Effects of Nitrogen Application Levels on Ammonia Volatilization and Nitrogen Utilization during Rice Growing Season

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1 Rice Science, 2012, 19(2): Copyright 2012, China National Rice Research Institute Published by Elsevier BV. All rights reserved Effects of Nitrogen Application Levels on Ammonia Volatilization and Nitrogen Utilization during Rice Growing Season LIN Zhong-cheng 1, 2, DAI Qi-gen 1, YE Shi-chao 1, 3, WU Fu-guan 2, JIA Yu-shu 3, CHEN Jing-dou 1, XU Lu-sheng 2, ZHANG Hong-cheng 1, HUO Zhong-yang 1, XU Ke 1, WEI Hai-yan 1 ( 1 Key Laboratory of Crop Genetics and Physiology of Jiangsu Province / Innovation Center of Rice Cultivation Technology in Yangtze Valley, Ministry of Agriculture, Yangzhou University, Yangzhou , China; 2 Agriculture Committee of Wujiang County, Jiangsu Province, Wujiang , China; 3 Agriculture Bureau of Guannan County, Jiangsu Province, Guannan , China) Abstract: We conducted field trials of rice grown in sandy soil and clay soil to determine the effects of nitrogen application levels on the concentration of NH + 4 -N in surface water, loss of ammonia through volatilization from paddy fields, rice production, nitrogen-use efficiency, and nitrogen content in the soil profile. The concentration of NH + 4 -N in surface water and the amount of ammonia lost through volatilization increased with increasing nitrogen application level, and peaked at 1-3 d after nitrogen application. Less ammonia was lost via volatilization from clay soil than from sandy soil. The amounts of ammonia lost via volatilization after nitrogen application differed depending on the stage when it was applied, from the highest loss to the lowest: N application to promote tillering > the first N topdressing to promote panicle initiation (applied at the last 4-leaf stage) > basal fertilizer > the second N topdressing to promote panicle initiation (applied at the last 2-leaf stage). The total loss of ammonia via volatilization from clay soil was kg/hm 2, equivalent to 10.92% 21.76% of the nitrogen applied. The total loss of ammonia via volatilization from sandy soil was kg/hm 2, equivalent to 11.32% 25.61% of the nitrogen applied. The amount of ammonia lost via volatilization and the concentration of NH + 4 -N in surface water peaked simultaneously after nitrogen application; both showed maxima at the tillering stage with the ratio between them ranging from 23.76% to 33.65%. With the increase in nitrogen application level, rice production and nitrogen accumulation in plants increased, but nitrogen-use efficiency decreased. Rice production and nitrogen accumulation in plants were slightly higher in clay soil than in sandy soil. In the soil, the nitrogen content was the lowest at a depth of cm. In any specific soil layer, the soil nitrogen content increased with increasing nitrogen application level, and the soil nitrogen content was higher in clay soil than in sandy soil. In terms of ammonia volatilization, the amount of ammonia lost via volatilization increased markedly when the nitrogen application level exceeded 250 kg/hm 2 in the rice growing season. However, for rice production, a suitable nitrogen application level is approximately 300 kg/hm 2. Therefore, taking the needs for high crop yields and environmental protection into account, the appropriate nitrogen application level was kg/hm 2 in these conditions. Key words: ammonia volatilization; nitrogen application level; soil type; nitrogen-use efficiency; rice Rice is an important food crop in China and nitrogen fertilizer plays a key role in obtaining high crop yields. However, much nitrogen is lost and nitrogen-use efficiency is generally low in rice production. Among the many types of nitrogen loss, ammonia volatilization is one of the most important losses (Peng et al, 2002). There are marked losses of nitrogen via ammonia volatilization after nitrogen fertilizer is applied into rice fields. The magnitude of this ammonia volatilization depends on the concentration of ammonium nitrogen in surface water, although it is also affected by temperature, wind speed, soil conditions, plant conditions, the ph value of the surface water, and light conditions, together with some other factors. The amount of ammonia volatilization