| Summary: | The first project involved the evaluation of different times of application on the fertilizer nitrogen recovery (FNR) in the soil-plant system in an irrigated upland cotton system, during two seasons. This was accomplished by using the isotopic dilution technique applying the 15N to microplots. No differences were observed in the total plant FNR, seed and stover, soil and the total FNR among the different 15N application times; however, despite no differences were observed in the plant the FNR value ranged from 30-38%, while the seed exhibited the highest FNR with an average over 50% of the 15N recovered in the plant. In the soil, the obtained average FNR value was over 40% while the total FNR (plant + soil) ranged from 70-80%, being reduced as the 15N application time was delayed.The second project was conducted during two seasons to examine the 15N and bromide distribution pattern in the soil profile as a function of the time of application. 15Nitrogen and bromide were applied to the soil at three different times in a cotton growth cycle; after that, soil samples were taken at the end of the cotton cycle to a depth of 1.80 m., and 15N and bromide recoveries were determined. Slightly higher FNR were obtained with the intermediate application time. The higher FNR were detected in the surface layer (0-30 cm) with an average of 40%. Below 30 cm depth, low 15N recoveries were obtained and even lower below the 60 cm soil layer. Bromide recovery behavior was related to the water movement in the soil profile: as the Br- application time was delayed more of the anionic tracer was found in the top of the soil profile, while less Br- was found in the surface soil for the early Br- application time.
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