GROWTH, YIELD AND YIELD COMPONENTS OF LOWLAND RICE AS INFLUENCED BY AMMONIUM SULFATE AND UREA FERTILIZATION

Journal of Plant Nutrition, 34:371 386, 2011 Copyright C Taylor & Francis Group, LLC ISSN: 0190-4167 print / 1532-4087 online DOI: 10.1080/01904167.2011.536879 GROWTH, YIELD AND YIELD COMPONENTS OF LOWLAND RICE AS INFLUENCED BY AMMONIUM SULFATE AND UREA FERTILIZATION N. K. Fageria, 1 A. B. dos Santos, 1 and A. M. Coelho 2 1 National Rice and Bean Research Center of EMBRAPA, Santo Antônio de Goiás, Brazil 2 National Maize and Sorghum Center, Empresa Brasileira de Pesquisa Agropecuaria (EMBRAPA), Sete Lagoas, Brazil Nitrogen (N) is one of the most important nutrients in increasing lowland rice yield. Two greenhouse experiments were conducted to evaluate influence of ammonium sulfate and urea fertilization on growth, yield and yield components of lowland rice. The nitrogen rates used were 0, 50, 100, 150, 300 and 400 mg N kg 1 of soil. Shoot dry weight and grain yield were significantly (P < 0.01) increased in a quadratic fashion when N rate increased from 0 to 400 mg kg 1 by ammonium sulfate as well as urea fertilization. Maximum grain yield was obtained at 168 mg N kg 1 soil by ammonium sulfate and at 152 mg N kg 1 soil by urea. Maximum grain yield at average N rate (160 mg kg 1 ) was 22% higher with the application of ammonium sulfate compared to urea, indicating superiority of ammonium sulfate compared to urea. Rice yield components, N uptake and use efficiency were significantly influenced with the increasing N rate from 0 to 400 mg kg 1 of soil by both the sources of N. Plant height, shoot dry weight, grain harvest index, 1000 grain weight and N uptake and use efficiency in shoot and grain had significant positive association with grain yield. However, spikelet sterility was negatively associated with grain yield. Soil ph, soil calcium, phosphorus, and potassium contents were significantly influenced by N treatments with urea fertilization. These soil properties were not influenced significantly by ammonium sulfate treatment, except P content. Keywords: Oryza sativa L., Inceptisol, soil ph, N utilization efficiency INTRODUCTION Rice is a staple food for more than 50% world s population, who live mostly in developing countries, and is arguably the most important crop worldwide (Fageria et al., 2003). After wheat, rice is most important cereal crop for human consumption. It provides about 21% of the total caloric intake of the world population (Fageria et al., 1997). Rice production is Received 26 February 2009; accepted 26 July 2009. Address correspondence to N. K. Fageria, National Rice and Bean Research Center of EMBRAPA, Caixa Postal 179, Santo Antônio de Goiás, GO, CEP 75375-000, Brazil. E-mail: fageria@cnpaf.embrapa.br 371

372 N. K. Fageria et al. concentrated in Asia, where more than 90% of the world s supply is produced. China and India are the leading producers as well as consumers of rice. Other major rice producing countries are Vietnam, Thailand, Japan and Indonesia. In South America, rice is eaten daily with dry beans. In Brazil, rice is grown on lowland as well as upland ecosystems. Upland rice is concentrated in the central part of Brazil locally known as Cerrado region. Lowland rice is mainly grown on Incetisols, locally known as Varzeas which are distributed throughout the country. There are about 35 million hectares of Varzea soils in Brazil. At present less than 2 million hectares are cultivated. Due to availability of water and favorable climatic conditions, these lands have very high potential for crop production. Soil fertility is one of the main yield limiting factors for rice production in Brazilian Inceptisols (Fageria and Baligar, 1996; Fageria et al., 2003). Nitrogen (N) is the most limiting nutrient for crop production in many of the world s agricultural areas, including Brazil, and its efficient use is important for the economic sustainability of cropping systems (Fageria and Baligar, 2005). Use of adequate rates and sources of nitrogen is very important for its efficient use. Such practice not only increases yield but also reduces cost of production and environmental pollution. Urea and ammonium sulfate are the main nitrogen carriers worldwide in annual crop production. However, urea is generally favored by the growers over ammonium sulfate due to lower application cost because urea has a higher N analysis than ammonium sulfate (46% vs. 21% N). In Brazil, these two N sources are commonly used in the rice cultivation. Data are limited or inconclusive in relation to efficiency of urea and ammonium sulfate in lowland rice production under Brazilian conditions. The objective of this study were to i) compare the effectiveness of ammonium sulfate and urea as sources of N in lowland rice culture, ii) determine association between yield and yield components, and iii) determine influence of urea and ammonium sulfate sources of N on soil chemical properties. MATERIALS AND METHODS Two greenhouse experiments were conducted simultaneously to evaluate the influence of ammonium sulfate and urea fertilizers on lowland rice production. The soil used in the two experiments was an Inceptisol. The chemical and physical characteristics of the soil were: ph 4.4, calcium (Ca) 3.9 cmol c kg 1, magnesium (Mg) 1.3 cmol c kg 1, aluminum (Al) 0.7 cmol c kg 1, phosphorus (P) 51.6 mg kg 1,potassium(K)61mgkg 1, copper (Cu) 4.8mgkg 1,zinc(Zn)1.3mgkg 1, iron (Fe) 450 mg kg 1, manganese (Mn) 67 mg kg 1 and organic matter content 23 g kg 1. The textural analysis was clay content 369 g kg 1, silt content 220 g kg 1 and sand content 411 g kg 1.