Received: 23 February 2011; Accepted: 20 May 2012 Corresponding author: DAI Qi-gen (qgdai@yzu.edu.cn) also differs markedly from year to year (Tian et al, 2001). Previous results showed that the amount of nitrogen lost via ammonia volatilization during the rice growing season accounted for 9% to 40% of the total nitrogen applied (Cai and Zhu, 1995). There have been some studies on the mechanisms of ammonia volatilization (Cao et al, 2000; Wu et al, 2009), including studies on measurement methods, nitrogen fertilizer types (Li et al, 2005; Gao et al, 2009), nitrogen application levels (Deng et al, 2006; Gao et al, 2009), water and fertilizer management, and the effects of different inhibitors (Cao et al, 2000; Su et al, 2003; Zhang et al, 2003; Song et al, 2004; Li et al, 2005; Deng et al, 2006; Tian et al, 2007; Gao et al, 2009; Wu et al, 2009). However, few comparative studies have been conducted in different soil types. This research was conducted on rice grown in two different soil types: sandy soil and clay soil. We

2 126 Rice Science, Vol. 19, No. 2, 2012 investigated the effects of different nitrogen application levels on ammonium nitrogen concentration in surface water, amount of ammonia volatilization, rice yield, nitrogen-use efficiency, and nitrogen content in the soil profile. The aim of the study was to determine whether there were differences in ammonia volatilization between different soil types under different levels of nitrogen application. These results will provide a theoretical basis for reducing nitrogen inputs, for improving nitrogen-use efficiency, and for protecting the environment. These data will help to determine optimum nitrogen application levels and management strategies to apply nitrogen for rice production in different soil types. MATERIALS AND METHODS Experimental design The experiments were conducted using large controllable bottom-leak pools (Lysimeter) located at the experimental farm of Yangzhou University, Yangzhou City, Jiangsu Province, China from 2008 to Each pool had a surface area of 3.06 m 2 (1.8 m length 1.7 m width) and a depth of 1.8 m with soil thickness of 1.4 m. At the bottom of the pool, there were a drainage system to control the leakage configuration and a device to collect the soil solution and quantify ammonia volatilization (Fig. 1). We used two experimental soil types: sandy soil and clay soil. The soil fertility in the 0 20 cm layer is shown in Table 1. Seeds of the experimental rice variety Nanjing 44 were sown on 18 May and seedlings were transplanted on 15 June with three seedlings per hill and a row spacing of 12 cm 26 cm. Nitrogen fertilizer was applied at the rates of 0, 100, 200, 300 and 400 kg/hm 2, which were designated as N0, N1, N2, N3 Fig. 1. NH 3 collection device in plots of experimental fields. Table 1. Soil fertility at 0 20 cm depth in sandy soil and clay soil. Soil fertility Sandy soil Clay soil Total nitrogen (g/kg) Available nitrogen (mg/kg) Available phosphorus (mg/kg) Available potassium (mg/kg) Organic matter (g/kg) ph value and N4, respectively. The treatments were arranged in a randomized design with two replications. The amount of nitrogen leaked per day was 5 mm. The ratio of basal-tillering fertilizer to panicle fertilizer (for promoting panicle initiation) was 5:5, and the ratio of basal fertilizer to tillering fertilizer (for promoting tillering) was 5:5. Panicle fertilizer was applied separately at the last 4-leaf stage and the last 2-leaf stage, with the same amount applied each time. Basal fertilizer was applied on 15 June before transplanting, tillering fertilizer on 21 June, panicle fertilizer at the last 4-leaf stage on July 24, and panicle fertilizer at the last 2-leaf stage on 4 August. Urea was used as nitrogen fertilizer. Phosphate fertilizer was superphosphate (P 2 O 5 ), which was applied as basal fertilizer at 90 kg/hm 2. Potassium fertilizer was KCl, which was applied twice, first as basal fertilizer at a rate of K 2 O 60 kg/hm 2, and then as panicle fertilizer applied at the last 4-leaf stage at a rate of K 2 O 60 kg/hm 2. The rice plants were irrigated with tap water and managed according to the standard cultivation practices for common high-yielding rice varieties. Measuring equipment and methods Soil fertility