Effects of Nitrogen Application Level on Rice Nutrient Uptake and Ammonia Volatilization

Rice Science, 2013, 20(2): 139 147 Copyright 2013, China National Rice Research Institute Published by Elsevier BV. All rights reserved DOI: 10.1016/S1672-6308(13)60117-1 Effects of Nitrogen Application Level on Rice Nutrient Uptake and Ammonia Volatilization YU Qiao-gang, YE Jing, YANG Shao-na, FU Jian-rong, MA Jun-wei, SUN Wan-chun, JIANG Li-na, WANG Qiang, WANG Jian-mei (Institute of Environment Resource and Soil Fertilizer, Zhejiang Academy of Agriculture Science, Hangzhou 310021, China) Abstract: The effects of different nitrogen application levels on nutrient uptake and ammonia volatilization were studied with the rice cultivar Zheyou 12 as a material. The accumulative amounts of nitrogen, phosphorus and potassium in rice plants across all growth stages showed a trend to increase with increasing nitrogen application levels from 0 to 270 kg/hm 2, but decreased at nitrogen application levels exceeding 270 kg/hm 2. Moreover, the accumulative uptake of nitrogen, phosphorus and potassium by the rice plants was increased by application of organic manure in combination with 150 kg/hm 2 nitrogen. The nitrogen uptake was high during the jointing to heading stages. Correlation analysis showed that rice yield was positively correlated with the accumulative uptake of nitrogen, phosphorus and potassium by the rice plants. The highest correlation coefficient observed was between the amount of nitrogen uptake and rice yield. The rate and accumulative amounts of ammonia volatilization increased with increasing nitrogen fertilizer application level. Compared with other stages, the rate and accumulative amount of ammonia volatilization were higher after base fertilizer application. The ammonia volatilization rates in response to the nitrogen application levels of 270 kg/hm 2 and 330 kg/hm 2 were much higher than those in the other treatments. The loss of nitrogen through ammonia volatilization accounted for 23.9% of the total applied nitrogen at the nitrogen application level of 330 kg/hm 2. Key words: rice; nitrogen; nutrient uptake; ammonia volatilization China is a major rice-producing country and accounts for one-fifth of the rice cultivation area in the world. Nitrogen is an essential nutrient for rice production and plays an important role in sustaining high yields (Guindo et al, 1994; Peng et al, 2002; Liu et al, 2007; Yang et al, 2010). Although the nitrogen fertilizer application level is very high in rice production in China, the nitrogen use efficiency is very low, thus resulting in serious ammonia volatilization (Fan et al, 2005; Lin et al, 2007; Ye et al, 2011). Furthermore, the high nitrogen loss induces non-point-source pollution and other environmental pollution. The question of how to reduce the nitrogen application level and improve nitrogen use efficiency, while rice plants maintaining high yield and good quality, has become an important research focus (Xie, 1998; Vlek and Byrnes, 2000; Zhu et al, 2005). Received: 24 April 2012; Accepted: 10 October 2012 Corresponding authors: MA Jun-wei (majw111@126.com); FU Jian-rong (fujr@mail.hz.zj.cn) Nutrient absorption characteristics differ with rice cultivar, fertilizer type, fertilization technology, soil type and environmental factors (Wopereis-Pura et al, 2002; Li et al, 2005; Liu et al, 2005; Zhang et al, 2006; Huang et al, 2008). The nutrient absorption amount varies with rice growth stage. Absorption is low at the seedling stage and peaks before the heading stage, then decreases as root activity declines (Guindo et al, 1994; Liu et al, 2007). Increasing the nitrogen application level could significantly increase rice production within limits. The highest nitrogen uptake is observed at the tillering stage, followed by the young panicle developmental stage. The stage of the highest phosphorus uptake is the young panicle developmental stage, followed by the tillering stage, but much phosphorus is also absorbed at the maturity stage. The period of the highest potassium uptake is before the heading stage and little is absorbed after heading (Moll et al, 1982; Wopereis-Pura et al, 2002; Zeng et al, 2007; Wang et al, 2011; Wu et al, 2011). Research on the effects of nitrogen application level

