Nitrogen rate effect on partitioning of nitrogen and dry matter by cotton

Increased understanding of the fate of N in cotton (Gossypium hirsutum L.) fields will improve N efficiency, optimize crop development and yield, and may help to avoid excessive N fertilization. This study quantified the effects of N fertilization rate (0, 84, and 168 kg ha-1) on seasonal uptake and partitioning of N and dry matter in field-grown cotton during 1989 and 1990. The N rates studied were part of a larger experiment initiated in 1987 where plots received annual preplant applications of 0, 28, 56, 84, 112, 140, or 168 kg N ha-1. Experiments were conducted on Commerce silt loam (fine-silty, mixed, thermic, nonacid, aeric Fluvaquent) on the Louisiana State University Agricultural Center Northeast Research Station near St. Joseph, LA. Plants were collected at five dates in each year at about 20-d intervals. Shed plant debris was also collected. The N content and aerial biomass of plant components were determined. Maximum N uptake occurred between 49 and 71 d after planting and was 2.9 and 4.3 kg ha-1 d-1 for cotton receiving 84 and 168 kg N ha-1, respectively. At maturity, plants receiving 84 kg N ha-1 contained 160 kg N ha-1 in aerial biomass and an additional 50 kg N ha-1 in abscised plant debris. The total amount of N assimilated by plants receiving 84 kg fertilizer N ha-1 averaged 111 kg N ha-1 more than plants receiving no fertilizer N for an apparent fertilizer efficiency greater than 100%. By the end of effective bloom, plants receiving 168 kg N ha-1 assimilated 15 to 40% more N, primarily in leaves and lower bolls, than plants receiving 84 kg N ha-1. This excess assimilated N was recovered in surface litter and in N-enriched plant components. Of the plant components studied, leaf-blades most consistently reflected the amounts of fertilizer N applied.