and nitrogen content of soil profile Top soil samples (0 20 cm depth) were collected before transplanting rice seedlings. The contents of total nitrogen, available nitrogen, available phosphorus, available potassium and organic matter were measured. The soil profile was analyzed in 10-cm depth increments to determine total nitrogen content after rice harvest. Inorganic nitrogen content in surface water After application of basal, tillering and panicle fertilizers, surface water samples were collected at the same time every day until the nitrogen content in surface water was similar in nitrogen-applied and no-nitrogen treatments. The water samples were collected using 50-mL syringes from five different locations in each section, and then mixed in 250 ml plastic bottles before adding five drops of oil of vitriol to acidify the solution to ph 2 4. The samples were stored in a freezer until analysis. Irrigation water and rainwater were also collected. The contents of nitrate and ammonium in

3 LIN Zhong-cheng, et al. Nitrogen Application Levels on Ammonia Volatilization and Nitrogen Utilization 127 samples were analyzed using the continuous flow analyzer produced by the German Instrument Company SEAL (BRAN + LUEBBE, AA3, Germany). We calculated the peak of ammonium nitrogen in surface water (kg/hm 2 ) as follows: peak of ammonium concentration in surface water (mg/l) depth of surface water (m) 10 4 m 2 /hm L/m kg/mg. The peak value occurred when the depth of surface water reached 5 cm. Measurement of ammonia volatilization The amount of ammonia lost via volatilization was measured using the closed chamber method. The principle was as follows: the volatilized ammonia was absorbed into a gas-washing bottle containing 2% boric acid. This was achieved by reducing pressure by exhausting air. Then, the NH 3 content in the bottle was determined by titration with standard acid, and the amount of ammonia lost via volatilization was calculated. The amount of air pumped into the vacuum pump was adjusted to ensure that the ventilation frequency of the confined space was times per minute. The device used to collect samples for measuring ammonia volatilization loss was designed by the Institute of Soil Science in Nanjing, China (Fig. 2). After application of basal, tillering and panicle fertilizers, samples for quantification of ammonia volatilization were collected at six times during 7:00 10:00 am and 3:00 6:00 pm each day. The data from these six samples was used to calculate the daily average, and was then converted into the amount of ammonia lost via volatilization in one day (pure N). The collection continued until the amount of ammonia volatilization became the same in nitrogen-applied and no-nitrogen treatments (Su et al, 2003; Deng et al, 2006; Tian et al, 2007). Measurement of nitrogen content in rice plants and grain yields Representative rice plants were sampled at the maturity stage. Dry matter accumulation was determined by drying plants at 80 C to constant weight. The nitrogen content was measured using the Kjeldahl method. Then, the amount of nitrogen accumulation in plants was calculated by multiplying the amount of biomass by the nitrogen content. We also measured rice grain yield and yield-related components including panicle number per unit area, grain number per panicle, total spikelet number, seed-setting rate and 1000-grain weight. Data processing and analysis Microsoft Excel was used for data processing and mapping, and the DPS statistical analysis software Fig. 2. Diagram of NH 3 measuring device. was used for statistical analysis. RESULTS Dynamic changes in NH 4 + -N concentration in surface water after applying nitrogen fertilizer As shown in Figs. 3 and 4, the NH 4 + -N concentration in surface water increased as the nitrogen application level increased. It peaked at 2 d after nitrogen application, then decreased gradually, becoming similar to the level in the no-nitrogen treatment after 5 7 d. The NH 4 + -N concentration in surface water was slightly lower in clay soil than in sandy soil. There was a low concentration of NH 4 + -N in surface water after application of basal nitrogen, because most of the NH 4 + -N was adsorbed by soil particles in the muddy water. In contrast, after application of tillering