Plant and Soil 47, 113-123 (1977) Ms. 3029 TRANSFORMATION OF SOIL AND FERTILIZER NITROGEN IN PADDY SOIL AND THEIR AVAILABILITY TO RICE PLANTS by TOMIO YOSHIDA* and BENJAMIN C. PADRE, JR. The International Rice Research Institute Los Bafios, Laguna, Philippines SUMMARY A pot experiment in the field showed that addition of ammonium sulfate increased the uptake of soil nitrogen. A-value was found to be independent of the rate of nitrogen application. The rice plant took up about 13 percent of the nitrogen in rice straw which was incorporated into the soil when nitrogen fertilizer was not added, and about 15 percent when 50 ppm N was added. Addition of different levels of fertilizer did not affect the release of immobilized fertilizer nitrogen. Recovery of fertilizer by the rice plant was low when nitrogen was added as basal (broadcast). Recovery was improved by incorporating fertilizer nitrogen before transplanting. Recovery of fertilizer nitrogen when topdressed at reproductive stages was much higher than when applied as basal. A fairly large portion of fertilizer nitrogen was immobilized into the soil. Availability of immobilized nitrogen in the soil appeared low. INTRODUCTION In Southeast Asia, the potential is great to increase rice production through fertilizer usage. In many rice-growing countries, however, fertilizer use is very limited. Greater efficiency in the use of fertilizer is a key factor for increasing rice production. Ammonium, a key compound in nitrogen transformation in plants and soil, is subjected to microbial, chemical, and physical reactions in paddy soils. Microbiological activities have been reported to be the major causes of inefficiency of fertilizer nitrogen use in some Philippine rice soils 7 s 16 17. The major microbial functions are: nitrification, subse- * Present address: National Institute for Environmental Studies, P.O. Yatabe, Ibaraki 300-21, Japan.
114 TOMIO YOSHIDA AND BENJAMIN C. PADRE, Jr. quent denitrification, and immobilization of the nitrogen, which makes it inefficient for rice plant growth. The following aspects of soil and fertilizer nitrogen transformation were studied: how the level of fertilizer nitrogen affects the availability of soil and fertilizer nitrogen for the rice plant, the fate of nitrogen in rice straw applied to the soil, and the fate of fertilizer nitrogen immobilized in the soil, and how the time of nitrogen application affects the rice plant under field conditions, using isotope technique. MATERIALS AND METHODS To measure the avialability of soil nitrogen to rice crops and to investigate how fertilizer nitrogen affects it under submerged conditions, 12 glazed porcelain pots were each filled with I0 kg of Maahas clay soil. The pots were placed into a lowland field so that the soil surface and water depth in the pot was level with the soil surface and water depth outside the pot. The pots received varying amounts of ammonium sulfate solution (9.59 atom % excess 15N) that would make the N levels equivalent to 0, 15, 30 and 45 ppm N in the soil. The fertilizer in the pots corresponded to applications of 0, 30, 60, and 90 kg N per hectare. Two 10-day-old seedlings were transplanted into each pot 1 day after the fertilized soil had been puddled. The water levels in the pots were maintained above the soil surface through harvest. The total nitrogen contents of the straw and panicles were analyzed separately by the Kjeldahl method at harvest. The soil in each pot was mixed thoroughly and a portion was taken for total nitrogen analysis. Nitrogen-15 content of the straw, panicles, and soil were analyzed. To investigate the availability of nitrogen in rice straw in submerged soil, glazed porcelain pots each containing 10 kg of Maahas soil were buried in the same manner as in the preceding experiment. Three groups of pots were prepared. One group was to be withdrawn from the soil at 30 days after transplanting, the second group at panicle initiation, and the last group at harvest. In each group, six pots received no nitrogen and the other six pots received 50 ppm N as ammonium sulfate. Three pots of each treatment with