Lowland Rice and N Sources 373 Soil analysis methods used are described in soil analysis manual of EMBRAPA (1997). Nitrogen rates used were 0, 50, 100, 150, 300 and 400 mg kg 1 of soil in both the experiments. Experiments were conducted in plastic pots with 7kgofsoilineachpot.Atthetimeofsowing,eachpotreceived200mgP and 200 mg K kg 1 of soil. Each pot also received 10 g dolomitic lime four weeks before sowing. The liming material was having 33% calcium oxide (CaO), 14% magnesium oxide (MgO) and 85% neutralizing power. The pots were subjected to wetting and drying cycles. The experimental design was a complete block with three replications. Cultivar shown was BRSGO Guará and there were four plants in each pot. After 14 days of sowing, pots were flooded with 2 3 cm water depth and drained five days before harvesting. Shoot and grain were separated at harvest and material was dried in an oven at 70 C to a constant weight. Grain harvest index, N harvest index and N use efficiency ratio were calculated by using the following formula (Fageria and Baligar, 2005): Grain harvest index = (Grain yield)/(grain + straw yield) Nitrogen harvest index = (N uptake in grain)/(n uptake in grain + straw) N use efficiency ratio = (Grain or straw yield)/(n uptake in grain or straw) Soil samples were taken from each pot after harvest of rice plants to evaluate influence of ammonium sulfate and urea sources of fertilizers on soil chemical properties. Data were analyzed by analysis of variance and regression analysis was performed. Appropriate regression model was selected on the basis of R 2. RESULTS AND DISCUSSION Plant Height, Shoot Dry Weight and Grain Yield Plant height was significantly increased only with the application of urea; however, shoot dry weight was significantly increased with ammonium sulfate as well as urea source of N (Table 1). The variation in plant height with urea fertilization was 95.7 to 106.3 cm, with an average value of 102.2 cm. The increase in plant height with urea application was quadratic in fashion and variability was 62%. Overall, ammonium sulfate produced about 2% higher plant height as compared to urea source of N. Increase in shoot dry weight was linear with increasing N rate in the range of 0 to 400 mg kg 1 of soil by ammonium sulfate and urea fertilization. The variation in shoot dry weight was 12.3 to 48.8 g plant 1, with an average value of 25.4 g plant 1 by ammonium sulfate and 10.6 to 44.2 g plant 1, with an average value of 24.4 g plant 1 by urea. Overall, ammonium sulfate produced 4% higher shoot dry weight compared to urea fertilization. Ammonium sulfate accounted 91%