140 on rice growth has been undertaken previously, but little information is available on the relationship between increasing nitrogen application level and nutrient uptake, especially in relation to nitrogen management practices and the nutritional characteristics. To provide a theoretical basis for development of methods to improve nitrogen use efficiency of rice, the nutrient uptake characteristics and loss of nitrogen through ammonia volatilization were studied in a rice field experiment on the northern plain of Zhejiang Province, China, with a late-season hybrid rice Zheyou 12 as a material. MATERIALS AND METHODS Experimental design The experiment was sited in the soil fertility field monitoring station of the Ministry of Agriculture, China, which is located on the northern plain of Zhejiang Province, China (30 26 24 N, 120 24 42 E). The area experiences a northern subtropical maritime moist climate with an average annual temperature of 15.9 C, average annual rainfall of 1 187 mm, and sunlight hours of 2 002.9 h. The soil type is a silt loam with the following physical and chemical properties: ph 7.41, soil bulk density 1.16 g/cm 3, total nitrogen 1.13 g/kg, available phosphate 98.0 mg/kg, available potassium 44.2 mg/kg, and organic material 13.5 g/kg. The selected rice cultivar Zheyou 12 is a late-season cultivar. Eight treatments were applied: 1) no fertilizer applied (N0); 2) nitrogen application of 90 kg/hm 2 (N90); 3) nitrogen application of 150 kg/hm 2 (N150); 4) nitrogen application of 210 kg/hm 2 (N210); 5) nitrogen application of 270 kg/hm 2 (N270); 6) nitrogen application of 330 kg/hm 2 (N330); 7) organic manure application of 3 000 kg/hm 2 and nitrogen application of 150 kg/hm 2 (M + N150), which equated to a nitrogen application level of 215.1 kg/hm 2 ; and 8) organic manure application of 3 000 kg/hm 2 and nitrogen application of 210 kg/hm 2 (M + N210), which equated to a nitrogen application level of 275.1 kg/hm 2. The total nitrogen content in the organic manure was 2.17%. All treatments were replicated three times. Each experimental plot was arranged randomly and comprised an area of 27.5 m 2 (5.5 m 5.0 m). Each plot was separated by a cement ridge and covered with plastic film, and had independent drainage and irrigation ditches, so as to prevent the spread of water and fertilizers between plots. All treatments Rice Science, Vol. 20, No. 2, 2013 received the same amounts of phosphate and potassium, applied as calcium superphosphate at 630 kg/hm 2 (P 2 O 5 75.6 kg/hm 2 ), and potassium chloride at 150 kg/hm 2 (K 2 O 90.0 kg/hm 2 ). All fertilizers that contained phosphate and potassium were applied as base fertilizers. The nitrogen fertilizer was urea, of which 60% was applied as base fertilizer and 40% as topdressing. Base fertilizers were applied one week before transplanting, and topdressings were applied twice: one week after transplanting and at booting. The rice seedlings were transplanted with an interplant spacing of 20 cm 20 cm in early July, and grains were harvested in November. Measuring items and methods Three hills of uniform rice plants in each plot were sampled at the jointing, heading and maturity stages, respectively. The rice plant samples were divided into straw and panicles and analyzed separately. Samples were dried at 105 C for 30 min, then at 60 C to constant weight. The dry weight was recorded and the contents of nitrogen, phosphorus and potassium were measured after the samples were ground to powder with a mortar. Plant samples were digested with H 2 SO 4 -H 2 O 2. Nitrogen content was measured with the alkali solution diffusion method, phosphorus content with the vanadium molybdenum yellow colorimetric method, and potassium content with the atomic absorption spectrometry method in accordance with Lu (1999). Ammonia volatilized from the soil was collected by a device made of 6-mm-thick, opaque black PVC tubing (Wang et al, 2002). At the top of the collection device, two pieces of sponge soaked with phosphoric acidglycerine solution were placed at a spacing of 5 cm, and the bottom of the collection device was inserted into the soil (Fig. 1). The collection device was placed Sponge Paddy soil Ø = 16 cm 20 cm 30 cm Fig. 1. Schematic illustration of ammonia volatilization collection device.

