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Abstract [Objectives]This study was conducted to explore the effects of different N application rates and densities on the growth and development of directseeding rape as well as on its yield.
[Methods]A field experiment was carried out by setting five N fertilizer treatments and three density levels.
[Results]Increasing seeding density and N application rate could improve the seed yield of rape. The suitable N application rate for the three densities were calculated using the fitted fertilizer efficiency models， respectively， to be 186.77， 221.35 and 236.14 kg/hm2， at which the yields were the highest. The results showed that in this area， the seeding density of directseeding rape could be selected in the range of 6.0×105-7.5×105 plants/hm2， and the suitable N application rate could be in the range of 221.35-236.14 kg/hm2.
[Conclusions]This study provides a theoretical basis for the maximization of spatial resource and efficient utilization of fertilizer.
Key words N application rate； Seeding density； Directseeding rape； Fertilizer response model
China is a major producer of rapeseed with a planting area of rape over 7 500 000 hm2 and an annual average yield over 14 000 000 t[1]. The planting area and total average yield both account for about 30% of those in the world. The main producing areas of rape are mainly distributed in the middle and lower reaches of Changjiang River， the Yunnan-Guizhou Plateau and Sichuan Basin[1]. In recent years， with the loss of rural labor force and the increase of labor cost， paddy field notillage directseeding rape has been developed rapidly. Compared with traditional seedling raising and transplanting method， directseeding rape has the advantages of labor saving， deeper root， difficult lodging and relatively lighter diseases[2]. However， in different areas and under different weather conditions， the optimal density and N application rate differ for directseeding rape in the highyield highefficiency cultivation mode. Therefore， the study on the effects of directseeding density and N application rate on the growth and development of directseeding rape and its yield is of great significance， and the exploration of the optimal combination of directseeding density and N application rate for paddyfield notillage directseeding rape will provide a scientific basis for the extension of the cultivation technique.
Materials and Methods
Experimental locations and materials An experiment about different N levels and seeding densities on paddysoil directseeding rape was carried out in Liuhe Village， Jiuzhen Town， Tianmen City. The tested cultivar was Huayouza 62， and the tested soil had medium fertility. The soil had a pH of 6.7， and contained organic matter 18.2 g/kg， alkalihydrolyzale nitrogen 115.6 mg/kg， rapidly available phosphorus 15.8 mg/kg and rapidly available potassium 131.2 mg/kg.
Experimental design
The experiment was designed with five N treatments and three density levels， with three replicates， so there were 45 treatments in total. The N treatments were pure N 0 kg/hm2， pure N 90 kg/hm2， pure N 180 kg/hm2， pure N 270 kg/hm2 and pure N 360 kg/hm2. For all the N fertilizer， 50% was applied as the base fertilizer， 10% was used as the hole applied fertilizer applied within 7-10 d after final singling， and the remaining 40% was applied as shooting fertilizer in early February. The three density levels were 4.5×105， 6.0×105 and 7.5×105 plants/hm2. The N treatments were in randomized block arrangement， and each plot had an area of 20 m2. K， P， and B fertilizers were all applied as the base fertilizer， according to K， P and B application rates of 180， 120 and 15 kg/hm2， respectively. The experimental field was selected from paddy soil with high elevations and lower underground water levels. The height of rice stubble was controlled lower than 10 cm， and weeding should be performed. In this experiment， seeding was performed on October 15， and thinning and final singling were performed after full stand and at the threeleaf stage according to experimental requirements. The growth stages of the tested rape were as follows： date of seeding： October 15， 2012， emergence stage： October 19， 2012， squaring stage： February 1， 2013， bolting stage： February 14， 2013， initial flowering stage： March 17， 2013， fullbloom stage： March 22， 2013， final flowering stage： March 30， 2013， and the maturation stage： May 7， 2013. The whole growth period was 204 d.
Determination indices and methods
Soil sample acquisition and analysis： Soil samples were collected after the harvest of rice about half a month before the application of the base fertilizer. The whole experimental field was the sampling unit， where 15 sampling points were distributed. The 0-20 cm arable layer soil was collected， and airdried， grinded and sieved with a 20mesh sieve， for the determination of the basic five elements of samples[3].
Investigation of growth stages： The growth and development of directseeding rape were investigated at wintering stage and maturation stage， and the investigated items include plant height， rhizome diameter， number of primary effective branches， number of secondary effective branches， number of pots per plant （number of all pods on single plant）， number of pods on the main inflorescence， number of seeds per pod （30 pods were randomly selected from each plant， and the average value was calculated） and 1 000seed weight （the weight of 1 000 seeds was determined randomly）. For the investigation， two replicates were selected from each treatment， and 10 reprehensive plants were selected from each plot. The average value of each item was calculated as the investigation result. Comparison of yield and yield components： At maturation stage of rape， five representative plants were selected from each plot， for the investigation of yield and yield components according to the method of laboratory test of standard field plot experiment.