374 N. K. Fageria et al. TABLE 1 Plant height and shoot dry weight as influenced by ammonium sulfate and urea fertilization Plantheight(cm) Shootdryweight(gplant 1 ) Nrate(mgkg 1 ) (NH 4 ) 2 SO 4 CO(NH 2 ) 2 (NH 4 ) 2 SO 4 CO(NH 2 ) 2 0 99.7 101.0 12.3 10.6 50 103.7 102.3 16.7 13.5 100 107.0 105.0 16.5 16.2 150 105.7 106.3 21.6 21.2 300 105.7 103.0 36.5 40.8 400 102.3 95.7 48.8 44.2 Average 104.0 102.2 25.4 24.4 F-test NS CV(%) 5.4 3.1 16.7 18.2 Regression analysis N rate CO(NH 2 ) 2 vs. plant height (Y) = 100.3987 + 0.0665X 0.00019X 2,R 2 = 0.6229 Nrate(NH 4 ) 2 SO 4 vs. shoot dry wt. (Y) = 10.2061 + 0.0912X, R 2 = 0.9082 N rate CO(NH 2 ) 2 vs. shoot dry wt. (Y) = 8.9314 + 0.0929X, R 2 = 0.9044 Significant at the 1% probability level. variability in shoot dry weight, whereas, urea accounted 90% variability in shoot dry weight (Table 1). Fageria and Barbosa Filho (2001) and Fageria and Baligar (2005) have reported the increase in plant height and shoot dry weight with increasing N rate. Fageria et al. (2003) reported that N is one of the major elements required for plant growth. Epstein and Bloom (2005) also reported that N deficiency retarded growth of crop plants. Plant height was having significant positive association with grain yield (Y = 403.4787 + 7.6832X 0.0351 2,R 2 = 0.2530 ). Similarly, shoot dry weight was also having significant positive association with grain yield (Figure 1). The increase in grain yield with increasing plant height and shoot dry weight was quadratic in fashion. Plant height accounted for 25% variability in grain yield and shoot dry matter accounted 72% variability in grain yield. Hence, increasing plant height and shoot dry weight can increase grain yield of lowland rice up to some extent. Fageria and Baligar (2001) and Fageria and Barbosa Filho (2001) reported that grain yield of lowland rice FIGURE 1 Relationship between shoot dry weight and grain yield.

Lowland Rice and N Sources 375 increased significantly and quadratically with increasing shoot dry weight. Hasegawa (2003) reported that higher yields of rice cultivars were associated with higher dry matter. Peng et al. (2000) also reported that the increasing trend in yield of rice cultivars released by the International Rice Research Institute in the Philippines before 1980 was mainly due to the improvement in grain harvest index, while an increase in total biomass was associated with yield trends for cultivars released after 1980. These authors also suggested that further increases in rice yield potential would likely occur through increasing biomass production rather than increasing harvest index. Grain yield significantly (P < 0.01) and quadratically increased with increasing N rate from 0 to 400 mg kg 1 of soil by ammonium sulfate as well as urea source (Figure 2). Maximum grain yield was obtained with the application of 168 mg N kg 1 of soil by ammonium sulfate and 152 mg N kg 1 of soil by urea. The variation in grain yield was 5.5 to 22.8 g plant 1, with an average yield of 15.9 g plant 1 by ammonium sulfate and 8.0 to 19.7 g plant 1, with an average value of 14.4 g plant 1 by urea fertilization. Ammonium sulfate accounted for 90% variability in grain yield, whereas, urea accounted 78% variability in grain yield. Across six N rates, ammonium sulfate produced 10% higher grain yield compared to urea. In addition, average across two N sources (160 mg N kg 1 ), ammonium sulfate produced 22.5 g grain yield per plant and urea produced 18.5 g grain yield per plant. The application of ammonium sulfate at the rate of 160 mg N kg 1 produced 22% higher grain yield compared to urea at the same rate of N. This means that ammonium sulfate was superior fertilizer for lowland rice grain yield compared to urea (Figure 2). Reddy and Patrick (1978) and Bufogle et al. (1998) reported no differences in straw or grain yield of lowland rice between the two N sources. In a greenhouse study, Phongpan et al. (1988) found no differences in grain and straw yields between urea and ammonium sulfate to an acid sulfate soil at low N rate (160 mg N kg 1 ), but at higher N rates (320 to FIGURE 2 Relationship between nitrogen rate applied by ammonium sulfate and urea and grain yield of lowland rice.