YU Qiao-gang, et al. Effects of Nitrogen Application Level on Rice Nutrient Uptake and Ammonia Volatilization 141 between rice plants immediately after fertilizer application. Volatile ammonia data were collected until the ammonia volatilization levels were similar to those in the N0 treatment. The amount of volatilized ammonia was sampled every 4 d after the base fertilizer application and 2 d after topdressing. The lower sponge in the device was removed and quickly placed into a plastic bag, and replaced by a fresh sponge soaked in the phosphoric acid-glycerine solution. The upper sponge in the device was replaced every 5 7 d depending on the moistness of the sponge. The ammonia content in the solution was measured with the ammonia diffusion method after 1.0 mol/l KCl solution (100 ml) was added to the sampled sponge. Total volatilized ammonia was calculated from the individual measurements. The rate of ammonia volatilization (V NH3-N, kg/(hm 2 d)) was calculated as: V NH3-N = 0.01M / (A D), where M is the average amount of ammonia determined for a single device at each time point (mg), A is the cross-sectional area of the collection device (m 2 ), and D denotes each successive collection time (d). Data analysis All data were processed with Microsoft Excel 2003 and DPS statistical software (Tang and Feng, 2002). RESULTS Effects of nitrogen application level on nitrogen content and uptake at different rice growth stages Within a certain nitrogen application range, the total nitrogen accumulation in the rice plants increased with the increasing nitrogen application level at the rice maturity stage, which followed the rank order N270 > N330 > N210 > N150 > N90 > N0 (Table 1). At the jointing stage, the total nitrogen accumulation in plants of the N0 treatment was significantly different from that of the other treatments. No significant difference in total nitrogen accumulation was observed among all nitrogen application treatments except the N90 treatment, and there was also no significant difference between the N90 and N150 treatments at the jointing stage. The total nitrogen accumulation in the N90 treatment was significantly different from that of the N150, N270 and N330 treatments at the heading stage. In addition, the N150 and N210 treatments differed significantly from the N270 and N330 treatments at this stage. At the maturity stage, the total nitrogen accumulation in the N90 treatment was significantly different from the N270 and N330 treatments, but no significant differences among the N150, N210, N270 and N330 treatments were observed. At the heading and maturity stages, the total nitrogen contents in the straw and panicles were higher in the M + N150 treatment than those in the N210 treatment (Table 1). However, the total nitrogen contents in the straw and panicles were lower in the M + N210 treatment than those in the N270 treatment except that in the panicles at the heading stage. These results indicated that combined application of organic and inorganic fertilizers stimulated nitrogen uptake in the rice cultivar Zheyou 12 under a nitrogen application level of 210 kg/hm 2. When the nitrogen application level exceeded 210 kg/hm 2, combined application of organic and inorganic fertilizers might reduce nitrogen uptake, resulting in heavy nitrogen loss. Effects of nitrogen application level