A closely spaced (1x1 m) cottonwood (Populus deltoides Bartr.) plantation was established to evaluate the effects of nutrient availability on fine-root dynamics. Time-release fertilizer (17N-6P-12K plus micronutrients) was applied to 225-m2 plots at rates of 0, 50, 100 and 200 kg N ha-1, and plots were monitored for two growing seasons. Fine-root production, mortality, live-root standing crop and lifespan were analyzed with monthly minirhizotron observations. Soil coring was used to measure fine-root biomass. Fine-root dynamics were controlled more by temporal, depth and root diameter factors than by fertilization. Cumulative fine-root production and mortality showed strong seasonal patterns. Production was greatest in the middle of the growing season and mortality was greatest after the growing season. Small diameter roots at shallow soil depths cycled more rapidly than larger or deeper roots. Strongest treatment effects were found in the most rapidly cycling roots. The standing crop of live roots increased with fertilizer treatment according to both minirhizotron and soil coring methods. However, production and mortality had unique treatment response patterns. While cumulative mortality decreased in response to increased fertilization, cumulative production was intermediate at 0 kg N ha-1, lowest with 50 kg N ha-1, and highest with 200 kg N ha-1. Aboveground growth responded positively to fertilization, yet growth was level after 50 kg N ha-1despite a threefold increase in application rate. Median fine-root lifespan varied from 307 to over 700 days and increased with depth, diameter and nutrient availability.