fertilizer, the NH 4 + -N concentration in surface water increased and peaked at mg/l in sandy soil (Fig. 3). When panicle fertilizer was applied at the last 2-leaf stage, the NH 4 + -N concentration in surface water was significantly lower than that when the panicle fertilizer was applied at the last 4-leaf stage (Fig. 4). The NH 4 + -N concentrations in surface water after application of nitrogen fertilizer at different stages were ranked in the following order (from the highest to the lowest): N application to promote tillering > the first N topdressing to promote panicle initiation (applied at the last 4-leaf stage) > basal fertilizer > the second N topdressing to promote panicle initiation (applied at the last 2-leaf stage). Dynamic changes of ammonia volatilization after applying nitrogen fertilizer in two soil types As shown in Figs. 5 and 6, the trends of ammonia volatilization were similar to the trends of NH 4 + -N concentration in surface water in each treatment. The

4 128 Rice Science, Vol. 19, No. 2, 2012 Fig. 3. Changes in NH 4 + -N concentration of surface water after nitrogen application at basal and tillering stages in sandy and clay soils. N fertilizer to promote tillering was applied at 6 d after basal fertilizer application. Total N application rates for N0, N1, N2, N3 and N4 were 0, 100, 200, 300 and 400 kg/hm 2, respectively, with quarter of the amount applied each time. amount of ammonia volatilization increased rapidly when the nitrogen application level increased, peaked at 2 3 d after application, and then decreased. In the same period, the amount of ammonia volatilization in clay soil was slightly less than in sandy soil. After four applications of the same amount of nitrogen fertilizer, the ammonia volatilization losses were ranked as follows (from the highest to the lowest): N application to promote tillering > the first N topdressing to promote panicle initiation (applied at the last 4-leaf stage) > basal fertilizer > the second N topdressing to promote panicle initiation (applied at the last 2-leaf stage). These results showed that the amount of ammonia volatilization was closely related to the concentration of NH 4 + -N in surface water, and both depended on the level of nitrogen application. Changes in amount of ammonia volatilization, its ratio to nitrogen application, ammonia volatilization peaks, and NH 4 + -N concentrations in surface water in two soil types As shown in Table 2, the amount of ammonia Days after the first N topdressing to promote panicle initiation (d) Fig. 4. Changes in NH 4 + -N concentration of surface water after topdressing to promote panicle initiation in sandy and clay soils. The second N topdressing was applied at 11 d after the first N topdressing to promote panicle initiation. Total N application rates for N0, N1, N2, N3 and N4 were 0, 100, 200, 300 and 400 kg/hm 2, respectively, with quarter of the amount applied each time. volatilized in each period significantly increased with increasing nitrogen application level. The ratio of the amount of ammonia volatilization to the nitrogen application level also increased significantly. The amount of ammonia volatilization and the ratio both peaked after applying tillering fertilizer. Ammonia volatilization showed similar trends in the two soil types. When the same amount of nitrogen was applied, the amount of ammonia volatilization was slightly lower in clay soil than in sandy soil, and the ratio showed similar trend. Similarly, in all four fertilizer applications, the amount of ammonia lost via volatilization and the ratio of this amount to nitrogen application level increased with increasing nitrogen application level. These results indicated that higher levels of nitrogen application may cause greater nitrogen losses via ammonia volatilization. As shown in Table 2, the total losses of ammonia via volatilization were kg/hm 2 in sandy soil and kg/hm 2 in clay soil. The ratios of