376 N. K. Fageria et al. 480 mg N kg 1 ), urea consistently produced higher yields than ammonium sulfate. This means that response of lowland rice to ammonium sulfate and urea depends on soil or climatic conditions (Bufogle et al., 1998). Panicle Number, Spikelet Sterility, 1000 Grain Weight and Grain Harvest Index The number of panicles increased significantly and quadratically with increasing N rates from 0 to 400 mg kg 1 of soil by both ammonium sulfate and urea sources of N (Figure 3). Panicle response to N fertilization was similar for both the N sources; however, magnitude of response was higher in case of ammonium sulfate. Ammonium sulfate accounted for 70% variability in panicle number, whereas, urea accounted about 57% variability in panicle number. This means that ammonium sulfate was superior fertilizer for panicle production in lowland rice compared to ammonium sulfate. Overall, ammonium sulfate produced 8% higher panicles compared to urea. Fageria and Baligar (2001) reported a significant and quadratic increase of number of panicles with increasing N rates in lowland rice. Panicle number had a quadratic association with grain yield, however, influence was not significant (Y = 16.4439 + 7.9655X 0.4861X 2,R 2 = 0.0514 NS ). Hasegawa (2003) also reported that panicle density was unrelated to the grain yield of high yielding rice cultivars. Spikelet sterility was significantly and quadratically increased with increasing N rate by ammonium sulfate as well as urea source of fertilizer FIGURE 3 Relationship between nitrogen application rate by ammonium sulfate and urea and number of panicles in lowland rice.

Lowland Rice and N Sources 377 TABLE 2 Spikelet sterility and 1000 grain weight as influenced by ammonium sulfate and urea fertilization Spikelet sterility (%) Weight of 1000 grain (g) Nrate(mgkg 1 ) (NH 4 ) 2 SO 4 CO(NH 2 ) 2 (NH 4 ) 2 SO 4 CO(NH 2 ) 2 0 20.0 16.8 23.7 24.6 50 18.8 25.2 23.5 23.3 100 23.1 26.0 22.9 22.0 150 24.2 32.8 23.7 22.7 300 45.9 58.2 20.9 24.0 400 52.7 48.4 21.5 21.1 Average 30.8 34.6 22.7 23.0 F-test CV(%) 18.7 27.0 1.8 4.3 Regression analysis Nrate(NH 4 ) 2 SO 4 vs. spikelet sterility (Y) = 18.0179 + 0.0388X 0.00013X 2,R 2 = 0.8596 Nratevs.CO(NH 2 ) 2 spikelet sterility (Y) = 14.6398 + 0.1817X 0.00022X 2,R 2 = 0.6912 Nrate(NH 4 ) 2 SO 4 vs. 1000 grain weight (Y) = 23.8632 0.00675X, R 2 = 0.6757 N rate CO(NH 2 ) 2 vs. 1000 grain weight (Y) = 23.7420 0.00468X, R 2 = 0.2125 NS,NS Significant at the 1% probability level and non-significant, respectively. (Table 2). Ammonium sulfate accounted 86% variability in spikelet sterility and urea accounted 69% variability in spikelet sterility. Overall, ammonium sulfate produced 12% lower spikelet sterility compared to urea. Furthermore, at maximum yield level (about 150 mg N kg 1 ), ammonium sulfate produced about 36% lower spikelet sterility compared to urea. The increase in spikelet sterility with increasing N rates may be associated with more spikelets produced per plant with increasing N rates and photoassimilate produced by source may not be sufficient to fill large number of spikelets. In other words, there was no appropriate balance between source and sink. Optimal yield may be achieved by successful regulation of source-sink relationships for production and utilization of photoassimilate within plants (Fageria et al., 2006). Yoshida (1981) reported that the percentage of ripened spikelets decreased when the number of spikelets per unit area increased. Hence, there appears to be an optimum number of spikelets for maximum grain yield under certain conditions and attempts to increase spikelet number per unit area will not result in increased grain yield (Yoshida, 1981). In addition, spikelet sterility is variety characteristic and genetically controlled. In lowland rice cultivar Metica 1 (a Brazilian cultivar), spikelet sterility decreases with increasing N rates (Fageria and Baligar, 2001). Spikelet sterility was having significant linear negative association with grain yield, as expected (Figure 4). The 1000 grain weight was significantly (P < 0.01) influenced by ammonium sulfate and urea fertilization (Table 2). Under ammonium sulfate fertilizer treatment, 1000 grain weight varied from 20.9 to 23.7 g, with an

