Soil Nitrogen Distribution and Plant Nitrogen Utilization in Direct-Seeded Rice in Response to Deep Placement of Basal Fertilizer-Nitrogen

2019-11-12WangDanyingYeChangXuChunmeiWangZaimanChenSongChuGuangZhangXiufu

Rice Science 2019年6期

Wang Danying, Ye Chang, Xu Chunmei, Wang Zaiman, Chen Song, Chu Guang,Zhang Xiufu

Research Paper

Wang Danying1, #, Ye Chang1, #, Xu Chunmei1, Wang Zaiman2, Chen Song1, Chu Guang1,Zhang Xiufu1

(State Key Laboratory of Rice Biology, China National Rice Research Institute, Hangzhou 310006, China; Key Laboratory of Key Technology on Agricultural Machine and Equipment, Ministry of Education / South China Agricultural University, Guangzhou 510642, China; These authors contributed equally to this work)

Deep placement of controlled-release fertilizer increases nitrogen (N) use efficiency in rice planting but is expensive. Few studies on direct-seeded rice have examined the effects of deep placement of conventional fertilizer. With prilled urea serving as N fertilizer, a two-year field experiment with two N rates (120 and 195 kg/hm2) and four basal N application treatments (B50, all fertilizer was broadcast with 50% as basal N; D50, D70 and D100 corresponded to 50%, 70% and 100% of N deeply placed as basal N, respectively) were conducted in direct-seeded rice in 2013 and 2014. Soil N distribution and plant N uptake were analyzed. The results showed that deep placement of basal N significantly increased total N concentrations in soil. Significantly greater soil N concentrations were observed in D100 compared with B50 at 0, 6 and 12 cm (lateral distance) from the fertilizer application point both at mid-tillering and heading stages. D100 presented the highest values of dry matter and N accumulation from seeding to mid-tillering stages, but it presented the lowest values from heading to maturity stages and the lowest grain yield for no sufficient N supply at the reproductive stage. The grain yield of D50 was the highest, however, no significant difference was observed in grain yield, N agronomic efficiency or N recovery efficiency between D70 and D50, or between D70 and B50, while D70 was more labor saving than D50 for only one topdressing was applied in D70 compared with twice in other treatments. The above results indicated that 70% of fertilizer-N deeply placed as a basal fertilizer and 30% of fertilizer-N topdressed as a panicle fertilizer constituted an ideal approach for direct-seeded rice. This recommendation was further verified through on-farm demonstration experiments in 2015, in which D70 produced in similar grain yields as B50 did.

direct-seeded rice; nitrogen fertilizer; deep placement; soil nitrogen distribution; nitrogen utilization

Nitrogen (N) is an essential macronutrient for the growth of rice (L.) and the major factor with respect to the development of high-yielding rice cultivars. In recent years, N fertilizers have been applied with increasing amounts in rice production in China, which has led to a drastic increase in N loss or removal resulting from N leaching, runoff, denitrification and ammonia volatilization (de Datta, 1995; Zhu et al, 2000; Choudhury and Kennedy, 2005; Yoon et al, 2006; Buresh et al, 2008). From 2004 to 2014, the average N recovery efficiency (NRE) and N agronomic efficiency (NAE) of applied N for rice production in China were only 39% and 13 kg/kg, respectively (Yu and Shi, 2015), both of which were significantly lower than the global average NRE (46%) and NAE (22 kg/kg) for rice (Ladha et al, 2005).

Plant nutrient demand varies with growth stage. Fertilization based on the nutrient requirements and uptake ability of plants can greatly enhance N use efficiency (NUE) and reduce atmospheric and aquatic pollution. N absorbed during different growth periods has different functions. The major roles of N absorbed at the seedling and tillering stages include the acceleration of tillering and promotion of early growth (Stutterheim and Barbier, 1995; Mae, 1997). Suitable N levels at the tillering stage shorten the leafing interval of the main culm and accelerate tiller development (Tanaka et al, 1965), while lower N concentrations in the shoots decrease the duration of the active tillering period (Stutterheim andBarbier, 1995). Fertilizer-N absorbed during the booting stage contributes to sink size by increasing both spikelet production and hull size and decreasing the amount of degenerate spikelets (Mae, 1997). Additional studies on nutrient uptake at different rice growth stages found that N uptake is low at the seedling stage but peaks at the tillering and booting stages, with approximately 34%–38% of the total N absorption occurring from jointing to heading (Yu et al, 2013).

