DIVISION S-4-SOIL FERTILITY AND PLANT NUTRITION Fate of Fertilizer Nitrogen in the Rice Root Zone

Poor N use efficiency by rice (Oryza sativa L.) in many regions of the world is due to the intensity of loss mechanisms functioning in the system. A study was conducted to determine the maximum potential loss of fertilizer "N in a Crowley (Typic Albaqualfs) silt loam soil where conditions are favorable for both nitrification and denitrification in the root zone of rice. Nitrogen losses were measured in the rhizosphere (soil core with rice plants) and nonrhizosphere (soil core without rice plants) soil systems. The potential N loss in the system with plants was found to be 193 mg N m d ', while N loss due to the rhizosphere effect alone was 143 mg N m 2 d ' (18% of the applied N). About 5% of the applied N was found to be lost due to upward movement of NH4 into overlying floodwater. Additional Index Words: nitrification, nitrogen loss, rhizosphere, paddy field, flooded soil. Reddy, K.R., and W.H. Patrick, Jr. 1986. Fate of fertilizer nitrogen in the rice root zone. Soil Sci. Soc. Am. J. 50:649-651. N GAINS AND LOSSES in a rice (Oryza sativa L.) plant-soil system occur as a result of a number of processes utilizing various pathways (Reddy and Patrick, 1984). Among the more poorly understood of these pathways are the transformations of N that take place in the root zone (Savant and DeDatta, 1982; Reddy and Patrick, 1984) of lowland rice and other aquatic plants. Because of the anaerobic nature of the flooded soil these plants obtain most of the O2 required for root respiration by diffusion from the atmosphere through the leaves and stems to the roots (Armstrong, 1964). If O2 diffusion through the aerenchyma tissue to the roots exceeds the plant respiratory requirement, O2 will diffuse from the root into the surrounding root zone (Barber et al., 1962; Mitsui and Tensho, 1952). This process results in an oxygenated rhizosphere surrounded by an O2-free anaerobic soil zone (Aimi, 1960; Armstrong, 1964; 1967). An extensive total area of aerobic-anaerobic interface exists because of the large total root surface involved. Armstrong (1964) measured the O2 diffusing from the roots of the rice plant at a rate of 12 g X 10~ cm~ root surface min~'. In a pot study, Rodriguez-Kabana et al. (1965) measured approximately 4 mg L~' O2 in the soil water adjacent to the roots of rice plants grown under submerged O2-free conditions. The development of two distinct soil layers in the 1 Joint contribution from the Univ. of Florida and Louisiana State Univ. Florida Agricultural Experiment Stations Journal Series no. 6498. Received 10 Mav 1985. 2 Professor, Univ. of Florida, Institute of Food and Agricultural Sciences, Central Florida Research and Education Center, Sanford, FL 32771, and Boyd Professor, Center for Wetland Resources, Louisiana State Univ., Baton Rouge, LA 70803. root zone can potentially influence N transformations. Ammonium N in the anaerobic soil adjacent to the oxidized rhizosphere can diffuse into the rhizosphere where it is oxidized to NOj. Because of its mobility, the Np^~ ion formed by this process can then diffuse back into the anaerobic zone where denitrification takes place. In recent years, several studies have attempted to evaluate the effect of rice plants on N loss from flooded soils (Broadbent and Tusneem, 1971; Reddy and Patrick, Jr., 1980; Fillery and Vlek, 1982; Smith and Delaune, 1984). The results presented in these studies did not arrive at any definitive conclusions on the effects of rice plants on N loss. The objective of this study was to determine the fate of applied fertilizer N in the root zone of rice plants and the surrounding anaerobic soil zone. The purpose of the study was not to assess the practical significance of the process, but to determine maximum potential loss in a system where experimental conditions are favorable for both nitrification and denitrification in the root zone. MATERIALS AND METHODS The soil used was a Crowley silt loam (Typic Albaqualfs) collected from the Rice Exp. Stn., Crowley, LA. It contained 0.8 g total N kg^, 7.0 g total C kg~', a cation exchange capacity (CEC) of 9.4 cmol (+) kg" of soil, and a pH of 5.8 (1:1 soil/water ratio). The soil had 10.8% clay, 70.7% silt, and 9.5% sand. Air-dried soil ground to pass through 0.84-mm mesh sieve was placed in polyvinyl chloride (PVC) tubes (20-cm long and 10-cm i.d.) and sealed at the bottom with a PVC cap (Fig. 1). Six rubber septa (three on each side placed 2 cm apart) had been installed at the bottom portion of the PVC tubes. Prior to the addition of soil, adequate deionized water containing N, P, and K was added to the tube to obtain saturated soil conditions and a final concentration of 50 mg N kg-', 25 mg P kg-', and 50 mg K kg-' of soil. All soil cores were placed in a greenhouse. In one set of cores, two healthy 20-d-old rice seedlings (var. 'Labonett') grown in sand culture were transplanted while the second set of cores was maintained with no plants. After the plants were grown for a period of 20 d, an additional 500 g of soil containing 1% rice straw was added to the core. Additional water was added to maintain saturated soil conditions. An additional 200 g of soil containing 2% rice straw was added to the core as a buffer zone. The main purpose for the addition of rice straw to the soil was to reduce N loss due to upward diffusion of N from the bottom portion of the core to the overlying floodwater. Results obtained in preliminary experiments indicated that it is important to prevent upward diffusion of NH^ if we are to be able to distinguish the effect of rice roots on N loss as compared to N losses due to nitrificationdenitrification reactions taking place at the soil-water interface. Because of the high C/N ratio of the rice straw (90:1), any N diffused will be readily immobilized. Labeled