378 N. K. Fageria et al. FIGURE 4 Relationship between spikelet sterility and grain yield. average value of 22.7 g. Similarly, under urea fertilizer treatment, 1000 grain weight varied from 21.1 to 24.6 g, with average value of 23.0 g. Fageria and Barbosa Filho (2001) reported 1000 grain weight of eight lowland rice genotypes varied from 22.5 to 29.1 g, with an average value of 26.0 g. Similarly, Peng et al. (2000) reported 1000 grain weight of 7 lowland rice genotypes varied from 19.3 to 28.0 g, with an average value of 23.3 g. Overall, 1000 grain weight was 1.3% higher under urea source of N compared to ammonium sulfate source of N. Regression analysis shows significant decreases in 1000 grain weight with increasing N rate by ammonium sulfate. However, regression equation shows no significant influence of urea fertilization on 1000 grain weight. The 1000 grain weight was having significant quadratic association with grain yield (Figure 5). However, R 2 value was quite low (R 2 = 0.1782 ). Yoshida (1981) reported that under most conditions, the 1000 grain weight of field crops is a very stable varietal character. Grain harvest index (GHI) was significantly influenced by both the sources of N fertilizers (Table 3). Ammonium sulfate increased GHI quadratically and it accounted 94% variation in the GHI. Increasing N rates by urea quadratically decreased GHI and it accounted 91% variation in GHI. The GHI varied from 0.10 to 0.55, with an average value of 0.42 by ammonium fertilization. In case of urea, GHI varied FIGURE 5 Relationship between 1000 grain weight and grain yield.

Lowland Rice and N Sources 379 TABLE 3 Grain harvest index (GHI) and N harvest index (NHI) in lowland rice as influenced by ammonium sulfate and urea fertilization Grain harvest index N harvest index Nrate(mgkg 1 ) (NH 4 ) 2 SO 4 CO(NH 2 ) 2 (NH 4 ) 2 SO 4 CO(NH 2 ) 2 0 0.52 0.55 0.86 0.81 50 0.52 0.54 0.84 0.85 100 0.55 0.55 0.86 0.85 150 0.51 0.48 0.87 0.81 300 0.31 0.22 0.77 0.71 400 0.10 0.16 0.51 0.63 Average 0.42 0.41 0.79 0.77 F-test CV(%) 11.9 3.5 10.8 2.4 Regression analysis Nrate(NH 4 ) 2 SO 4 vs.ghi (Y) = 0.5203 + 0.00052X 0.0000039X 2,R 2 = 0.9370 N rate CO(NH 2 ) 2 vs. GHI (Y) = 0.5729 0.00054X 0.000014X 2,R 2 = 0.9051 Nrate(NH 4 ) 2 SO 4 vs. NHI (Y) = 0.8312 + 0.00090X 0.000041X 2,R 2 = 0.9376 N rate CO(NH 2 ) 2 vs. NHI (Y) = 0.8252 + 0.000224X 0 0000018X 2,R 2 = 0.9301 Significant at the 1% probability level. from 0.16 to 0.55, with an average value of 0.41. At maximum grain yield level ( 165 mg N kg 1 ), GHI was about 0.50. Snyder and Carlson (1984) reviewed GHI of rice and noted variations from 0.23 to 0.50. However, Kiniry et al. (2001) reported that rice GHI values varied greatly among cultivars, locations, seasons, and ecosystems, and ranged from 0.35 to 0.62. Fageria and Barbosa Filho (2001) reported GHI values of lowland rice genotypes varied from 0.33 to 0.44. The limit to which GHI can be increased is considered to be about 0.60 (Austin et al., 1980). The GHI is significantly and quadratically associated with grain yield (Figure 6). Maximum grain yield of about 19 g plant 1 was obtained with GHI of 0.50 (Figure 6). The term GHI was introduced by Donald (1962), and since has been considered to be an important trait for yield improvement in field crops. Several authors have reported that GHI is an important trait in FIGURE 6 Grain harvest index and grain yield.

