Effects of Different Nitrogen Application Rates on 15N Absorption of Rice

Jianhui LI, Baoan XU, Zhenming CANG, Qiuju WANG, Feng JIAO*, Ruichang ZHAI

1. Heilongjiang Bayi Agricultural University, Daqing 16319, China;

2. Heilongjiang Shuguang Farm, Jiamusi 15451, China;

3. Heilongjiang Beidahuang Rice Industry Group Co. Ltd., Harbin 15090, China

Nitrogen is an essential nutrition to plants, as it exerts significant impacts on the growth and yield of crops[1-4]. The rice yield largely depends on the accumulation and distribution of nitrogen in the nutritional organs and growth organs of plants[5-6].Generally speaking,the tiller stage to the flowering period are critical time for the rice to absorb nitrogen.During this period, the absorbed nitrogen is stored in the leaves[7].When rice enters into the flowering period, the nitrogen in the nutrition organs began to circulate to the seeds, in order to satisfy the seeds’needs of nitrogen[8-9].The absorption and circulation of nitrogen in the rice production need further investigation, and this in turn contributes to the further investigation and management of rice growth.

With regard to the current agricultural production, farmers apply chemical fertilizers reasonably,and enhance the fertilizer use efficiently. Therefore,the effect of nitrogen application on15N absorption and distribution of Longjing 31 and Kongyu 131 rice varieties was studied by 15N isotopic trace technique in this paper.

Materials and Methods

Tested materials and tests design

The field experiment was carried out in Heilongjiang Beidahuang Scientific and Technological Park. The soil was Albic planosol. The foundation fertility of soil before the experiment was 31.13 g/kg organics,163.51 mg/kg available nitrogen, 31.14 mg/kg available phosphorus, 116.65 mg/kg available potassium, 6.02 pH. We set out five treatments as N0,N1,N2,N3,and N4, and we applied 0, 90, 150, 210 and 270 kg/hm2of nitrogen for each treatment, as the percentage of base fertilizer,tiller fertilizer and panicle fertilizer was 4∶3∶3. The nitrogen source was urea(N,46%),phosphorus fertilizer was calcium superphosphate(P2O5, 17%), and 60 kg/hm2was applied. Besides, 90 kg/hm2of potassium sulfate (K2O, 50%) was applied.Phosphorus fertilizer was applied as the base fertilizer. 60% potassium was applied as base fertilizer, and the rest 40% was applied as the panicle fertilizer.

Meanwhile,small regions were divided for each treatment.PVC cylinder was put into the soil for 45 cm deep,and the PVC cylinder was 60 cm high and 25 cm wide. There was one spot for a seedling in each cylinder.The15N isotopic trace technique was adopted in each area, and the isotope was the15N trace urea(abundance 5%,Shanghai Research Institute of Chemical Industry). The experiment was repeated four times. In each treatment, the diameter of the hole for rice seedlings was 14 cm, the line distance was 30 cm. There were three strains in each hole.There were 7 lines in each treatment and the line length was 10 m.

The tested rice species include Longjing 31 and Kongyu 131. The seedlings were planted on May 15.The field management in other experiments was the same as that in the large field.

Sample collection and analyses

During the four critical growth periods,namely the tillering period,jointing period, flowering period and maturing period, there were three places to plant rice in each area. We isolated different organs based on stalk length,leaves and seeds. In the experiment,rice was harvested after complete maturing. The rice plants include stem leaves, seeds and roots based on different organs.All plants samples were cleaned with clean water and ionized water for many times.The plant samples were baked at 75 ℃. We analyzed the nutritional content in the plants.

The nitrogen in the plants was boiled with H2SO4-K2SO4-CUSO4[10],and was determined by the KjelFlex K360 nitrometer produced by BUCHI Company.The15N isotopic trace technique was determined by the Integra CN special isotope mass spectrometer produced by Europa Scientifi company.The nitrogen ratio in the plants was the result of plant N atom percent and the fertilizer N atom percent.

Data process and statistics analyses

All experiment data was processed by Excel 2003 and was analyzed by SAS9.2.