and without ammonium sulfate were amended with rice straw (0.67% N) at the rate of 0.5 per cent. Two 10-day-old seedlings were transplanted into each pot 1 day after incorporation of the rice straw and ammonium sulfate. The soil remained submerged throughout the growing period of the rice crop. The soil was analyzed for available nitrogen at the beginning of the experiment and at 18, 30, and 44 days after transplanting. The nitrogen content of the plants was analyzed at 30 days after transplanting, at panicle initiation stage, and at harvest. The plant samples were also analyzed for 15N content. To study the availability of immobilized nitrogen and how nitrogen application affects the availability of the immobilized nitrogen, a soil containing tagged nitrogen was pulverized and thoroughly mixed (the soil was collected
N TRANSFORMATION AND AVAILABILITY 115 from an experimental field where 15N was used). The 15N atom per cent excess of the soil nitrogen was 0.13. The 15N atom percent excess detected in the rice straw and grains at harvest was in the range of 0.13 to 0.19 percent. Ten-kg lots of this soil were weighed into 12 glazed percolain pots. The pots were again buried in a lowland field. The rate of nitrogen application in the pots was varied: 0, 15, 30, and 45 ppm N as ammonium sulfate. The soil was continuousiy submerged through harvest. The panicles and straw at harvest were analyzed separately for total nitrogen and 15N content. A field experiment was conducted to trace the fate of fertilizer nitrogen. The uptake of fertilizer nitrogen by the rice crop and its effect on grain yield when fertilizer is applied at different stages of crop growth were also determined. Twenty-one plots, each 5 8 m, were prepared. Three plots without nitrogen were included as controls. An area of 1 1.25 m in the center of each plot was enclosed with plastic sheets. Tagged ammonium sulfate was added in this area at the rate of 50 kg N/ha. Untagged ammonium sulfate fertilizer was applied outside the plastic sheets. Fertilizer was incorporated into three plots 1 day before transplanting, and was broadcast into three other plots just after transplanting. Fertilizer was topdressed into the remaining plots at maximum tillering, at panicle initiation, at booting, and at flowering stages. The soil in all plots was kept submerged throughout the growth of the rice crop. Plant samples were collected 10 days after each fertilizer application and the total nitrogen and 15N content was analyzed. At harvest the total nitrogen was analyzed in both the straw and grain and in the soil. Samples were also prepared and analyzed for 15N content. IR 20 was used in all experiments. All the treatments were triplicated. The samples for lsn analysis were prepared by the method used by Manguiat and Y o s hi d a 10. Nitrogen-15 gas was generated from these samples using the Rittenberg method described by P roksch14. Isotope nitrogen-15 was analyzed by emission spectrometry using a JASCO NIA-1 15N analyzer. RESULTS The measurement of availability of soil nitrogen indicated that the A-value (availability index of soil nitrogen) was not affected much by the rate of nitrogen application. The A-values were 206, 201, and 216 ppm at 15, 30, and 45 ppm N application, respectively. The values are 14 to 15 per cent of the total soil nitrogen. Mineralization of the soil nitrogen appeared to increase by the addition of inorganic nitrogen fertilizer, as shown by the increase of soil nitrogen uptake by the rice crop when the rate of fertilizer nitrogen was 30 ppm N or more (Table I). The fertilizer nitrogen uptake by the rice crop was much greater when calculated by the differential method than by the isotope method at fertilizer application rates of 30 ppm N or more. The uptake of fertilizer N by rice crops increased
