Comparison of Organic and Inorganic Nitrogen Sources for Rice1

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Abstract
Rice (Oryza sativa L.) responds well to green manure‐N under flooded conditions, but more quantitative data are needed for comparisons to inorganic N. This study compared the effects of vetch (Vicia benghalensis L.) and (NH4)2SO4 on patterns of N availability and plant uptake, applied‐N recovery at harvest, and grain yield in flooded rice culture. The sources were incorporated in a Clear Lake clay (Typic Pelloxerert) in factorial combination of 60 kg N/ha increments up to a maximum rate of 180 kg N/ha. Grain yields were determined in plots of 18.9 m2 while N balance was assayed in microplots (0.27 m2) treated with 15N‐labeled materials. Ammonium sulfate was more effective in raising extractable‐N concentrations early in the growing season and made a greater contribution to plant N status throughout the season. There was no evidence that vetch extended the period of N availability to plants. Grain yields were highly dependent on N uptake; they maximized at 10.1 Mg/ha with 120 kg N/ha as (NH4)2SO4 but responded linearly to vetch up to 8.4 Mg/ha with the highest rate of addition. Ammonium sulfate effected greater labeled‐N recovery (19% across rates) and apparent N recovery (55.6%) by plants than vetch (10.1 and 27.9%, respectively). More vetch‐N was recovered in the soil at harvest; total recoveries were, therefore, not significantly different between N sources. The rate of green manure‐N mineralization early in the growing season appears to be the limiting factor in comparison to inorganic N.