To align the application of N fertilizer with N requirements during rice growth, Ten Berge et al (1997) recommended frequent and small doses of fertilizer applications. However, Jing et al (2007) found that splitting fertilizer-N applications with more than three times has only small effects on grain yield in transplanted rice. In direct-seeded rice, although Xu et al (2018) found that applying N fertilizer with four times during rice growth (basal : seedling : tillering : panicle = 5 : 2 : 2 : 1) results in significantly higher grain yield compared to application with three times (basal : tillering : panicle = 5 : 4 : 1). Ni et al (2003) found that when the percentage of panicle fertilizer N is increased to 20%–30% (e.g. the ratio of basal : seedling : tillering : panicle N was 4 : 2 : 2 : 2 in conjunction with four split fertilizer applications and the ratio of basal : tillering : panicle was 4 : 3 : 3 in conjunction with three split fertilizer applications), no significant differences are observed in grain yield, effective panicle number per plant, number of filled grains per panicle and 1000-grain weight between the two split N application methods. Lin et al (2014) and Wang et al (2017) compared the uptake of fertilizer applied at different growth stages and found that the uptake of fertilizer-N at early vegetative stage is limited because of the immature root system, however, when fertilizer is applied at the booting stage, the roots of the plants are well developed, and most of the fertilizer-N is absorbed within several days after application (Mae, 1997). Qi et al (2012) also found that delaying the first urea application significantly increases plant growth, grain yield, above-ground N uptake and NRE of dry direct-seeded rice. Thus, from the perspective of reducing labor costs and increasing NRE, N fertilizer applied to direct-seeded rice is also recommended to omit seeding fertilizer and applied as three split applications, such as basal, tillering and panicle applications. Usually, basal fertilizer is broadcast and incorporated into puddled soils shortly before direct seeding, tillering fertilizer is broadcast (topdressed) at the initial tillering stage, and panicle fertilizer is topdressed at the panicle initiation or heading stage. In direct-seeded rice, N application rates typically range from 150–300 kg/hm2, and the basal fertilizer-N : tillering fertilizer-N : panicle fertilizer-N is typically 40%–60% : 20%–30% : 20%–30%.

Both the deep placement of fertilizer and the adoption of slow/controlled-release fertilizer can dramatically improve NUE (Cao et al, 1984; Katyal et al, 1985; Chalam et al, 1989; Savant and Stangel, 1990; Mohanty, 1999; Ke et al, 2018; Zhang S G et al, 2018). N recovery of 50% was reported in the plants receiving prilled urea treatments at a 10 cm depth (at 58 kg/hm2) whereas 32% for broadcasting (Cao et al, 1984). Urea supergranules and urea briquettes (two slow-release fertilizers) applied to the soil at a depth of 5 cm can result in greater grain yields than soil surface-applied prilled urea (Eriksen and Nilsen, 1982; Singh et al, 1988). However, few farmers have adopted the deep placement of fertilizers, because the manual technique needed is very laborious. Manual deep placement of fertilizer requires approximately 40 h/hm2whereas topdressing requires only less than half of the labor and time (Scholten, 1992; Cassman et al, 1998). Economic considerations have also hindered the acceptance of slow-release and controlled-release fertilizers. At present, although there are many so-called slow/controlled- release fertilizers in the fertilizer market in China, fertilizers that produce good effects are expensive. Common N-P-K compound fertilizers and urea applied as basal fertilizers or for topdressing are still the most commonly used for rice production in China.