5 LIN Zhong-cheng, et al. Nitrogen Application Levels on Ammonia Volatilization and Nitrogen Utilization 129 Days after the first N topdressing to promote panicle initiation (d) Fig. 5. Changes in NH 3 volatilization after N application of basal and tillering fertilizer in sandy and clay soils. N fertilizer to promote tillering was applied at 6 d after basal fertilizer application. Total N application rates for N0, N1, N2, N3 and N4 were 0, 100, 200, 300 and 400 kg/hm 2, respectively, with quarter of the amount applied each time. Fig. 6. Changes in NH 3 volatilization after N application after topdressing for promoting panicle initiation in sandy and clay soils. The second N topdressing was applied at 11 d after the first N topdressing to promote panicle initiation. Total N application rates for N0, N1, N2, N3 and N4 were 0, 100, 200, 300 and 400 kg/hm 2, respectively, with quarter of the amount applied each time. this loss to total nitrogen application were 11.32% 25.61% in sandy soil and 10.92% 21.76% in clay soil. As shown in Fig. 7, the amount of ammonia lost via volatilization increased markedly as the nitrogen application level increased to kg/hm 2 in both soil types. With increased nitrogen application level, the ammonia volatilization increased exponentially, Table 2. Effects of nitrogen application level and soil type on NH 3 -N volatilization loss and percentage of loss to nitrogen application level. N application time and Sandy soil Clay soil NH 3 -N volatilization loss N0 N1 N2 N3 N4 N0 N1 N2 N3 N4 Basal fertilizer NH 3 -N volatilization (kg/hm 2 ) 0.07 d 1.09 c 4.18 c 8.65 b a 0.07 e 1.14 d 2.75 c 9.42 b a Ratio of NH 3 loss to N applied (%) c 8.37 bc ab a b 5.50 b a a N application for promoting tillering NH 3 -N volatilization (kg/hm 2 ) 0.14 e 7.82 d c b a 0.16 e 6.58 d c b a Ratio of NH 3 loss to N applied (%) b b a a d c b a The first N topdressing for promoting panicle initiation NH 3 -N volatilization (kg/hm 2 ) 0.09 e 1.38 d 6.31 c b a 0.06 d 1.76 d 6.92 c 9.73 b a Ratio of NH 3 loss to N applied (%) c b a a c ab b a The second N topdressing for promoting panicle initiation NH 3 -N volatilization (kg/hm 2 ) 0.10 e 1.02 d 6.00 c b a 0.12 e 1.44 d 3.72 c 9.11 b a Ratio of NH 3 loss to N applied (%) d c a b d 7.45 c b a Total amount of NH 3 -N volatilization NH 3 -N volatilization (kg/hm 2 ) 0.39 e d c b a 0.42 e d c b a Ratio of NH 3 loss to N applied (%) c b a a d c b a Total N application rates for N0, N1, N2, N3 and N4 were 0, 100, 200, 300 and 400 kg/hm 2, respectively. Within a row for a soil type, data followed by the common lowercase letter indicate no significant difference at 0.05 level.

6 130 Rice Science, Vol. 19, No. 2, 2012 Fig. 7. Relationship between total NH 3 volatilization loss and nitrogen application rate. and occurred more readily in sandy soil than in clay soil. As shown in Table 3, the peaks of ammonia volatilization losses and NH 4 + -N concentrations in surface water after nitrogen application occurred on the same day, and both increased significantly with increasing nitrogen application level. The maximum ammonia volatilization loss and NH 4 + -N concentration in surface water were observed when nitrogen was applied to promote tillering. At this time, the amounts of ammonia lost via volatilization and NH 4 + -N concentration in surface water were kg/hm 2 and kg/hm 2 in sandy soil, respectively, with the ratio ranging from 26.75% to 33.65%. In clay soil, these values were kg/hm 2 and kg/hm 2, respectively, with the ratio ranging from 23.76% to 27.94%. The peaks of ammonia volatilization losses and NH 4 + -N concentrations in surface water all increased with increasing nitrogen application level. However, the magnitude of the peaks differed among the different application times. For example, there was a significant difference in the peak sizes after application of basal fertilizer and tillering fertilizer, but no difference in the size of the peaks between the two panicle fertilizer applications. At the same level of nitrogen application, the peaks of ammonia volatilization losses and NH 4 + -N concentrations in surface water were slightly lower in clay soil than in sandy soil. The ratio of the peaks of ammonia volatilization loss to NH 4 + -N concentration in surface water was also slightly lower in clay soil (except for the nitrogen application at the last 2-leaf stage) than in sandy soil. Effects of soil types and nitrogen application levels on rice yield and its components As shown in Table 4, rice yields were slightly higher in clay soil than in sandy soil, and the rice yields in both soil types showed