380 N. K. Fageria et al. improving rice yield (Peng et al., 2000; Fageria and Barbosa Filho, 2001; Fageria et al., 2006). N Harvest Index, N Uptake and Use Efficiency Nitrogen harvest index (NHI) was significantly and quadratically increased with increasing N rate from 0 to 400 mg kg 1 by ammonium and urea source of N fertilizers (Table 3). The NHI values varied from 0.51 to 0.87, with an average value of 0.79 for ammonium sulfate and from 0.63 to 0.85, with an average value of 0.77 for urea. Ammonium sulfate accounted for 94% variation in NHI and Urea accounted for 93% variation in NHI. Overall, ammonium sulfate produced about 3% higher NHI compared to urea fertilization. Fageria and Baligar (2005) reported that the NHI values varied from crop species to crop species and among genotypes of the same species. Fageria and Barbosa Filho (2001) reported that NHI values varied from 0.44 to 0.66 in lowland rice depending on genotypes. The NHI had a significant linear association with grain yield (Figure 7). This means that increasing N uptake in grain can increase grain yield of lowland rice. Nitrogen concentration (content per unit of dry weight) in shoot and grain of rice was significantly influenced by ammonium sulfate and urea sources of fertilization (Table 4). Shoot as well as grain concentrations increased significantly in a quadratic fashion with increasing N rates from 0 to 400 mg kg 1 in both the sources of N fertilizers. Rice plants fertilized with urea were having 9% more N in the shoot compared to plants fertilized with ammonium sulfate. However, in grain N concentration was 8% higher in plants fertilized with ammonium sulfate compared to urea fertilization. Higher N in grain means higher yield as discussed earlier. Hence, ammonium sulfate is better source of N fertilizer for improving rice yield. The N concentration in shoot varied from 3.6 to 9.5 g kg 1, with an average value of 5.5gkg 1 for ammonium sulfate and 4.1 to 8.9 g kg 1, with an average value of6.0gkg 1 for urea. Similarly, N concentration in grain varied from 9.4 to 15.8 g kg 1, with average value of 12.2 for ammonium sulfate and 7.5 to FIGURE 7 Relationship between nitrogen harvest index and grain yield.

Lowland Rice and N Sources 381 TABLE 4 Nitrogen concentration in shoot and grain of lowland rice as influenced by ammonium sulfate and urea fertilization (NH 4 ) 2 SO 4 CO(NH 2 ) 2 N rate N conc. in N conc. in N conc. in N conc. in (mg kg 1 ) shoot (g kg 1 ) grain (g kg 1 ) shoo(g kg 1 ) grain (g kg 1 ) 0 3.6 10.6 4.3 7.5 50 4.2 9.4 4.1 9.3 100 4.5 10.6 4.6 9.9 150 4.8 12.5 6.0 11.3 300 6.6 14.1 8.2 14.1 400 9.5 15.8 8.9 15.5 Average 5.5 12.2 6.0 11.3 F-test CV(%) 6.0 12.4 8.2 4.6 Regression analysis Nrate(NH 4 ) 2 SO 4 vs. N conc. in shoot (Y) = 0.3227 + 0.00137X, R 2 = 0.9158 Nrate(NH 4 ) 2 SO 4 vs. N conc. in grain (Y) = 0.9641 + 0.00151X, R 2 = 0.6955 N rate CO(NH 2 ) 2 vs. N conc. in shoot (Y) = 0.3803 + 0.00132X, R 2 = 0.9158 N rate CO(NH 2 ) 2 vs. N conc. in grain (Y) = 0.7986 + 0.00196X, R 2 = 0.9034 Significant at the 1% probability level. 15.5 g kg 1, with an average value of 11.3 g kg 1 for urea. Fageria (2003) reported that optimum N concentration for maximum shoot yield in lowland rice was 6.5 g kg 1 and for maximum grain yield was 10.9 g kg 1. Nitrogen uptake (dry weight X concentration) in shoot and grain was significantly influenced by ammonium sulfate and urea sources of fertilization (Table 5). The increase in shoot N uptake was linear for both the N sources, while N uptake in grain was quadratic for ammonium sulfate as well as urea sources of N. The increase in N uptake in shoot and grain followed shoot dry weight and grain yield pattern for both the sources of fertilization. The N uptake in shoot was having significant quadratic association with grain yield, while N uptake in grain was having significant linear association with grain yield (Figures 8 and 9). Fageria (2003) reported a significant quadratic association with N uptake in shoot and grain and grain yield of lowland rice. N use efficiency or N use ratio (dry weight/n uptake) in grain was significantly influenced under both the sources of N fertilizers (Table 6). However, N use efficiency was non-significant for shoot dry weight for ammonium sulfate as well as urea fertilization. The N use efficiency in shoot as well as grain was having significant quadratic association with grain yield (Figures 10 and 11). However, shoot N use efficiency accounted for 26% variation in grain yield and variability in grain yield was 57% due to N use efficiency in grain (Figures 10 and 11). This means that N use efficiency in grain is more important than N use efficiency in shoot in improving grain yield of rice.