Results and Analyses

The amount of nitrogen in the rice plants during different growth period

According to Table 1, the amount of nitrogen was the most in the rice plants during the tiller period. With the growth of rice, the dry materials in rice accumulated, while the nitrogen in the plants gradually declined. This might because the rapid accumulation of dry materials in the plants diluted the nitrogen in the plants, and the nitrogen amount in the plants during the maturing stage was the lowest. During the final period of rice growth,the nitrogen in the nutritional organs moved to the storing organs, thus the nitrogen amount in the organs such as straws declined dramatically. The effects of different nitrogen level on the amount of N in rice were significant.The amont of nitrogen in the N0 treatment was lower than that of other treatments,and although the nitrogen content in different organs varied, the value increased along with the enhanced skills in nitrogen application. When the plants reached maturing period, it was shown that the nitrogen in Longjing 31 was higher than Kongyu 131. The difference of nitrogen in two rice species reached 0.01 significant level.

Table 1 The amount of N in the rice plants during different growth period %

The influence of different treatments on the accumulation of nitrogen in the rice plants

It is shown in Table 2 that nitrogen in the plant accumulates as the rice grows. Between the tiller period and jointing period, the amount of nitrogen in the plants was the largest, which was almost 40% of the amount of nitrogen during the entire growth period,while there was another peak of nitrogen accumulation during the early period of growth. Therefore, the absorption of nitrogen in the rice generally took place in the early period of growth, and the absorption of nitrogen in the growth period was little.The application of nitrogen treatment obviously facilitated the rice plants to absorb nitrogen. In terms of the rice plants at the same growing stage, the more nitrogen was applied, the more nitrogen accumulated in the plants. During the final period of rice growth,compared to N0, in the N1 treatment to N4 treatment, the absorption of nitrogen in Longjing 31 rose by 42.0%, 91.2%,116.9% , and 121.0% . Therefore,Longjing 31 was more sensitive than Kongyu131 in the aspect of nitrogen absorption.

The absorption and distribution of 15N in rice plants

It is suggested in Table 3 that amount of 15N in different parts of plants differed, and the amount of seeds in the plants was the highest,followed by that in the straw and leaves. The amount of nitrogen in the roots was the least. The reason for such phenomena was that the amount of dry materials in each organs of plants was different,and the amount of dry materials in the roots was particularly little.The dry materials in the harvest organs of Longjing 31 was 5.2 to 6.2 times that of the underground roots, while the dry materials in the stem and leaves were 4.4 to 5.5 times that of the roots. By comparing different species,we found that the nitrogen in the harvest organs of Longjing 31 in N1, N2, N3 and N4 treatment was higher than that of Kongyu 131 by 27.9%,37.5%,23.9%and 33.0%, respectively. There were no distinct dif-ferences in other organs. The amount of15N differed in the aspect of organs,stems and leaves. The accumulation of15N in the harvest organs of Longjing 31 was 3.1% to 7.5% higher than Kongyu 131,while the accumulation of stems and leaves in other organs decreased by 3.2%to 7.9%.

Table 2 The absorption of N during different growth periods

Table 3 15N absorption and distribution

Conclusions and Discussions

So far, the study of nitrogen has largely focused on the accumulation dynamics of nitrogen in the rice at different growth stage. Cao Hongsheng et al.[11]suggested that the growth of tiller and spike needs nitrogen the most.Wang Xiuqing[12]believed that the absorption of nitrogen in the rice plants peaked from the jointing period to the spike period. 50% nitrogen was absorbed in the growth of rice. Experiment showed that the amount of nitrogen was the most in the early period,which was 40% of the total amount during the entire growth period. The peak of nitrogen came before the tiller period. Therefore, the demand of nitrogen was pretty high in the early stage of growth.An environment full of nitrogen can promote the growth of leaves, enlarge leaf area and provide insurance for the successful photosynthesis[13], and promote the addition of effective number of tiller, in order to reduce the unsuccessful grouting in some seeds,and to raise the yield[14].

The isotopic trace experiment indicated that with the addition of nitrogen, the accumulation of15N decreased in the harvest organs of rice,but the accumulation in the stem and leaves rose. The15N in Longjing 31 and Kongyu 131 differed insignificantly.The nitrogen improved the nutrition surroundings of rice, and enhance the accumulation of nitrogen in the nutritional organs. Experiment also suggested that the nitrogen in the fertilizer was distributed as seeds ＞stems and leaves ＞roots, and the accumulation of15N in the rice decreased. There were two reasons for such phenomenon. First of all, the biological yield of the root was little,which directly led to the low accumulation of15N.Secondly,with the rapid growth of nutritional organs, those nitrogen moved upward to the aboveground[1].The accumulation of15N in the seeds of Longjing 31 was higher than Kongyu 131,while on the contrary,the amount of15N in the stem part of Longjing 31 was lower than Kongyu 131. In general,the redistribution and rotating efficiency of nitrogen in the rice are essential to the yield of Longjing 31.

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