116 TOMIO YOSHIDA AND BENJAMIN C. PADRE, Jr. TABLE 1 Nitrogen uptake of the rice crop at different levels of fertilizer nitrogen application Fertilizer Fertilizer N (ppm) Soil Total N applied Differential Isotope N N (ppm) method method (ppm) (ppm) 0 - - 72.6 72.6 15 5.4 5.3 72.8 78.1 30 19.5 11.9 80.2 92.1 45 32.3 18.1 86.8 105.0 with increased nitrogen application. The amounts of fertilizer N taken up by rice, measured by the isotope method were: 35.0 per cent of the applied fertilizer at 15 ppm; 39.7 per cent at 30 ppm; and 40.3 per cent at 45 ppm. But when the differential method was used the values were calculated as 36.3, 65.0, and 71.8 per cent. Table 2 shows the recovery of tagged fertilizer N at harvest at different rates of nitrogen application. Plant recovery of fertilizer N was about 5 percent lower in the 15-ppm N application than at 30- and 45-ppm N applications. The amount of fertilizer N that remained in the soil increased as the rate of application increased. But the percentage of total fertilizer nitrogen applied that remained in the soils decreased as the rate of nitrogen application increased. The values were 51.5 percent at 15 ppm N application; 40.4 percent at 30 ppm and 28.9 percent at 45 ppm. The recoveries of total tagged nitrogen in soil and rice plant were 86.5 percent at 15 ppm N application; 80.1 percent at 30 ppm; and 69.2 percent at 45 ppm. Fer- TABLE 2 Recovery of tagged fertilizer nitrogen in plants and soil at harvest at different levels of nitrogen application Fertilizer N N Total N recovered recovered (%) applied in plant in soil (ppm) (%) (%) 15 35.0 51.5 86.5 30 39.7 40.4 80. l 45 40.3 28.9 69.2
N TRANSFORMATION AND AVAILABILITY 117 tilizer nitrogen losses were 13.5, 19.9, and 30.8 percentages, respectively. Results of the experiment on straw nitrogen availability to the rice crop showed that tagged nitrogen in rice straw appeared to be gradually available to the rice plant from tillering stage to harvest. The rates of straw nitrogen that became available to the rice plant were 5.1 percent at tillering stage; 7.7 percent at panicle initiation; and 12.9 percent at harvest. When ammonium sulfate was added to the soil, however, the mineralization of the straw nitrogen appeared to increase. The rates of straw nitrogen that became available to the rice plant when 50 ppm N as ammonium sulfate was added to the soil increased to 6.4 percent at filleting stage; 14.0 percent at panicle initiation; and 15.3 percent at harvest. The dry matter weights at different growth stages revealed that adding rice straw to the soil retarded rice growth. Nitrogen uptake by the rice plant was reduced when rice straw was added to the soil till the panicle initiation stage. But the amount differed little at harvest. Adding rice straw and inorganic nitrogen did not retard plant growth. Rice straw added to the soil apparently immobilized the available soil nitrogen into organic form. Amounts of available soil nitrogen decreased significantly at 30 days after transplanting (Table 3). But the amount of available nitrogen in the straw-treated soil was slightly higher than that in the control at 44 days after transplanting. A result of the experiment on the availability of fertilizer nitrogen that was immobilized into soil organic nitrogen showed that 7.2 per- TABLE 3 Available nitrogen in submerged soils with and without tagged rice straw (0.5%) that contains 0.67% nitrogen (ppm) Fertilizer treatment (50 ppm N) Days after transplanting 0 18 30 44 Without rice straw No treatment 21.8 54.1 10.1 2.3 Nitrogen added 66.4 99.9 47.7 3.8 With rice straw No treatment 21.1 17.2 8.2 7.5 Nitrogen added 65.9 52.5 22.6 4.1
118 TOMIO YOSHIDA AND BENJAMIN C. PADRE, Jr. cent of the nitrogen was taken up by the rice plant at 0 ppm N application; 7.9 percent at 15 ppm; 7.5 percent at 30 ppm; and 8.1 percent at 45 ppm. The different amounts of fertilizer nitrogen added to the soil did not significantly affect the release of the immobilized nitrogen. Table 4 shows the recovery of fertilizer nitrogen that was applied in the field in single doses at different growth stages. The recovery of fertilizer nitrogen by the rice plant increased at later growth stages until about the booting stage. Recovery was low when fertilizer was broadcast as basal. Analysis of tagged nitrogen in rice plants 10 days after application indicated that except in the basal applicacations, most of the nitrogen detected