significant increase with increasing nitrogen application level. The rice yields peaked at kg/hm 2 and kg/hm 2 in clay and sandy soils, respectively, at the nitrogen application rate of 300 kg/hm 2. The yields showed a downward trend as the nitrogen application rate increased beyond 300 kg/hm 2 due to lodging of rice plants. As shown in Table 4, with the increase in nitrogen application level, the rice panicle number and total spikelet number per unit area increased, but the grain number per panicle, seed-setting rate and 1000-grain weight decreased. Rice panicle number per unit area, total spikelet number per unit area and seed-setting Table 3. Maximum NH 3 -N volatilization loss (MAL), maximum concentration of NH + 4 -N in surface water (MNSW) and their ratio under different nitrogen application levels. N application time and Sandy soil Clay soil measuring item N0 N1 N2 N3 N4 N0 N1 N2 N3 N4 Basal fertilizer (17 June) MAL (kg/hm 2 ) 0.01 e 0.43 d 1.78 c 3.08 b 5.01 a 0.01 e 0.25 d 0.88 c 3.23 b 3.90 a MNSW (kg/hm 2 ) 0.11 e 2.34 d 4.50 c 6.00 b 8.58 a 0.11 e 2.09 d 4.52 c 6.83 b 8.77 a Ratio of MAL to MNSW (%) 9.17 e d c b a 8.77 c c b a a N application to promote tillering (23 June) MAL (kg/hm 2 ) 0.01 e 2.82 d 6.00 c 8.93 b a 0.01 d 1.68 c 5.04 b 5.69 b 9.14 a MNSW (kg/hm 2 ) 0.14 e 9.26 d c b a 0.15 e 6.06 d c b a Ratio of MAL to MNSW (%) 7.97 c ab a b a 7.41 b a a a a The first N topdressing to promote panicle initiation (26 July) MAL (kg/hm 2 ) 0.01 e 0.57 d 2.50 c 4.32 b 5.53 a 0.01 e 0.34 d 2.28 c 3.17 b 5.65 a MNSW (kg/hm 2 ) 0.11 e 3.06 d 8.70 c b a 0.09 e 2.19 d 7.92 c b 23.9 a Ratio of MAL to MNSW (%) c b a a a d cd a bc ab The second N topdressing to promote panicle initiation (6 August) MAL (kg/hm 2 ) 0.01 c 0.32 c 1.12 b 2.39 a 2.47 a 0.01 e 0.31 d 0.75 c 1.46 b 2.89 a MNSW (kg/hm 2 ) 0.09 e 1.75 d 3.07 c 8.54 b a 0.13 e 0.77 d 2.33 c 3.73 b 9.09 a Ratio of MAL to MNSW (%) d c a b c 9.45 c a b a b Dates in the table are the time when MAL and MNSW reached the maximum. Total N application rates for N0, N1, N2, N3 and N4 were 0, 100, 200, 300 and 400 kg/hm 2, respectively. Within a row for a soil type, data followed by the common lowercase letter indicate no significant difference at 0.05 level.

7 LIN Zhong-cheng, et al. Nitrogen Application Levels on Ammonia Volatilization and Nitrogen Utilization 131 Table 4. Effects of different nitrogen application levels and soil types on rice yield and yield components. Soil type Treatment Panicle number (No./m 2 ) Grain number per panicle (No./panicle) Spikelet number ( 10 3 /m 2 ) Seed-setting rate (%) 1000-grain weight (g) Actual yield (kg/hm 2 ) Sandy soil N e a e a a d N d b d b a c N c bc c c b b N b cd b d c a N a d a e c a Clay soil N e a e a a c N d b d b b b N c bc c c c a N b bc b d d a N a c a e d a Total N application rates for N0, N1, N2, N3 and N4 were 0, 100, 200, 300 and 400 kg/hm 2, respectively. Within a column for a soil type, data followed by the same lowercase letter indicate no significant difference at 0.05 level. rate were higher in clay soil than in sandy soil, but the grain number per panicle and 1000-grain weight were slightly lower. Under these experimental conditions, the relationship between rice yield and nitrogen application level fitted the following quadratic equations: for sandy soil, y = x x (r = **), and the optimal nitrogen application level was kg/hm 2 ; for clay soil, y = x x (r = **), and the optimal nitrogen application level was kg/hm 2. Effects of different soil types and nitrogen application levels on dry matter accumulation, nitrogen accumulation and nitrogen-use efficiency As shown in Table 5, increased levels of nitrogen application were associated with increased dry matter accumulation and nitrogen accumulation in both soil types, but decreased in nitrogen apparent efficiency and nitrogen agronomic efficiency. The dry matter accumulation and nitrogen accumulation were greater in clay soil than in sandy soil. In both sandy soil and clay soil, the highest level of nitrogen application (400 kg/hm 2 ) resulted in the highest nitrogen accumulation in rice ( kg/hm 2 in sandy soil, kg/hm 2 in clay soil), but the lowest apparent nitrogen recovery efficiency (38.61% in sandy soil, 41.48% in clay soil) and the lowest nitrogen agronomic