient loss from the plant in addition to the amounts that are lost by mowing or grazing. We hypothesized that grass species from nutrient-poor habitats had longer root life spans and consequently lost smaller quantities of nutrients through root turnover. 2. In a garden experiment, root life spans and root diameters were measured by repeated observations in minirhizotrons placed in monocultures of Lolium perenne L. and Arrhenatherum elatius L. (characteristic of fertile soils) and Molinia caerulea L. and Nardus stricta L. (preferring nutrient-poor soils). 3. Average root life spans were 14 weeks in L. perenne, 40 weeks in A. elatius, 53 weeks in M. caerulea and 58 weeks in N. stricta. Root life spans of species from fertile habitats were significantly shorter than the root life spans of species from low fertility habitats. 4. In addition, there were significant differences in root diameter among species, root diameter being positively correlated to root life spans. Root diameter decreased during root ageing in all species, while the decline in diameter occurred more slowly in N. stricta than in A. elatius and M. caerulea. 5. An increase in the abundance of plant species adapted to fertile habitats will, because of the greater C and nutrient returns to the soil in root turnover, increase soil fertility. This effect may constitute a positive feedback between changes in plant species composition and nutrient cycling.