at harvest had been recovered by the rice plant within 10 days after application. At harvest, 60 percent of the fertilizer N was recovered from the soil and rice plants when it was applied by broadcasting as basal. When the fertilizer was incorporated into the soil, the recovery by the rice plant increased but more nitrogen remained in the soil. Total nitrogen recovery in soil and plant increased when nitrogen was applied at later stages of rice growth. The effect of time of nitrogen application on the yield, nitrogen uptake of rice plant and A-value was shown in Table 5. The increased utilization of the added fertilizer did not correspondingly increase the grain yield. But the nitrogen topdressed at panicle initiation increased grain number and grain yield, although nitrogen applied as basal and incorporated gave equally high grain number and grain yield. TABLE 4 Recovery of tagged fertilizer nitrogen applied at different growth stages of the rice crop Tagged N recovered (kg/ha) Time of application 10 days after application Plant At harvest Plant Soil Plant and soil Total N recovered (%) Basal (broadcast) 1.4 Basal (incorporated) 1.3 Maximum filleting (broadcast) 19.1 Panicle initiation (broadcast) 25.3 Booting (broadcast) 29.1 Flowering (broadcast) 26.9 7.6 22.4 30.0 60 16.2 25.8 42.0 84 16.3 17.9 34.2 68 20.8 19. l 39.9 80 25.8 16.6 42.4 85 25.1 17.0 42.1 84
N TRANSFORMATION AND AVAILABILITY ] 19 TABLE 5 Effect of time of nitrogen application on the yield, nitrogen uptake of the rice plant and A-value Time of application Grain Grains Total nitrogen A-value yield (no./~q,m) uptake (t/ha) (kg/ha) Basal (broadcast) 5.0 39103 101.5 309 B asal (incorporated) 5.9 41641 129.4 174 Maximum tillering (broadcast) 5.4 35351 119.3 158 Panicle initiation (broadcast) 5.6 42643 117.9 117 Booting (broadcast) 5.3 33799 123.1 94 Flowering (broadcast) 4.9 31492 121.9 96 O N (control) 4.5 28310 86.0 -- Total nitrogen uptake was highest when nitrogen was applied as basal and incorporated into the soil because of high soil nitrogen uptake. Highest A-value was obtained in the basal (broadcast) treatment. The A-value decreased as the fertilizer application was delayed. DISCUSSION Kyuma and Kawaguchi 9 reported that paddy soils in the Philippines generally showed high inherent fertility potential. B r o a db e n t and R e y e s 7 also found that in four Philippine paddy soils, A values, sometimes used as estimates of available soil N, were quite high and not much influenced by fertilizer level. In a pot experiment by Broadbent and Reyes 7 using Maahas clay soil, the rice plant took up 8 ppm more soil nitrogen at the 50- ppm level of fertilizer N than in the unfertilized control pots. Our results showed that the rice plant took up 14.2 ppm more soil nitrogen at the 45-ppm level of fertilizer N than in the control pots. Assuming that there are 2 million kilograms of soil per hectare furrow slice, about 16 kg N/ha in the former study and 28 kg N/ha in the latter study were released from the soil by the addition of fertilizer nitrogen. Although the two experiments were conducted under different conditions, the difference in the amounts of soil nitrogen released by fertilizer addition can likely be explained by the amounts of soil used in the experiments. The field experiment indicated that the additional amount of soil
120 TOMIO YOSHIDA AND BENJAMIN C. PADRE, Jr. nitrogen taken up by rice plant over the treatment without nitrogen was 27.2 kg N/ha at the rate of 50 kg N/ha fertilizer application (basal incorporated). The nitrogen concentration of 50 kg N/ha is assumed to be 25 ppm N on a soil basis. Thus, more soil nitrogen appears to have been released by fertilizer application in the field than in the greenhouse pot experiment. Tokunaga et al. 15 suggested that root zone development and increased of root activity are major causes of the priming effect (the increase uptake of soil nitrogen as a result of fertilizer application). The priming effect appeared to be greater as the root zone of the rice plant increased. The uptake of fertilizer N by the rice crop increased as nitrogen applications