on phosphorus uptake at different rice growth stages Total phosphorus accumulation in the rice plants differed significantly between the N0 and all nitrogen application treatments except that in the panicles at the Table 1. Total nitrogen accumulation in rice plants at different growth stages in response to different nitrogen application levels. kg/hm 2 Jointing stage Heading stage Maturity stage Straw Total Straw Panicle Total Straw Panicle Total N0 25.82 c 25.82 c 62.86 d 10.40 cd 73.26 e 20.17 c 54.27 d 74.44 d N90 45.35 b 45.35 b 97.67 c 15.53 a 113.19 d 39.83 b 91.54 c 131.36 c N150 54.34 ab 54.34 ab 122.49 b 14.38 abc 136.87 c 61.24 ab 141.07 abc 202.32 abc N210 64.18 a 64.18 a 119.94 b 11.13 bcd 131.07 cd 68.48 ab 153.46 ab 221.94 abc N270 65.01 a 65.01 a 170.20 a 10.04 d 180.24 a 103.43 a 172.15 a 275.58 a N330 60.07 a 60.07 a 164.51 a 7.88 d 172.38 ab 90.29 a 147.75 ab 238.04 ab M + N150 60.06 a 60.06 a 127.76 b 14.64 abc 142.40 c 81.63 ab 174.84 a 256.47 ab M + N210 56.11 ab 56.11 ab 134.06 b 14.93 ab 148.98 bc 70.29 ab 109.27 bc 179.56 bc N0, No fertilizer applied; N90, Nitrogen applied at 90 kg/hm 2 ; N150, Nitrogen applied at 150 kg/hm 2 ; N210, Nitrogen applied at 210 kg/hm 2 ; N270, Nitrogen applied at 270 kg/hm 2 ; N330, Nitrogen applied at 330 kg/hm 2 ; M + N150, Organic manure applied at 3 000 kg/hm 2 with urea-nitrogen of 150 kg/hm 2 ; M + N210, Organic manure applied at 3 000 kg/hm 2 with urea-nitrogen of 210 kg/hm 2. Values followed by the same letter in the same column do not differ significantly between the treatments at the 5% significant level.

142 Rice Science, Vol. 20, No. 2, 2013 Table 2. Total phosphorus accumulation in rice plants at different growth stages in response to different nitrogen application levels. kg/hm 2 Jointing stage Heading stage Maturity stage Straw Total Straw Panicle Total Straw Panicle Total N0 7.80 b 7.80 b 20.12 c 1.73 bc 21.75 c 4.12 d 19.14 d 23.26 d N90 10.43 a 10.43 a 22.94 bc 2.49 a 25.43 ab 4.97 c 22.98 bc 27.95 c N150 11.43 a 11.43 a 26.01 abc 2.39 a 28.41 ab 11.23 bc 32.55 ab 43.79 ab N210 13.31 a 13.31 a 22.12 bc 1.76 bc 23.89 b 9.33 c 31.09 abc 40.42 abc N270 13.22 a 13.22 a 29.38 a 1.54 c 30.92 a 17.70 a 32.42 ab 50.12 a N330 12.00 a 12.00 a 27.86 ab 1.25 c 29.11 ab 15.60 ab 31.23 abc 46.83 ab M + N150 12.37 a 12.37 a 28.40 ab 2.40 a 30.80 a 15.67 ab 35.83 a 51.51 a M + N210 12.05 a 12.05 a 25.14 abc 2.30 ab 27.44 ab 11.53 bc 22.76 c 34.29 bc N0, No fertilizer applied; N90, Nitrogen applied at 90 kg/hm 2 ; N150, Nitrogen applied at 150 kg/hm 2 ; N210, Nitrogen applied at 210 kg/hm 2 ; N270, Nitrogen applied at 270 kg/hm 2 ; N330, Nitrogen applied at 330 kg/hm 2 ; M + N150, Organic manure applied at 3 000 kg/hm 2 with urea-nitrogen of 150 kg/hm 2 ; M + N210, Organic manure applied at 3 000 kg/hm 2 with urea-nitrogen of 210 kg/hm 2. Values followed by the same letter in the same column do not differ significantly between the treatments at the 5% significant level. heading stage (Table 2). At the jointing stage, the highest total phosphorus accumulation was observed in the N210 treatment, whereas at the heading and maturity stages, the total phosphorus accumulation was the highest in the N270 treatment. Organic manure application in the N150 treatment stimulated phosphorus uptake in Zheyou 12. However, organic manure application in the