In direct-seeded rice, with the development of direct seeding machine which can conduct mechanical seeding and deep fertilization synchronously, labor intensity is no longer a restriction for fertilizer deep placement. In mechanical hill-seeded rice production, basal fertilizer can be mechanically placed in the soil between two planting rows during seeding. Researchers have identified that the optimum fertilization depth in direct-seeded rice is 8–10 cm (Chen et al, 2014; Liu et al, 2016). Using artificial fertilization, Liu et al (2016) compared the effects of different fertilizer application depths on N accumulation and transfer in direct- seeded rice and found that fertilization at 8 cm below the soil surface results in higher N accumulation in plants than that at other depths (4, 12 and 16 cm). With respect to the mechanical deep-fertilization method, the optimum ditch depth is identified as 10 cm by combined operations of ditching, deep fertilizing and seeding, and the fertilizer distributions in the 0–5 and 5–10 cm soil layers at this ditch depth are consistent with the distribution of roots in the soil (Chen et al, 2014). Although much research has been performed concerning the deep placement of fertilizer, effective and labor-saving fertilization methods of deeply placing common compound fertilizers or urea as basal fertilizers have seldom been reported. In this study, via the use of common compound fertilizers and common prilled urea as N fertilizers, field experiments with two N rates and different ratios of deeply placed basal fertilizer and topdressing were conducted to explore the effects of deep placement of basal fertilizer on soil N distribution at different rice growth stages, rice growth and NUE, and to identify a labor-saving and efficient N fertilizer application method for direct-seeded mechanical rice production.

MATERIALS AND METHODS

Field experiments

Field experiments were conducted by artificial seeding and fertilization in 2013 and 2014 at the experimental farm of the China National Rice Research Institute (CNRRI) (120.2′ E, 30.3′ N, 11 m above the sea level), Hangzhou, China. Before seeding and fertilizing, five soil cores were collected diagonally from the 0–20 cm soil layer at each site, and the primary soil properties were analyzed (Lu, 2000). The soil consisted of 1.81 mg/gtotal N, 18.3 mg/gorganic matter, 0.078 mg/gexchangeable K, 0.016 mg/gavailable P and 0.023 mg/g available N, with the pH of 6.83. From sowing to maturity, the average daily temperature was 26.5 ºC and 25.1 ºC in 2013 and 2014, respectively, the average daily radiation was 17.6 and 13.9 MJ/(m2·d), respectively, and the total rainfall from sowing to maturity was 944.2 and 754.1 mm in 2013 and 2014, respectively.

Chunyou 84, a predominant single-season hybrid rice variety bred by the CNRRI, was used. It has high lodging resistance and high grain yield. The experiment was arranged in a split-plot design with three replicates. The main plots represented the N rate, and the subplots (20 m2each) represented different ratios of deeply placed basal N. Prilled urea at two N rates, 120 kg/hm2(N1) and 195 kg/hm2(N2), were applied during the entire plant growth period. In addition,N0 treatment (without N applied) was implemented as a control to calculate NAE and NRE. Basal fertilizer was deeply placed as mechanical seeding and deep fertilization synchronously method (Chen et al, 2014). A fertilizer ditch (10 cm depth) was dug between two planting rows, and basal fertilizer-N was deeply placed into the fertilizer ditch at the time of seeding. There were three ratios of basal N deep placement (B50, D50 and D100) in 2013. In B50, 50% fertilizer-N was applied as basal N, which was broadcast before seeding, 30% fertilizer-N was applied as tillering fertilizer, which was topdressed at the tillering stage at 20 d after seeding (DAS), and another 20% fertilizer-N was applied as panicle fertilizer, which was topdressed at panicle initiation at 71 DAS. In D50, 50% fertilizer-N was deeply placed as basal fertilizer, and the other fertilizer-N was topdressed as both tillering (30%) and panicle (20%) fertilizers. In D100, 100% fertilizer-N was manually deeply placed at seeding. In 2014, there was an additional treatment, D70, in which 70% fertilizer-N was deeply placed (as it was in 2013), and the other 30% fertilizer-N was topdressed as panicle fertilizer.

In both years, the fields were puddled under pond conditions and drained 2 d before seeding. Pregerminated seeds were hill seeded manually on the surface of the puddled soil on 26 May 2013 and 22 May 2014 at a density of 25 cm × 17 cm (between and within rows) wiht three seeds per hill. This spacing was chosen because it can result in the highest grain yield (Wang et al, 2014). Before sowing, the seeds were soaked for 24 h and then incubated for 24 h for proper germination, after which they were treated with chemical pesticides to avoid attack by soil insects, birds and rodents.

