Dynamics of nitrogen and dry-matter partitioning and accumulation in broccoli (Brassica oleracea var. italica) in relation to extractable soil inorganic nitrogen

The effects of six rates of N fertilization (0, 125, 250, 375, 500 and 625 kg N ha(-1)) on the dynamics of N utilization relative to extractable inorganic N in the soil profile were determined for broccoli in three growing seasons. The amount of pre-existing extractable inorganic N in the soil was lowest for the spring planting, followed by the early-summer then late-summer plantings. During the first 2 wk after transplanting, plant dry-matter (DM) and N accumulation rates were low, and because of the mineralization of soil organic N the extractable soil inorganic N increased over that added as fertilizer, especially in the top 30 cm. From 4 wk after transplanting until harvest, DM and N accumulation in the plants was rapid and corresponded to a rapid depletion of extractable inorganic N from the soil. At high N-fertilization rates, leaf and stem DM and N accumulations at harvest were similar among the three plantings. However, the rates of accumulation in the two summer plantings were higher before and lower after inflorescence initiation than those in the spring planting. Under N treatments of 0 and 125 kg ha(-1), total N in leaf tissue and the rate of leaf DM accumulation decreased while inflorescences developed. There was little extractable inorganic soil-N during inflorescence development in plots receiving no N fertilizer, yet inflorescence dry weights and N contents were greater than or equal to 50 and greater than or equal to 30%, respectively, of the maxima achieved with N fertilization. These results indicate that substantial N is translocated from leaves to support broccoli inflorescence growth under conditions of low soil-N availability.