N210 treatment reduced the total phosphorus accumulation. These results indicated that organic manure application promoted phosphorus uptake when the nitrogen application level was under 150 kg/hm 2. The variation in phosphorus accumulation was similar to that in nitrogen accumulation. The total accumulated amounts of nitrogen and phosphorus showed an increasing tendency with plant development, whereas the total nitrogen and phosphorus contents showed a decreasing tendency in the straw and an increasing tendency in the panicles from heading to maturity stage. Consequently, the phosphorus content was higher in the grain than in the straw at the harvest stage. At the heading and maturity stages, the total phosphorus contents of the straw and panicles were higher in the M + N150 treatment than those in the N210 treatment (Table 2), whereas the total phosphorus contents of the straw and panicles were lower in the M + N210 treatment than those in the N270 treatment except that in the panicle at the heading stage. These results indicated that combined application of organic and inorganic fertilizers might promote phosphorus uptake in Zheyou 12 under the nitrogen application level of 210 kg/hm 2. When the nitrogen application level exceeded 210 kg/hm 2, combined application of organic and inorganic fertilizer might reduce phosphorus uptake in rice. Effects of nitrogen application level on potassium uptake at different rice growth stages The total potassium accumulation in the rice plants showed a trend to increase with increasing nitrogen application level at different growth stages (Table 3). The total potassium accumulation in all nitrogen application treatments showed a significant difference from the N0 treatment except that in the panicles at the heading stage. At the jointing stage, the highest total potassium accumulation was observed in the N210 treatment. However, at the heading and maturity stages, the total potassium accumulation was the highest Table 3. Total potassium accumulation in rice plants at different growth stages in response to different nitrogen application levels. kg/hm 2 Jointing stage Heading stage Maturity stage Straw Total Straw Panicle Total Straw Panicle Total N0 43.67 b 43.67 b 119.95 c 7.20 b 127.15 c 93.47 c 18.03 c 111.50 c N90 63.64 a 63.64 a 158.68 ab 10.96 a 169.64 ab 117.47 b 30.94 b 148.40 b N150 68.72 a 68.72 a 159.41 ab 10.57 a 169.98 ab 171.84 ab 44.41 ab 216.25 ab N210 74.89 a 74.89 a 139.54 b 7.85 b 147.39 b 164.56 ab 45.51 ab 210.07 ab N270 67.19 a 67.19 a 180.99 a 7.17 b 188.16 a 203.22 a 49.29 a 252.51 a N330 65.77 a 65.77 a 178.36 a 6.08 b 184.45 a 176.53 ab 43.70 ab 220.24 ab M + N150 71.87 a 71.87 a 171.82 a 11.01 a 182.83 a 214.35 ab 50.62 a 264.97 a M + N210 69.77 a 69.77 a 167.25 ab 10.71 a 177.96 ab 161.97abd 33.32 b 195.29 ab N0, No fertilizer applied; N90, Nitrogen applied at 90 kg/hm 2 ; N150, Nitrogen applied at 150 kg/hm 2 ; N210, Nitrogen applied at 210 kg/hm 2 ; N270, Nitrogen applied at 270 kg/hm 2 ; N330, Nitrogen applied at 330 kg/hm 2 ; M + N150, Organic manure applied at 3 000 kg/hm 2 with urea-nitrogen of 150 kg/hm 2 ; M + N210, Organic manure applied at 3 000 kg/hm 2 with urea-nitrogen applied of 210 kg/hm 2. Values followed by the same letter in the same column do not differ significantly between the treatments at the 5% significant level.