In both years, potassium was applied in the form of KCl as two split applications at a rate of 150 kg/hm2K2O, with 70% applied as a basal dressing and 30% applied as topdressing at panicle initiation. Phosphorus was applied as a basal dressing in the form of calcium superphosphate at a rate of 45 kg/hm2P2O5. Water management followed standard farmers’ practices. There was no standing water on the soil surface from sowing until the three-leaf stage of rice, after which point the field was reflooded, and a water depth of 1–3 cm was maintained until the end of the tillering period. Afterward, the water was drained for 7–10 d to control unproductive tillers, and later, a water depth of 3–5 cm was maintained until the grain-filling stage. Weed and insects were intensively controlled by chemicals to avoid biomass and yield losses.

In both years, 12 hills of plants were sampled from each subplot at the mid-tillering (53 DAS), heading (105 DAS) and maturity. The plants sampled at mid- tillering were divided into leaf blade (leaf) and stem plus sheath (stem) tissues, and the plants sampled at heading were divided into leaf and panicle tissues, while the plants sampled at maturity were divided into straw and grain. The dry weight of each part was determined after oven drying at 70 ºC for more than 48 h until constant weight. The plant dry weight was considered as the sum of the weights of all the parts. Dry matter accumulations from mid-tillering to heading and from heading to maturity were calculated as the difference in total aboveground dry weight between the mid-tillering and heading stages and between the heading and maturity stages. At maturity, the grain yield in each subplot was determined by harvesting an area of 5 m2and converting the yield to kg/hm2at 14% moisture content. Yield components were calculated from the abovementioned 12 hills. The panicles were hand threshed, and filled spikelets were separated from unfilled spikelets by submerging them in tap water. The dry weights of the straw, rachises, and filled and unfilled spikelets were determined after oven drying at 70 ºC to a constant weight. Three subsamples of 20 g filled spikelets were counted to determine the grain weight. All the unfilled spikelets were counted, and the number of total spikelets (filled and unfilled) was calculated. Last, spikelets per panicle (the ratio of total spikelet number to panicle number) and seed-setting rate were calculated.

Soil samples were collected on the day before seeding and fertilizing, mid-tillering (53 DAS), heading (105 DAS) and after harvest. In the deeply placed basal N treatments (D100, D70 and D50), soil samples were collected from five places in each subplot at the 6 and 12 cm (lateral distance) away from deeply placed fertilizer line. At each location, soil cores were collected at 0–10 cm and 10–20 cm depths. In the basal N broadcast treatment (B50), five soil cores were collected diagonally in each plot at 0–10 cm and 10–20 cm depths. The samples were carefully mixed to provide composite subsamples for the analyses of soil total N and available N.

The total N concentrations in the plant parts and soil samples were determined by Kjeldahl digestion and steam distillation methods. Total N accumulations in the aboveground dry matter at mid-tillering, heading and maturity were calculated. NUE was represented by two different terms: NAE (the yield increase per unit of applied N, kg/kg) and NRE (the increase in N absorbed in aboveground biomass per unit of applied N, %).

On-farm demonstration experiments

On-farm demonstrations were conducted by synchronous mechanical seeding and fertilization in 2015 in Fuyang (120.0′ E, 30.1′ N; 14 m above sea level) and Yuhang (120.3′ E, 30.4′ N; 40 m above sea level), Hangzhou, China. High-yielding hybrid rice varieties Nei2you 6 and Yongyou 12 were used in Fuyang and Yuhang, respectively, which are widely used in South China single-rice cropping systems. N was applied as D70 in 2014. Via the use of a newly developed hill-seeder that can synchronously perform ditch digging, hill seeding and fertilizing, seeds were mechanically hill seeded; 70% N, 70% K and all the P fertilizers were applied in the form of a compound fertilizer (N:P2O5:K2O = 22:10:16, 668 kg/hm2) to a 10 cm deep fertilizer ditch between the two sowing lines, which was then covered with soil while seeding, and another 30% N (63.0 kg/hm2) and 30% K (45.8 kg/hm2) was topdressed in the form of urea and KCl, respectively, as panicle fertilizer at the initial panicle initiation stage. The planting area of D70 for each variety was 3000 m2. In addition, another 667 m2was mechanically hill seeded, and fertilizer was applied as B50 at each site, which served as a control.