382 N. K. Fageria et al. TABLE 5 Nitrogen uptake in shoot and grain of lowland rice as influenced by ammonium sulfate and urea fertilization (NH 4 ) 2 SO 4 CO(NH 2 ) 2 N rate N uptake in N uptake. in N uptake in N uptake in (mg kg 1 ) shoot (mg plant 1 ) grain (mg plant 1 ) shoot (mg plant 1 ) grain (mg plant 1 ) 0 22.2 143.2 23.1 96.9 50 31.1 170.2 26.8 146.8 100 34.4 211.8 35.5 195.4 150 41.7 284.9 26.8 209.3 300 64.7 219.5 66.3 162.0 400 82.9 86.4 72.7 124.7 Average 46.2 186.0 45.5 155.9 F-test CV(%) 16.2 19.0 13.7 12.6 Regression analysis N rate ((NH 4 ) 2 SO 4 ) vs N uptake in shoot (Y) = 21.2793 + 0.1493X, R 2 = 0.9158 N rate ((NH 4 ) 2 SO 4 ) vs N uptake in grain (Y) = 125.3801 + 1.4860X 0.00393X 2,R 2 = 0.7600 N rate CO(NH 2 ) 2 vs N uptake in shoot (Y) = 20.3343 + 0.1959X, R 2 = 0.9177 N rate CO(NH 2 ) 2 vs. N uptake in grain (Y) = 104.8821 + 1.0113X 0.00247X 2,R 2 = 0.7568 Significant at the 1% probability level. FIGURE 8 Relationship between N uptake in shoot and grain yield. FIGURE 9 Relationship between N uptake in grain and grain yield.

Lowland Rice and N Sources 383 TABLE 6 Nitrogen utilization ratio in shoot and grain of lowland rice as influenced by ammonium sulfate and urea fertilization (NH 4 ) 2 SO 4 CO(NH 2 ) 2 N rate N use ratio in shoot N use ratio in grain N use ratio in shoot N use ratio in grain (mg kg 1 ) (mg mg 1 ) (mg mg 1 ) (mg mg 1 ) (mg mg 1 ) 0 140.2 102.3 114.5 133.6 50 134.3 106.6 126.3 108.2 100 120.0 94.6 114.1 100.6 150 130.5 80.3 108.8 88.9 300 143.8 70.7 156.2 70.7 400 153.0 63.5 154.0 64.5 Average 137.0 86.3 129.0 94.4 F-test NS NS CV(%) 21.1 17.1 19.2 4.6 Regression analysis Nrate(NH 4 ) 2 SO 4 vs. N use ratio in shoot (Y) = 129.0233 + 4.7760X, R 2 = 0.0671 NS Nrate(NH 4 ) 2 SO 4 vs. N use ratio in grain (Y) = 104.3718 0.10849X, R 2 = 0.5730 N rate CO(NH 2 ) 2 vs. N use ratio in shoot (Y) = 137.6486 0.12625X + 0.00043X 2,R 2 = 0.1268 NS Nrate(CO(NH 2 ) 2 vs. N use ratio in grain (Y) = 129.8208 0.3427X + 0.00045X 2,R 2 = 0. 9618,NS Significant at the 1% probability level and non-significant, respectively. FIGURE 10 Relationship between N utilization efficiency in shoot and grain yield. FIGURE 11 Relationship between N utilization efficiency in grain and grain yield.

