Effects of nitrogen fertilization on growth and yield of cassava in Hawaii: I dynamics of soil nitrogen

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Abstract
A field experiment was conducted in 1987 on a Vertic Haplustoll on the Island of Oahu (Hawaii) to monitor the dynamics of 1M KCl extractable NH4‐N and NO3‐N, and organic carbon (OC) and total nitrogen (N) contents following the application of increasing rates of N fertilizer as urea (0, 50, 100, and 150 kg N/ha) and subsequent cropping with irrigated cassava. Soil samples were taken at 0–15, 15–30, 30–45, and 45–60 cm soil depths at 60, 120, 180, 240, and 300 days after planting (DAP). Cumulative rainfall values for the periods 0–60, 60–120, 120–180, 180–240, and 240–300 DAP were 151, 0, 156, 142, and 1032 mm, respectively. Two waterlogged periods occurred, one just before 180 DAP and the other after 240 DAP. NH4‐N accumulated to high levels after the first waterlogged period but no NO3‐N was detected. NH4‐N declined sharply after the second waterlogged period. Both OC and total N contents decreased over time and were not affected significantly by N fertilization. Changes in OC and total N contents over time were controlled primarily by temperature and C/N, respectively.