Both varieties were seeded on June 2 at a spacing of 25 cm × 17 cm with 3–5 seeds per hill. Management operations such as weeding, irrigation and plant protection measures were performed as needed. At the maturity stage, a 5-m2area (6 replications) was harvested diagonally in each treatment, and the grain yield was calculated at 14% moisture content.

Statistical analysis

Statistical analysis was conducted using the SPSS statistics software (version 22). Analysis of variance (ANOVA) was carried out separately for each trait in the experiment using a split-plot design in 2013 and 2014 after testing for normality, and the grain difference between D70 and B50 in 2015 was analyzed using one-way ANOVA.Significant differences among the measured variables between treatments were compared using the least significant difference (LSD) test at the 0.05 level.

Fig. 1.Distribution of total nitrogen (N) within soil profiles, as affected by deep placement of N as a basal fertilizer in 2014.

B50, 50% fertilizer-N was broadcast before seeding as basal fertilizer; D50, 50% fertilizer-N was deeply placed as basal fertilizer; D70, 70% fertilizer-N was deeply placed as basal fertilizer; D100, 100% fertilizer-N was deeply placed as basal fertilizer; N1, 120 kg/hm2N; N2, 195 kg/hm2N.

Bars show standard deviation (= 5) and different lowercase letters above the bars indicate significant differences among different N application methods (< 0.05).

RESULTS

Soil N content and distribution at different plant growth stages

Higher soil total N content was observed at mid-tillering than at heading at both the two rates (Fig. 1). Average soil total N content in 0–20 cm soil layer at the N1 and N2 rates at mid-tillering was 2.17 and 2.28 mg/g, respectively, and at heading, it was 2.04 and 2.15 mg/g, respectively. In the deeply placed fertilizer treatments, the soil total N content varied vertically and horizontally. The total N in 0–10 cm depth soil was greater than that in 10–20 cm depth soil by 10.2% at mid-tillering and by 9.2% at heading, peaking at the deep fertilizer placement point, and then gradually decreased with an increase in lateral distance. For example, at the N1 rate at mid-tillering, the soil total N within 0–10 cm depth at 12 cm distance in D100 was 3.46% and 2.19% lower than that at 0 and 6 cm distances, respectively.

All deeply placed fertilizer treatments had greater soil total N content than B50, and the maximum value was found in D100 at both N rates, followed by D70 and D50. Further comparing the differences among different sites, it was found that the difference among treatments was most obvious at the fertilization point, and the farther away from the fertilization point, the smaller the difference was. Nevertheless, at the rice growing site 12 cm away from the fertilization site, the average soil N content of D100 was 5.2%, 3.9% and 3.0% higher than that of D70, D50 and B50, respectively.

Fig. 2. Nitrogen (N) concentration in different plant parts at different growth stages.

Bars show standard deviation (= 5) and different lowercase letters above the bars indicate significant differences among different N application methods (< 0.05).

Plant N content at different growth stages

N concentrations in different plant parts at mid-tillering, heading and maturity stages are shown in Fig. 2. At mid-tillering, D100 showed the highest N content in the stem and leaves at both N rates and years. At heading and maturity, the opposite trend was observed. In 2013, N content at heading under D50 was 9.4%, 14.9% and 11.4% higher than that under B50 in the stems, leaves and panicles at the N2 rate, respectively, and the straw N content at maturity was greater in D50 than in B50 by 7.7% and 7.6% at the N1 and N2 rates, respectively. In 2014, no significant difference in N content was observed among B50, D50 and D70 in any plant part at heading and maturity stages.

Plant dry matter and N accumulation at different growth stages

At maturity, dry matter accumulation was the highest in D50 in 2013, followed by B50, and that in D100 was 15.4% and 9.2% lower than that in D50 and B50, respectively, at the N1 rate, and 10.1% and 5.6% lower at the N2 rate, respectively (Fig. 3). In 2014, D100 had significantly lower values at both N rates, whereas differences among D70, D50 and B50 were not significant.