increased. Calculation of results by the differential method gave higher values than did calculation by the isotope method. This is probably so because using the isotope method, the actual fertilizer nitrogen uptake is determined, while using the differential method, the fertilizer N and the priming effect are determined. Patnaik n, Broadbent and Reyes 7, and Andreeva and S c h e gl o v a 2 also reported that recovery was lower using the isotope method than using the differential method and attributed it to the stimulation of soil nitrogen mineralization by the inorganic nitrogen fertilizer. Broadbent 3 found that rates of exchange resulting from biological activity were increased when the fertilizer N added to the soil was increased and that ionic exchange was negligible between fixed and exchangeable NH4 +, and between exchangeable NHa +, and amino-n. By checking the stimulating effect in unplanted soils, which gave positive results, B r o a db e n t also demonstrated that the priming effect cannot be attributed solely to the rhizosphere microflora activity. B r o a d b e n t 8 also found that osmotic effects appear to be a contributing factor. Sapozhnikov et al. 12 tested how the addition of fertilizer N affects the uptake of soil N, using a split root technique. He found that application of fertilizer N increased the plant utilization of soil N although there was no contact between the fertilizer and the soil. They attributed this to the increased metabolism of the fertilized plants and to the increased absorbing power of the root system. Uptake of soil N, however, was even higher when the fertilizer was placed in contact with the soil. The amount of immobilized nitrogen increased as the rate of applied nitrogen increased. When this amount was calculated as per-
N TRANSFORMATION AND AVAILABILITY 121 cent of the original nitrogen added, however, higher values were obtained with the lower rates of applied nitrogen. This could explain why plant recovery of fertilizer N was lower at 15 ppm N application rate than at 30 or 45 ppm N application. Less of the added fertilizer was available to the plant at lower rates of nitrogen application. About 13 percent of the straw nitrogen became available to the rice plant. The addition of ammonium sulfate increased the availability of the straw nitrogen to 15.3 percent. The addition of nitrogen did not increase the mineralization of straw nitrogen much probably because the added rice straw caused available nitrogen to become immobilized. In a California soil, a small difference was observed in the quantities of nitrogen immobilized in anaerobic flooded conditions and in aerobic conditions 6. The scientists also found that nitrogen immobilized in submerged soil amended with rice straw containing 1.17% N was as great as that in rice straw with 0.47% N. This study also showed that adding rice straw that contained 0.67% N immobilized the available nitrogen for about a month after flooding the soil. The experiment of Broadbent and Reyes 7 showed that the addition of rice straw containing 0.84% N retarded nitrogen uptake by the rice plant but the effect was overcome 90 days atter transplanting. In an earlier experiment 17, only a small amount of tagged N was found in a second crop of rice. Tyler and Broadbent 13 likewise failed to obtain data to support the view that adding untagged fertilizer might stimulate the release of previously immobilized tracer nitrogen. Broadbent and Nakashima 4 reported that fertilizer nitrogen was gradually transformed into more stable forms following its incorporation into the soil organic nitrogen. The experiment, designed to measure uptake of nitrogen applied at different rice growth stages, showed that when ammonium fertilizer was applied to the surface of the soil, the rice plant recovered only 15 percent of the nitrogen. But incorporating the fertilizer into the soil increased the nitrogen recovery; applying nitrogen at later stages of rice growth increased it still more. In a laboratory experiment, Y o s h i d a and P a d r e 16 found that high amounts of fertilizer N were lost in the upper 2.5 cm layer of flooded soil; in the lower layers, no loss of fertilizer N was detected. Nitrification and subsequent denitrification was considered the major cause of nitrogen losses in submerged soils lo 16 17. Wada et al. 18 reported that most of