Data reduction and statistical analysis were performed with EXCEL and SAS 9.4， as well as variance analysis. The fitting analysis of linear plus plateau fertilizer efficiency model was performed with SPSS 19.
Results and Analysis
Effects of different treatments on growth of directseeding rape at wintering stage
It could be seen from Table 1 that at wintering stage， increasing N application rate could promote the growth and development of rape， and under different seeding density levels， the plant height， rhizome diameter and maximum leaf length all increased with the increase of N application rate. Under the N application rate lower than 270 kg/hm2， there were very significant differences between different N application levels， while there were no significant differences between the 270 and 360 kg/hm2 levels， indicating that the N application rate of 270 kg/hm2 had already satisfied the demand for N element by the growth and development of plant. Though the leaf number， maximum leaf width and expansion all increased with the N application rate increasing under the different planting densities， but the variation trends were not accordant. Furthermore， under the same N application rate， the low seeding density treatment grew better， and the high seeding density treatment showed the poorest growth. The main reason might be that the increase of seeding density caused the decrease of N element acquired by individual plant and the reduction of growth space， and finally affected plant growth.
Effects of different treatments on yield of directseeding rape
It could be seen from the data in Table 2 that under the three different densities， the increase of N application rate could promote the growth and development of rape. The plant height， rhizome diameter and height of the branching part all increased with the increase of N application rate， and the variation trends were basically the same as those at wintering stage. Under the same N application rate， the increase of density would cause the decreases in plant height and rhizome diameter， because the increase of the density would reduce the fertilizer amount absorbed by individual plant， and the growth space would be reduced correspondingly. The number of primary branches increased with the N application rate increasing， and tended to be stable at a certain rate. Among the yield components， with the increase of N application rate， the effective pods per plant， the number of seeds per pod and 1 000seed weight all increased， and when the N application rate reached a certain value， the effective pods per plant and number of seeds per pod tended to be stable， while the 1 000seed weight was on the decrease. At the seeding density of 4.5×105 plants/hm2， the effective pods per plant， number of seeds per pod and 1 000grain weight were the highest， of 73 pods/plant， 22 seeds/pod and 3.69 g， respectively. As to yield， the application of nitrogen could significantly improve yield， and when the N application rate reached a certain level， the yield tended to a steady state， which accords with the previous research results[4]. The suitable N application rate differed according to seeding density， and could be deduced through the fertilizer efficiency model. It could be seen from Table 3 that when no nitrogen was applied， the yield would be reduced with the seeding density increasing. At the seeding density of 4.5×105 plants/hm2， the yield of rape was 2 049.5 kg/hm2 under no nitrogen application， and when the N application rate reached 360 kg/hm2， the yield was the highest of 2 659.1 kg/hm2. However， the yield at the N application rate of 180 kg/hm2 tended to be stable. The rape planted at the densities of 6×105 plants/hm2 and 7.5×105 plants/hm2 showed the yields of 2 028.4 and 2 007.1 kg/hm2 under no nitrogen application， respectively， and exhibited the highest yields of 3 336.3 and 3 781.9 kg/hm2 at the N application rate of 270 kg/hm2， respectively. The yield tended to a steady state around the N application rate of 270 kg/hm2. Further analysis showed that increasing the seeding density of rape could effectively improve yield. The yield at the density of 7.5×105 plants/hm2 was significantly higher than those at other two densities， and when the N application rate was 270 kg/hm2， the yield was the highest of 3 781.9 kg/hm2. Therefore， in production， in order to improve yield， the seeding density could be properly improved， and N fertilizer could be applied at a proper amount.
The optimal N application rate for the highest yield under different densities
Fertilizer efficiency fitting analysis was performed on N application rate and yield of rape using linear plus plateau model[5-7]， and the optimal N application rate was determined.
Linear plus plateau model： y=a+bx （x≤C）
y=P （x>C）
wherein y is seed yield （kg/hm2）， x is fertilizer application rate （kg/hm2）， and a is intercept； and b is regression coefficient， C is the point of intersection of the linear plus plateau， and P is plateau yield （kg/hm2）. When b>Px/Py， C is the optimal fertilizer application rate， and when b≤Px/Py， the recommended fertilizer application rate is 0 kg/hm2.
Fertilizer efficiency fitting analysis was performed on N application rate and yield of rape planted according to different seeding densities using linear plus plateau model， obtaining the fertilizer efficiency models of the two （Fig. 1）.