Fig. 3. Dry matter accumulation at maturity in 2013 and 2014.

Bars show standard deviation (= 5) and different lowercase letters above the bars indicate significant differences among different N application methods (< 0.05).

Fig. 4. Dry matter accumulations during different growth stages in 2013 and 2014.

Bars show standard deviation (= 5) and different lowercase letters above the bars indicate significant differences among different N application methods (< 0.05).

When the dry mater accumulation at different growth durations were compared (Fig. 4), D100 had the highest values from seeding to mid-tillering but the lowest value from heading to maturity in both years, which were significantly lower than those of D50 and B50 by 15.5% and 14.7% at the N1 rate and by 31.8% and 30.9% at the N2 rate in 2013, respectively. A similar trend was also observed in 2014. From mid- tillering to heading, significant differences in dry matter accumulation were also observed among other N application treatments. In 2013, D50 had a significantly greater value than B50 at the N1 and N2 rates (13.8% and 7.7%, respectively). In 2014, D50 and D100 showed nearly similar dry matter accumulations, and D70 and B50 showed nearly similar values. From heading to maturity, no significant difference was found between D50 and B50 in 2013 at N2 rate or among B50, D50 and D70 in 2014 at either N rate.

As shown in Fig. 5, the changes in plant N accumulation with plant growth were similar to those in plant dry matter accumulation. D100 had the highest N accumulation at mid-tilling but had the lowest values at heading and maturity. In 2013, the N accumulation at mid-tillering in D50 and B50 was 12.8% and 32.8% lower compared with that in D100 at the N1 rate, respectively, and 17.1% and 26.9% lower at the N2 rate, respectively. At the heading and maturity stages in 2013, D50 showed the highest N accumulation at both N rates, followed by B50, and both showed a significantly higher N accumulation than D100. In 2014, there were no significant differences in N accumulation among D50, B50 and D70 at the N1 rate at any of the three plant growth stages. Although N accumulation in D70 was 13.0% and 14.3% higher than those in D50 and B50, respectively, at mid-tillering at the N2 rate, the differences among B50, D50 and D70 at heading and maturity were relatively small.

Grain yield and fertilizer-N use efficiency

The grain yield and yield components between N rates and among N application treatments are shown in Table 1. Significant differences were observed between N rates for grain weight and grain yield in 2013 and for panicle number per square meter, spikelet number per panicle, seed-setting rate and grain yield in 2014. Except for grain weight in 2014, all the measurements showed significant differences among the N application methods. However, the interaction between the N rate and the N application method was not significant for all the measurements.

In both years, D50 had the highest grain yield, followed by B50 in 2013 and D70 in 2014, while compared with the other treatments. D100 had significantly lower grain yield and spikelet number per panicle but higher panicle number per square meter and seed-setting rate in both years. There was no significant difference in spikelet number per panicle or panicle number per square meter among B50, D50 and D70 at either N rate.

Fig. 5. Nitrogen (N) accumulation in aboveground plant parts at different growth stages.

Bars show standard deviation (= 5) and different lowercase letters above the bars indicate significant differences among different N application methods (< 0.05).

In both years, NAE and NRE decreased with increasing N rates, and D100 had a significantly lower value than the other treatments at both N rates (Fig. 6). No significant difference in NAE was observed among B50 and D50 at either N rate in 2013, and among B50, D50 and D70 at the N2 rate in 2014. The effect of deeply placed basal N on NRE was more clear. D50 had a significantly higher (8%) NRE than B50 at both N rates in 2013 and a significantly higher one (9.9%) at the N2 rate in 2014. However, further increasing the proportion of basal N to 70% was not beneficial for the NRE, as no significant differences were observed between D50 and D70 or between D70 and B50.

On-farm demonstration experiments

In the on-farm demonstration experiments in 2015, both D70 and B50 were mechanically hill seeded. The grain yields of Nei2you 6 and Yongyou 12 were 9 128 and 8 616 kg/hm2, respectively, and no significant difference was observed between the D70 and B50 treatments (Fig. 7). The results showed that the two fertilizer application method had the same effect on grain yield.