YU Qiao-gang, et al. Effects of Nitrogen Application Level on Rice Nutrient Uptake and Ammonia Volatilization 143 in the N270 treatment. The total potassium content increased with plant development and showed a trend to increase in the straw and panicles in conjunction with plant development. At the maturity stage, the total potassium content in the straw remained higher than that in the panicles. At the heading and maturity stages, the total potassium contents of the straw and panicles were higher in the M + N150 treatment than those in the N210 treatment (Table 3). However, the total potassium contents of the straw and panicles were lower in the M + N210 treatment than those in the N270 treatment except that in the panicles at the heading stage. These results indicated that combined application of organic and inorganic fertilizers increased potassium uptake in Zheyou 12 under the nitrogen application level of 210 kg/hm 2. Relationship between nutrient uptake and yield Significant correlations existed between yield and total nitrogen, phosphorus and potassium absorption amounts, and the respective correlation coefficients were 0.80, 0.73 and 0.77 (Fig. 2). The highest correlation coefficient was observed between yield and total nitrogen absorption, which indicated that the nitrogen levels directly affected the rice yield. The average nitrogen absorption levels in the rice plants were 57.87, 88.57 and 68.59 kg/hm 2 in the periods from transplanting to jointing, jointing to heading, and heading to maturity, respectively (Table 4). This result indicated that across the whole growth period, the rice plants accumulated the highest amount of nitrogen from the jointing to heading stages. Moreover, nitrogen uptake was the highest in the N270 treatment at all growth stages. However, nitrogen uptake increased more slowly and even declined when the nitrogen Table 4. Effects of different nitrogen application levels on nitrogen uptake by rice plants at different growth stages. kg/hm 2 Transplanting to Jointing to Heading to jointing heading maturity N0 25.82 d 21.62 d 1.18 e N90 45.35 c 67.84 c 18.17 d N150 54.34 b 82.53 b 65.45 b N210 64.18 a 66.89 c 90.87 a N270 65.01 a 115.23 a 95.34 a N330 60.07 ab 112.31 a 65.66 b M + N150 60.06 ab 82.34 b 114.07 a M + N210 56.11 b 92.87 ab 30.58 c N0, No fertilizer applied; N90, Nitrogen applied at 90 kg/hm 2 ; N150, Nitrogen applied at 150 kg/hm 2 ; N210, Nitrogen applied at 210 kg/hm 2 ; N270, Nitrogen applied at 270 kg/hm 2 ; N330, Nitrogen applied at 330 kg/hm 2 ; M + N150, Organic manure applied at 3 000 kg/hm 2 with urea-nitrogen of 150 kg/hm 2 ; M + N210, Organic manure applied at 3 000 kg/hm 2 with urea-nitrogen of 210 kg/hm 2. Values followed by the same letter in the same column do not differ significantly between the treatments at the 5% significant level. application level exceeded 270 kg/hm 2, which indicated that the amount of nitrogen uptake might increase with increasing nitrogen application level at all rice growth stages when the nitrogen application level was below 270 kg/hm 2 for Zheyou 12. The amount of nitrogen absorbed directly affected the nitrogen utilization efficiency and rice yield. A significant positive correlation was observed between nitrogen accumulation and yield at the different growth stages, especially from the transplanting to jointing stages (Fig. 3). The correlation coefficients were 0.89, 0.70 and 0.75 at the transplanting to jointing, jointing to heading, and heading to maturity stages, respectively. Effects of nitrogen application level on ammonia volatilization from soil Ammonia volatilization rates from the soil at different nitrogen application levels are listed in Table 5. The ammonia volatilization rates showed a trend to increase Fig. 2. Correlation between rice yield and uptake of nitrogen, phosphorus and potassium in response to different nitrogen application levels. **, P < 0.01.

