Root harvests and root windows were used to study the influence of fire, mowing and nitrogen additions on root lengths, biomass, and nitrogen content in tall-grass prairie. Four years of nitrogen additions (10 g m2 yr−1) increased below-ground mass by 15 % and nitrogen concentration in that mass by 77 %. In general, live roots and rhizomes exhibited greater increases in nitrogen concentrations than detrital roots and rhizomes. After four years of treatment, live roots and rhizomes immobilized an additional 1.5 to 5 g/m2 of nitrogen, depending upon specific treatment, while dead roots and rhizomes immobilized an additional 3 to 3.5 g/m2. Average root growth parameters, as measured with root windows, were positively correlated with above-ground peak foliage biomass; however, the only significant correlation was between average new root growth and above-ground peak foliage biomass (r = 0.73, p ≤ 0.04). Root growth and decay, as measured by annual mean values for eight root windows over a four year interval, were insensitive to climatic and treatment effects.