DISCUSSION

Paddy field fertilizer-N can be lost from leaching, runoff, denitrification and ammonia volatilization (de Datta, 1995; Zhu et al, 2000; Choudhury and Kennedy, 2005; Yoon et al, 2006; Buresh et al, 2008; Zhang Y F et al, 2018). In irrigated paddy fields, where anaerobic conditions prevail under the top layer, inorganic N is maintained mainly as ammonium (Freney et al, 1985), and ammonium in surface water is susceptible to loss by ammonia volatilization and water outflow. Previous studies have shown that deep placement of fertilizer-N can decrease N losses by influencing N transformation. Deep placement of N not only decreases soil urease activity (Liu et al, 2015) but also increases the contact between NH4+and small clay particles, and more NH4+isretained in the soil as nonexchangeable NH4+(Cao et al, 1984; Mohanty et al, 1999).

Similar to previous studies (Savant and Stangel, 1990; Mohanty et al, 1999), this study also found that deep placement of N significantly increased soil total N concentrations. Higher N content in the 0–20 cm soil layer was observed in D100 treatment compared with B50 treatment at both the mid-tillering and heading stages. Different from previous deep placement studies (Cao et al, 1984; Singh et al, 1989; Jaiswal and Singh, 2001; Chen et al, 2008; Kapoor et al, 2008; Pang et al, 2017) in which all N fertilizers were applied at one time, different percentages of fertilizer- N were deeply placed as basal fertilizers in our study. We found that the greater the ratio of deeply placed basal fertilizer to topdressing, the higher the soil total N content in the 10–20 cm soil layer.

Table 1. Grain yield and yield components of rice grown under different nitrogen (N) application methods in 2013 and 2014.

Within a column for each N rate, means followed by the same lowercase letters are not significantly different according to LSD0.05. * denotes significant difference at the 5% level. ns, No significant difference.

Plants absorb N from the soil, and the soil N supply affects plant growth. D100 showed the highest soil N content both at tillering and heading, however, it showed the lowest dry matter accumulation and grain yield at maturity. This result is different from the findings of Lawal and Lawal (2002), Wang (2004), Bulbule et al (2005) and Kapoor et al (2008), who found that deeply placed N results in higher grain yields compared with N broadcasting. This discrepancy may be attributed to the difference in fertilizer type and fertilizer application time. Prilled urea was used as fertilizer-N in this study, whereas a controlled-release supergranule fertilizer was used in other studies. In addition, the N fertilizer was split into basal and topdressing applications in this study, while all of the fertilizer-N was applied simultaneously at seeding in other studies. However, our results are in accordance with the findings of Liu et al (2015), where fertilizer was also applied in split applications (50% of fertilizer-N was deeply placed as basal fertilizer, and 50% was applied as topdressing), and the same prilled urea was used. Both studies found that, in response to split N applications, deep placement of basal fertilizer resulted in higher grain yields compared with N broadcasting.

Unlike slow or controlled-release fertilizers, which are coated with a polymer membrane, most N fertilizers, such as common compound fertilizers and prilled urea, have a relatively fast nutrient release rate. The hydrolysis of urea can peak within 3–5 d for pellets and within 24–48 h for urea in solution or as a fine powder (Lyster et al, 1980; Whitehead and Raistrick, 1990). NH4+-N losses start within a few hours after N application and constitute more than 50% of the applied urea N within 7 to 10 d after N application (de Datta and Buresh, 1989). In the present study, when all the fertilizer was deeply placed as basal fertilizer in the D100 treatment, a higher total N concentration was observed in the 0–20 cm soil layer, especially at the mid-tillering stage (Fig. 1), which promoted plant growth and tillering development during the vegetative phase. D100 presented high N concentrations in the stems and leaves at mid-tillering (Fig. 2) and increased dry matter and N accumulation from the seeding to mid-tillering stages (Figs. 4 and 5).