efficiency (11.57 kg/kg in sandy soil, kg/kg in clay soil). This result indicates that the increase in nitrogen does not lead to a synchronous increase in rice yield, and the nitrogenuse efficiency decreases with increased nitrogen application level. Effects of soil types and nitrogen application levels on total nitrogen content in soil profile at maturity stage As shown in Table 6, the upper soil layer had the highest nitrogen content. Soil nitrogen content decreased with increasing soil depth. At cm depth, the soil nitrogen content reached the minimum (e.g., 0.31 g/kg in no-nitrogen treatment in sandy soil). As the depth extended lower than 50 cm, there was a slight increase in nitrogen content, possibly because few rice roots reached this layer, and therefore, less nitrogen was absorbed. Alternatively, it could reflect nitrogen infiltration. With the increase in nitrogen application level, the nitrogen content in each soil layer showed an Table 5. Effects of different nitrogen application levels and soil types on dry matter accumulation, nitrogen accumulation and nitrogen-use efficiency. Soil type Treatment Dry matter accumulation N accumulation N apparent recovery N agronomic efficiency (kg/hm 2 ) (kg/hm 2 ) efficiency (%) (kg/kg) Sandy soil N d e - - N c d a a N b c b b N a b c c N a a d d Clay soil N d e - - N c d a a N b c b b N a b c c N a a d d Total N application rates for N0, N1, N2, N3 and N4 were 0, 100, 200, 300 and 400 kg/hm 2, respectively. Within a column for a soil type, data followed by the common lowercase letter indicate no significant difference at 0.05 level.

8 132 Rice Science, Vol. 19, No. 2, 2012 Table 6. Effects of different nitrogen application levels on total nitrogen content in soil profile. g/kg Soil type Soil depth (cm) Treatment N0 N1 N2 N3 N4 Sandy soil a 0.43 a 0.47 a 0.53 a 0.57 a b 0.42 ab 0.46 ab 0.42 b 0.45 b c 0.40 bc 0.39 bc 0.38 cd 0.41 b d 0.34 ef 0.37 c 0.37 de 0.40 b f 0.33 f 0.36 c 0.35 e 0.43 b ef 0.38 cd 0.37 c 0.41 bc 0.45 b de 0.36 de 0.37 c 0.40 bcd 0.41 b de 0.34 ef 0.39 abc 0.40 bcd 0.41 b Mean 0.33 e 0.37 d 0.40 c 0.41 b 0.44 a Clay soil a 0.51 a 0.53 a 0.60 ab 0.63 a b 0.49 a 0.51 bc 0.61 a 0.62 a cd 0.45 bc 0.50 c 0.54 cd 0.58 c e 0.44 c 0.49 d 0.45 f 0.57 c f 0.42 c 0.46 e 0.49 ef 0.58 bc de 0.41 c 0.47 e 0.50 def 0.61 ab c 0.43 c 0.52 b 0.52 cde 0.59 bc b 0.48 ab 0.51 bc 0.56 bc 0.59 bc Mean 0.40 e 0.45 d 0.50 c 0.53 b 0.60 a Total N application rates for N0, N1, N2, N3 and N4 were 0, 100, 200, 300 and 400 kg/hm 2, respectively. For mean values, data followed by the same lowercase letters within a row indicate no significant difference at 0.05 level, and for other values, data followed by the same lowercase letters within a column indicate no significant difference at 0.05 level. increasing trend, and the total average nitrogen content in the depth of 0 80 cm increased significantly. In clay soil, the nitrogen content in each soil layer was higher than that in sandy soil. DISCUSSION Ammonia volatilization is an inevitable process of nitrogen loss in farmland. It occurs in both paddy fields and dry lands, and is affected by many factors (Song and Fang, 2003; Chen et al, 2007; Peng et al, 2009). In this study, the amount of ammonia volatilization in clay soil was lower than in sandy soil, and the amount of nitrogen lost as a proportion of that applied was also lower. To a great extent, these outcomes were strongly related to the concentration of NH 4 + -N in surface water after nitrogen fertilizer application in the two soil types, because the concentration of NH 4 + -N in surface water in clay soil was lower than that in sandy soil at the same period. In clay soil, migration of nitrogen into the soil layers also helped to reduce the loss of nitrogen via ammonia volatilization (Huang et al, 2006; Zhang and Wang, 2007; Li et al, 2008). In addition to soil type, the ammonia volatilization loss in paddy fields was closely related to the amount and timing of nitrogen application. The amount of ammonia volatilization and its ratio to the nitrogen application level increased with the nitrogen application level increased. This was mainly due to the great increase in the NH 4 + -N concentration in surface water caused by the increased nitrogen application level. Nitrogen