144 Rice Science, Vol. 20, No. 2, 2013 0.70** Fig. 3. Correlation between nitrogen accumulation and rice yield at different growth stages. **, P < 0.01. with increasing nitrogen application level after the fertilizer was applied. The ammonia volatilization rates were higher in the N270 and N330 treatments compared with the other treatments after topdressing at the tillering and panicle initiation stages. This result indicated that ammonia volatilization was sharply increased if the nitrogen application level exceeded 270 kg/hm 2. The accumulative ammonia volatilization from the soil showed significant differences among the three fertilizer application periods (Fig. 4). The largest accumulative amount of ammonia volatilization was observed after application of the base fertilizer, then following topdressing at the tillering stage, and the lowest accumulative amount was observed after topdressing at the panicle stage. The accumulative amount of ammonia volatilization was greatly affected by the nitrogen application level. The accumulative amounts of ammonia volatilization after base fertilizer application and after topdressing at the tillering stage showed more distinct difference. The accumulative amount of ammonia volatilization in the N330 treatment was 7.30 kg/hm 2, which accounted for 23.9% of the base nitrogen fertilizer. The percentages of ammonia volatilization relative to the total amount of base Fig. 4. Effect of different nitrogen application levels on accumulative amount of ammonia volatilization. nitrogen fertilizer were 19.06%, 21.23%, 24.74% and 22.06% in the N90, N150, N210 and N270 treatments, respectively. Overall, the accumulative amounts of ammonia volatilization from the soil increased with increasing nitrogen application level. The loss of ammonia was serious at early stages of rice growth, but low at advanced developmental stages. DISCUSSION A shortage in soil nitrogen supply is one of the main limiting factors for achieving high rice yields, and Table 5. Ammonia volatilization from the soil of rice fields in response to different nitrogen application levels applied as a base fertilizer or topdressing. kg/(hm 2 d) Base fertilizer Topdressing at tillering stage Topdressing at panicle stage 1 4 d 5 8 d 9 12 d Average 1 2 d 3 4 d 5 6 d Average 1 2 d 3 4 d 5 6 d Average N0 0.10 e 0.25 d 0.11 c 0.15 0.11 e 0.07 d 0.07 c 0.08 0.17 d 0.10 c 0.09 b 0.12 N90 1.77 d 0.47 d 0.15 c 0.80 0.29 d 0.09 d 0.09 c 0.16 0.13 d 0.17 c 0.10 b 0.13 N150 3.00 c 1.12 c 0.56 b 1.56 0.37 cd 0.32 c 0.18 c 0.29 0.37 c 0.20 bc 0.10 b 0.22 N210 4.10 b 2.47 b 1.08 a 2.55 0.52 c 0.53 c 0.19 c 0.41 0.47 bc 0.23 bc 0.10 b 0.27 N270 4.98 ab 2.53 b 1.21 a 2.91 2.93 b 1.47 b 1.12 b 1.84 0.57 ab 0.30 ab 0.11 b 0.33 N330 5.81 a 4.24 a 1.49 a 3.85 4.88 a 4.40 a 2.67 a 3.98 0.72 a 0.34 a 0.18 a 0.41 N0, No fertilizer applied; N90, Nitrogen applied at 90 kg/hm 2 ; N150, Nitrogen applied at 150 kg/hm 2 ; N210, Nitrogen applied at 210 kg/hm 2 ; N270, Nitrogen applied at 270 kg/hm 2 ; N330, Nitrogen applied at 330 kg/hm 2. Values followed by the same letter in the same column do not differ significantly between the treatments at the 5% significant level.

YU Qiao-gang, et al. Effects of Nitrogen Application Level on Rice Nutrient Uptake and Ammonia Volatilization 145 nitrogen fertilizer is an important component in rice production. The key period for nitrogen absorption by rice plants is from tillering to flowering, during this period the absorption of soil nitrogen is at its maximum rate. Most of the absorbed nitrogen is stored in the leaves and may be transported to the grains during grain filling (Jiang et al, 2004; Duan et al, 2005). Cao et al (1992) reported that the largest amount of nitrogen absorption occurs at the tillering and booting stages. Wang et al (2003) indicated that the peak period of nitrogen uptake is from the jointing to heading stage, which accounts for 34% 38% of the total nitrogen absorbed during the whole rice growth period. The present study showed that the nitrogen uptake peaked in Zheyou 12 from jointing to heading, and the amount of nitrogen uptake showed a trend to increase with increasing nitrogen application levels under 270 kg/hm 2 at all growth stages. The accumulative amount of total nitrogen, phosphorus and potassium increased with increasing nitrogen application level up to 270 kg/hm 2, but decreased when the nitrogen application level exceeded 270 kg/hm 2. The total nitrogen content of plants in the N330 treatment was less than that in the N270 treatment. The reason might be due to that the nitrogen application level of 330 kg/hm 2 exceeds an optimal nitrogen application level, which is unfavorable for the normal growth of rice root system