In order to model growth and nitrogen (N) uptake by the rice plant as well as improve nitrogen fertilizer management, a description of the temporal behavior of nitrogen in a flooded soil cropped to rice is needed. The objective of this study was to characterize the seasonal changes in soil, water, and plant nitrogen in a flooded rice culture. Lebonnet rice (Oryza sativa L.), a short-season variety, was grown on a Crowley silt loam soil (a fine, montmorillonitic, thermic Typic Albaqualf). Twelve plots were established utilizing a randomized complete block design with six controls, and six receiving 130 kg N/ha as urea in a three-way split. Amounts of inorganic soil nitrogen, as well as nitrogen uptake and dry matter production by the different plant fractions, were monitored at weekly intervals for 13 weeks.
Nearly 90 % of the preflood application and 95 % of all subsequent inorganic N peaks were exchangeable NH4-N until the flood was removed. Although the preflood nitrogen application was accounted for in the soil, the midseason applications were virtually undetected. An increase in inorganic soil nitrogen occurred later in the season which was thought to be due to the decomposition of rice straw or algae. In the fertilized plants, 35% of the total dry matter production at harvest was found in the dead tissue, with 31% in the active-vegetative tissue and 34% in the reproductive tissue. Dry matter production for control plants was 23, 34, and 43% of the total, respectively, for the dead, active-vegetative, and reproductive tissues. Nitrogen concentrations in fertilized plants were significantly higher than controls until after tillering. Active-vegetative tissues in fertilized and control plants showed decreases in N concentration with time, while reproductive and dead tissue nitrogen concentrations remained relatively constant after the initial sample dates. Patterns of N uptake were similar to dry matter production. Sixty kg of N per hectare accumulated in control plants and 150 kg of N per hectare in fertilized plants. At harvest, 23, 28, and 49% of the total nitrogen uptake was in the active-vegetative, dead, and reproductive tissues of the fertilized plants, respectively. In control plants at harvest, 25, 16, and 59% of the uptake was in the active-vegetative, dead, and reproductive tissues, respectively. In a plot of plant N vs. soil N, the efficiency of N fertilizer use was 55% for the preflood fertilizer application. A relationship was found to exist whereby N in the reproductive tissue increased as the inverse of the N in the active-vegetative tissue increased.