fertilizer was applied in equal amount at four times during the rice growing period. There were differences among the different application times in terms of the rate of ammonia volatilization after nitrogen application. From the highest to the lowest, the ammonia volatilization losses at the different stages were ranked as follows: N application to promote tillering > the first N topdressing to promote panicle initiation (applied at the last 4-leaf stage) > basal fertilizer > the second N topdressing to promote panicle initiation (applied at the last 2-leaf stage). These results were consistent with the findings of Huang et al (2006) and others, which showed that the maximum ammonia volatilization losses were at the tillering-fertilizer stage. Ammonia volatilization occurred more easily at the early tillering stage because the plant roots were not well developed, and plants were distributed sparsely in the fields. Our results of the amount of ammonia volatilization after application of basal fertilizer and panicle fertilizer are more controversial. Song et al (2004) reported greater ammonia volatilization losses after basal fertilizer application than after panicle fertilizer application. However, Huang et al (2006) showed that the maximum ammonia volatilization losses occurred after application of tillering fertilizer, followed by panicle fertilizer, and the minimum ammonia volatilization losses occurred after basal fertilizer application. The above differences were strongly related to the timing and rate of panicle fertilizer

9 LIN Zhong-cheng, et al. Nitrogen Application Levels on Ammonia Volatilization and Nitrogen Utilization 133 application, as well as the different degrees of field sheltering, which resulted from differences in the plant population size (Tian et al, 2007). Furthermore, if the urea reached the soil layer, it would significantly reduce ammonia volatilization (Cao et al, 2000). The difference between our study and previous studies is that we applied the equal amount of fertilizer at each application, whereas different amounts were applied at different times in previous studies. Therefore, we were able to directly compare the amount of ammonia volatilization among the four applications, and directly compare the results between the two soil types. Previous studies (Fan et al, 2005; Zhang et al, 2006; Lin et al, 2007) and the results of this study showed that the concentration of NH 4 + -N in surface water increased rapidly after nitrogen fertilizer application in both soil types. Taking the 300 kg/hm 2 nitrogen application level as an example, the highest NH 4 + -N concentration in surface water was at the tillering period (66.74 mg/l), when the surface water was 5-cm deep. The amount of nitrogen in the surface water was kg/hm 2, accounting for 34.61% of the total amount of nitrogen applied. This indicates that there is a large amount of nitrogen in the surface water after nitrogen application. Therefore, active drainage should be avoided within a week of nitrogen application. Nitrogen application before heavy rains should also be avoided, thus reducing nitrogen loss and improving nitrogen-use efficiency. Rice yield and nitrogen-use efficiency were higher in clay soil than in sandy soil, with the increase in rice production more prominent in the 300 kg/hm 2 fertilizer treatment. These results were closely related to the fertility of soil (Ye et al, 2005). At the same time, the difference in nitrogen losses between the two soil types was an important factor influencing nitrogen absorption and utilization and its conversion into rice yield. Li et al (2005) found that the combination of organic and inorganic fertilizers not only improved rice yield but also reduced the negative effects of nitrogen on the environment. That result suggested that increasing organic fertilizer to increase organic matter in the soil may be a promising way to lower nitrogen loss from soil. Therefore, the application level and the manner of nitrogen fertilizer application should be optimized for each soil type in rice production. This will reduce the amount of nitrogen lost via ammonia volatilization, improve nitrogen-use efficiency, and protect ecological environment. 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