A high N uptake requirement occurs from mid-tillering to flowering during rice growth (Mae, 1997; Lin et al, 2014). During this plant growth period, although significantly higher soil total N was observed in D100 both at mid-tillering and heading, the soil N supply in D100 still could not meet the great N demand of the plants. Kamiji et al (2011) analyzed the variation in spikelet numbers in different cultivars growing under various regimens of N and found that the variation in spikelet numbers is caused mainly by plant N status at the late spikelet differentiation stage. The ‘feast or famine’ pattern of N supply in D100 during the reproductive phase resulted in short panicles, low dry matter and N accumulations from heading to maturity (Figs. 4 and 5), and D100 resulted in the lowest grain yield and NUE in this study. Similar results were reported by Setter et al (1994), who found that increased early N uptake and dry matter accumulation do not increase grain yields. Peng et al (1996) also reported that the contribution of N acquired during the early vegetative period to grain and total biomass production at maturity is less important than the contribution of N uptaken after mid-tillering when the crop demand is the greatest and reproductive growth begin.

Fig. 6. Nitrogen (N) agronomic efficiency (NAE) and N recovery efficiency (NRE) in 2013 and 2014.

Bars show standard deviation (= 5) and different lowercase letters above the bars indicate significant differences among different N application methods (< 0.05).

Fig. 7. Grain yield of two high-yielding hybrid rice varieties in response to different nitrogen (N) application methods in 2015.

B50, 50% fertilizer-N was broadcast before seeding as basal fertilizer; D70, 70% fertilizer-N was deeply placed as basal fertilizer.

Bars are SD (= 5) and the same lowercase letters above the bars indicate no significant differences among different N application methods (< 0.05).

To align N supply and crop demand, N fertilizer in China is typically applied in three time as basal, tillering and panicle fertilizers. However, with the increasing cost of labor, farmers would like to reduce the number of fertilizer applications. When all the N fertilizer was applied as basal fertilizer in D100, the uptake of N in the vegetative growth stage exceeded the plant requirements, while in the reproductive stage, plants had no enough N. Therefore, to synchronize the N supply with the plant N demand, based on the results of 2013, 70% N fertilizer was deeply placed as basal fertilizer, and the remaining 30% was topdressed as panicle fertilizer in D70 in 2014. A decrease in N input at seeding significantly decreased the soil total N content and N supply from seeding to mid-tillering, and compared with D100, D70 showed significantly fewer tiller numbers. However, no significant differences were observed in tiller development, grain yield, N or dry matter accumulations between D50 and D70. The results showed that it was possible to apply fertilizer two times in direct-seeded rice growing in a clay loam soil, with 70% N fertilizer deeply placed as basal fertilizer and the remaining 30% broadcast as panicle fertilizer. Tillering fertilizer topdressing requires approximately 8 h/hm2by manual or 2 h/hm2by machine, therefore, more than 20 $/hm2can be saved in China if fertilizer is applied only two times (as basal and panicle fertilizers). Moreover, D70 resulted in a significantly higher NRE than the broadcasting treatment.

CONCLUSIONS

Deep placement of N fertilizer affects plant grain yield and NUE by influencing soil N distribution and plant N uptake. When conventional prilled urea was used as N fertilizer, all N deeply placed as basal fertilizer at seeding resulted in both low grain yields and NUE because of high N supplies during the vegetative phase and low N supplies during the reproductive phase. Compared with split broadcast applications of N fertilizer, deep placement of 50% and 70% N as basal fertilizer at a ditch of 10 cm depth combined with the application of the remaining N as topdressing resulted in higher grain yields and NUE because the latter method both synchronizes N supply with plant demand for N and ensures prolonged N availability in the soil until flowering. Given the labor requirements for manual topdressing, N fertilizer applied two times, in which 70% N is deeply placed as basal fertilizer and 30% N is topdressed as panicle fertilizer, is a good approach to synchronously increase grain yield and NUE and save labor in direct-seeded rice grown in clay loam soils.

Acknowledgements

This research was supported by the National Key Research and Development Program of China (Grant No. 2016YFD0300108), the National Natural Science Foundation of China (Grant Nos. 31671630 and 31371581), and the National Rice Industry Technology System (CARS-01-04A) in China.

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28 July 2018;

26 December 2018

ZHANG Xiufu (zhangxiufu@caas.cn)

This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/)

Peer review under responsibility of China National Rice Research Institute

http://dx.doi.org/10.1016/j.rsci.2018.12.008

(Managing Editor: Li Guan)

Rice Science

2019年6期