384 N. K. Fageria et al. Fageria and Baligar (2005) reported significant quadratic association with N use efficiency and grain yield of lowland rice. Soil Chemical Properties Soil ph, Ca, and K contents were significantly influenced by increasing N rates by urea but effect was non-significant for these soil chemical properties when N was added with ammonium sulfate fertilization (Table 7). There was a quadratic increase in soil ph with increasing N rates with urea (Y = 5.3212 + 0.000501X 0.00000095X 2,R 2 = 0.3903 ). Both ammonium sulfate and urea are acidity producing fertilizers when applied to soil, and ammonium sulfate produces more acidity compared to urea. Ammonium sulfate slightly decreases soil ph with increasing N rate as expected. The increase in ph with increasing N rate with urea might be associated with original ph of urea fertilizer. Original ph of ammonium sulfate fertilizer used in the experiment was 5.0 and urea fertilizer was having original ph 7.2. Although both the fertilizers might have produced acidic reaction in the soil but due to addition of lime at sowing and urea fertilizer having high ph, still soil ph increased. Soil ph across two fertilizer sources was positively associated with grain yield (Y = 1577.9340 + 605.2022X 57.3635X 2,R 2 = 0.2983 ). Based on regression equation, maximum grain yield was obtained TABLE 7 Influence of ammonium sulfate and urea on soil ph, Ca, P and K contents of soil after harvest of lowland rice Nrate(mgkg 1 ) ph Ca (cmol c kg 1 ) P mg (kg 1 ) K (cmol c kg 1 ) (NH 4 ) 2 SO 4 0 5.2 4.1 76.4 0.15 50 5.2 3.6 74.5 0.04 100 5.2 3.9 81.0 0.04 150 5.3 4.7 77.4 0.04 300 5.1 4.1 61.3 0.04 400 5.1 4.9 61.3 0.05 Average 5.2 4.2 72.0 0.06 F-test NS NS NS CV (%) 3.5 12.1 9.1 20.7 CO(NH 2 ) 2 0 5.2 3.6 79.4 0.09 50 5.6 4.1 79.1 0.05 100 5.4 3.3 83.9 0.04 150 5.3 3.4 80.7 0.04 300 5.6 4.4 63.5 0.05 400 5.7 4.1 73.2 0.06 Average 5.4 3.8 76.6 0.05 F-test CV (%) 2.2 8.3 4.6 13.3,NS Significant at the 1% probability level and non-significant, respectively.

Lowland Rice and N Sources 385 at ph 5.3. Rice is tolerant to soil acidity (Fageria et al., 2006) and Fageria and Baligar (1999) reported that relative shoot dry matter yield of lowland rice was obtained at ph 4.9 in Brazilian Inceptisol. The difference in present study for ph compared to earlier study may be due to different cultivars used. Soil Ca content was significantly and linearly increased with increasing N rates with urea fertilizer (Y = 4.2697 + 0.3078X, R 2 = 0.5314 ). The increase in Ca content may be associated with an increase in soil ph. Association between Ca content in the soil and grain yield was significantly linear and negative (Y = 26.7311 2.8653X, R 2 = 0.1056 ) indicating no beneficial effect of increasing Ca level in the soil on rice yield. Soil P was significantly and quadratically increased in case of ammonium sulfate with increasing soil N (Y = 77.3684 + 0.0038X 0.00013X 2,R 2 = 0.5394 )butincaseof urea P was significantly and linearly decreased with increasing N rate (Y = 82.0211 0.0324X, R 2 = 0.3944 ). The decrease in soil P at higher N rate may be associated with P immobilization by high Ca content at higher ph (Fageria et al., 1997). Phosphorus content in the soil was having significantly linear association with grain yield (Y = 6.0203 + 0.2850X, R 2 = 0.2352 ), indicating soil under investigation was low in available P for rice growth and development. Increasing soil N by ammonium sulfate did not influence soil K significantly. However, increasing N rate significantly influenced extractable soil K by urea (Table 7). The relationship between N rate and soil K was significant and quadratic (Y = 0.0786 0.00044X + 0.00000101X 2,R 2 = 0.7123 ), indicating decrease in K content of soil and then increase. The K decrease may be associated with high uptake of this element with high yield of shoot dry matter and grain yield of rice with increasing N rates. Uptake of potassium is maximum by rice compared with other macronutrients (Fageria et al. 1997). Association between soil K and grain yield was significant and quadratically negative (Y = 26.7208 304.1100X + 1313.7190X 2,R 2 = 0.2443 ), indicating that soil in which rice plants grown was not K deficient. CONCLUSIONS Nitrogen fertilizer applied through ammonium sulfate and urea significantly increased grain yield of lowland rice. Rice response to both the sources was similar, however, magnitude of response was different and ammonium sulfate at maximum yield level N rate produced about 22% more grain yield compared to urea. Higher grain yield under ammonium sulfate fertilizer was associated with high shoot dry weight, higher panicle number, lower spikelet sterility, high grain harvest index, high N harvest index and lower spikelet sterility compared to urea fertilization. The plant growth and yield components which were associated with increase grain yield were shoot dry weight

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