122 TOMIO YOSHIDA AND BENJAMIN C. PADRE, Jr. the nitrogen applied at the heading stage was taken up by rice within 3 to 4 days. Alberda 1 reported that the growth of the root mat started at the end of tillering stage and that the mat became very thick with many root hairs, at about booting stage The root mat might enhance the plant absorption of nitrogen. The increase of fertilizer nitrogen uptake from maximum tillering to booting stage may indicate agreement with A 1 b e r d a's findings 1. p a t n ai k 11 also reported higher recovery of fertilizer N when topdressed at booting stage than at tillering stage. Received 17 November 1975 REFERENCES 1 Alberda, Th., Growth and root development of lowland rice and its relation to oxygen supply. Plant and Soil 5, 1-28 (1954). 2 Andreeva, E A. and Scheglova, G. M., Uptake of soil nitrogen on application of nitrogen fertilizers and nitrification inhibitors as revealed by greenhouse pot experiment using 15N. Trans. 9th Int. Cong. Soil Sci. 2, 523-532 (1968). 3 Broadbent, F. E., Effect of fertilizer nitrogen on the release of soil nitrogen. Soil Sci. Soc. Am. Prec. 39, 692-696 (1965). 4 Broadbent, F. E. and Nakashima, T., Plant recovery of immobilized nitrogen in greenhouse experiments. Soil Sci. Soc. Am. Prec. 39, 55-60 (1965). 5 Broadbent, F. E., Variables affecting A values as a measure of soil nitrogen availability. Soil Sci. U0, 19-23 (1969). 6 Broadbent, F. E. and Nakashima, T., Nitrogen immobilization in flooded soils. Soil Sci. Soc. Am. Prec. 34, 218-221 (1970).? Broadbent, F. E. and Reyes, O. C., Uptake of soil and fertilizer nitrogen by rice in some Philippine soils. Soil Sci, ll2, 200-205 (1971). 8 Broadbent, F. E. and Tusneem, M. E., Losses of nitrogen from some flooded soils in tracer experiments. Soil Sol. Soc. Am. Prec. ;}5, 922-926 (1971). 9 Kyuma, K. and Kawaguchi, K., Fertility evaluation of paddy soils in South and Southeast Asia - Second approximation: Evaluation of three independent constituents of soil fertility - Discussion Paper No. 40, SEAS,!Kyoto University, Kyoto, Japan (1972). I0 Manguiat, I. J. and Yoshida, T., Nitrogen transformations of ammonium sulfate and alanine in submerged Maahas clay. Soil Sci. Plant Nutr. 19, 95-I02 (1973). I I Patnaik, S., N Is tracer studies on the utilization of fertilizer nitrogenwby rice in relation to time of application. Prec. Indian Acad. Sci. 61B 31-38 (1965). 12 Sapozhnikov, N. A., Nesterova, E. I., Rusinova, I. P., Sirota, L. B. and Livanova, T. K., The effect of fertilizer nitrogen on plant uptake of nitrogen from different podzolic soils. Trans. 9th Int. Cong. Soil Sci. 2, 467-474 (1968). 13 Tyler, K. B. and Broadbent, F. E., Nitrogen uptake by rye grass from three tagged ammonium fertilizers. Soil Sci. Soc. Am. Prec. 23, 231-234 (1958). 14 Proksch, G., Application of mass and emission spectrometry for 14N/15N ratio determination in biological material. 'Isotopes and Radiation in Soil-Plant Relationships Including Forestry' International Atomic Energy Agency, Vienna, pp. 217-225 (I 972).
N TRANSFORMATION AND AVAILABILITY 123 15 Tokunaga, Y., Miyama, K., Kitahara, K. and Kusano, S., Dynamic behavior of nitrogen in fresh organic matter and fertilizer applied to upland soil. J. Cent. Agric. Exp. Stn. 20, 1-58 (1974). 16 Yoshida, T. and Padre, B. C. Jr., Nitrification and denitrification in submerged Maahas clay soil. Soil Sci. Plant Nutr. 20, 241-247 (1974). 17 Yoshida, T. and Padre, B. C., Jr., Effect of organic matter application and water regimes on the transformation of fertilizer nitrogen in a Philippine soil. Soil Sci. Plant Nutr. 21, 281-292 (1975). 18 Wada, G., Shoji, S., Takahashi, J., Saito, K., and Shinbo, I., The fate of fertilizer nitrogen applied to the paddy field and its absorption by rice plant. III. Fate of top-dressed nitrogen in the soil and its absorption by rice plant. Proc. Crop Sci. Soc. Japan 40, 287-293 (1971).