in the northern area of Zhejiang Province, China, and thus results in reduced nitrogen absorption. The absorbed nitrogen used for rice straw and leaf growth at the tillering stage is transported to the panicles at advanced developmental stages (Guindo et al, 1994; Liu et al, 2007). Therefore, the total amount of nitrogen in the straw decreases, while both the panicles and plants increase with progressive development. Consequently, the nitrogen content in the grains is higher than that in the straw before harvest. Correlation analysis showed that yield was positively correlated with the accumulative amounts of nitrogen, phosphorous and potassium in the rice plants. Among these factors, the highest correlation coefficient was observed between nitrogen uptake amount and yield, which was similar to the results of Zhang et al (2005). An important strategy to sustain and enhance soil fertility and improve fertilizer utilization efficiency is to combine application of chemical and organic fertilizers (Xu et al, 2008). Yang et al (2004) studied the effect of application of a single chemical fertilizer and combined application of chemical and organic fertilizers on dry matter accumulation and distribution of nutrients in rice plants. The results showed that combined application of organic and inorganic fertilizers promoted the transfer of nutrients to the grains and improved rice yields. The present study showed that organic fertilizer of 3 000 kg/hm 2 in combination with a nitrogen application level of 150 kg/hm 2 promoted the uptake and utilization of nitrogen, phosphorus and potassium by rice Zheyou 12 plants. With application of a single chemical fertilizer, dry matter accumulation and nutrient uptake in rice were mainly concentrated at the tillering and booting stages, but were mainly concentrated from the heading to maturity stage in response to combined application of chemical and organic fertilizers, which could increase the number of panicles per unit area and the number of grains per panicle (Guindo et al, 1994; Yang et al, 2004; Yang et al, 2010). Ammonia volatilization from rice farmland is an inevitable source of nitrogen loss that is closely related to the nitrogen application level and the duration after fertilizer application (Song et al, 2004; Ye et al, 2011). The amount and rate of ammonia volatilization increased with increasing nitrogen application level, which might reflect the dramatically increased ammonia nitrogen content in the surface water of rice fields with increasing nitrogen application level. The most rapid ammonia volatilization occurred after base fertilizer application, and the slowest was observed after fertilizer topdressing at the panicle stage. A possible explanation for this phenomenon is that the applied nitrogen level of the base fertilizer was three times higher than that of the topdressing fertilizer applied at the tillering and panicle stages, and the soil ammonia nitrogen declined quickly because of the strong absorption of soil nitrogen by the rice root system at advanced developmental stages (Song et al, 2004; Ye et al, 2011). The rate of ammonia volatilization in the present study was high in the N270 and N330 treatments after application of the base fertilizer and topdressing at tillering, which might reflect an excessive nitrogen application level. Therefore, the suggested nitrogen application level for Zheyou 12 plants should be less than 270 kg/hm 2 in rice agricultural production systems. The present study showed that the rate and accumulated amount of ammonia volatilization increased with increasing nitrogen application level. The most rapid ammonia volatilization rate and the highest accumulative amount of ammonia volatilization were observed after base fertilizer application. The ammonia

146 Rice Science, Vol. 20, No. 2, 2013 volatilization rates in response to the two highest nitrogen application levels (N270 and N330) were much higher than those of the other treatments after application of base fertilizer and topdressing at the tillering stage. The loss of nitrogen through ammonia volatilization was 23.89% of the total applied nitrogen at the nitrogen application level of 330 kg/hm 2. The present results also showed that the amount of ammonia volatilization was the largest after base fertilizer application, followed by topdressing at the tillering stage, and was the lowest after topdressing at the panicle stage, which was in accordance with the results of Song et al (2004). This phenomenon might be due to the plant root system is still not well developed at the early tillering stage, and thus conditions will be more favorable for ammonia volatilization. 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