Seeding density 4.5×105 plants/hm2：
y=3.08x+2 067.22（x<186.77）
y=2 642.86（x≥186.77）
Seeding density 6×105 plants/hm2：
y=5.93x+2 021.57（x<221.35）
y=3 334.2（x≥221.35）
Seeding density 7.5×105 plants/hm2：
y=7.52x+2 000.46（x<236.14）
y=3 776.25（x≥236.14）
According to the fitted fertilizer efficiency models， the correlation coefficients of the three models were 0.991， 0.995 and 0.999， respectively， all reaching the very significant level. The N application rate suitable for the directseeding rape at the three planting densities was calculated， respectively. The calculated suitable N application rates for the three planting densities （4.5×105， 6×105 and 7.5×105 plants/hm2） were， respectively， 186.77， 221.35 and 236.14 kg/hm2， at which the theoretical yields reached the maximum values， which were 2 642.86， 3 334.20 and 3 776.25 kg/hm2， respectively. It could be seen from Fig. 1 that the lower the seeding density， the less the suitable nitrogen the rape required. The yields all increased with the N application rate increasing， and after the N application rate reached a certain level， the yield kept a steady state with the N application rate increasing. Conclusions and Discussion
The results of this study showed that the growth and development of rape differed to certain degrees under different densities. Under the same N application level， the rape grew better under low density than under high density. The rape grown under low density showed plant height， rhizome diameter， effective pods per plant， number of seeds per pod and 1 000seed weight and yield index better than the rape grown under high density. These differences between different densities were mainly due to the growing space for individual plants under low density was larger than that for plants under high density； the spacing between rape plants under high density was small， resulting in restriction on lateral growth； and under the highest density， the nutrients absorbed by individual plants were less than those absorbed by those under low density when the total amounts of the nutrients were fixed. Comprehensively， the traits including plant height， rhizome diameter， effective pods per plant， number of seeds per plant， and 1 000seed weight and the yield index of lowdensity rape were all better than those of highdensity rape.
Increasing N application rate could significantly improve the yield of rape， and increasing seeding density also could significantly improve yield. The results of this study showed that at the seeding density of 4.5×105 plants/hm2， the highest yield of rape was 2 659.1 kg/hm2， and when the seeding density was increased to 6×105 plants/hm2， the highest yield was 3 336.3 kg/hm2， which increased by 25.5%； and when the seeding density was increased to 7.5×105 plants/hm2， the highest yield was 3 781.9 kg/hm2， which increased by 13.4%. It could be seen that with the further increase of seeding density， the gain in yield caused by the increase of seeding density decreased gradually. In production， yield could be improved by properly improving seeding density. When the density reaches a certain value， the increase of seeding density would affect the growth of rape， and the effective pods per plant， number of seeds per pod and 1 000seed weight would decrease， thereby resulting in decrease of yield[8-9 ].
When the seeding density is increased， the required suitable N application rate also increases gradually， and when the seeding density reaches a certain value， the increase of the suitable N application rate decreases gradually. In this study， the suitable N application rate was 186.77 kg/hm2 at the density of 4.5×105 plants/hm2， 221.35 kg/hm2 at 6×105 plants/hm2， and 236.14 kg/hm2 at 7.5×105 plants/hm2， which only increased by 14.79 kg/hm2 compared with that at 6.0×105 plants/hm2. The main reason was that the increase of density affected the growth of rape， which correspondingly restricted the improvement of rape biomass and yield. The results of this study showed that in this area， the seeding density of directseeding rape could be selected in the range of 6.0×105-7.5×105 plants/hm2， and the suitable N application rate could be in the range of 221.35-236.14 kg/hm2， so that maximization of spatial resource and efficient utilization of fertilizer could be effectively realized. References
[1]China statistical yearbook[M]. Beijing： China Statistics Press， 2016. （in Chinese）
[2]HU WX， LI ZX， XU BQ， et al. Cultivation Techniques of High Efficiency of Directed Sowing Rapeseed in Rice Fields with Tillage Free[J]. Journal of Anhui Agricultural Sciences， 2007， 35（10）： 2883-2883. （in Chinese）
[3]BAO SD. Soil agrochemistry analysis[M]. Beijing： China Agriculture Press， 2000. （in Chinese）
[4]WANG Y， LU JW， LI XK， et al. Study on Nitrogen Fertilizer Effect and Optimum Fertilizer Ratefor Transplanting and DirectSeeding Rapeseed[J]. Scientia Agricultura Sinica， 2011， 44（21）： 4406-4414. （in Chinese）
[5]WANG XR， CHENG XP， ZHANG FS， et al. Application of fertilization models in recommended fertilization in China[J]. Journal of Plant Nutrition and Fertilizer， 1998， 4（1）： 67-74. （in Chinese）
[6]WANG SR， CHEN XP， GAO XZ， et al. Study on simulation of "3414" fertilizer experiments[J]. Journal of Plant Nutrition and Fertilizer， 2002， 8（4）：409-413. （in Chinese）
[7]ZHANG MQ， LI J， KONG QB， et al. Progress and Prospect of the Study on CropresponsetoFertilization Function Model[J]. Acta Pedologica Sinica， 2016， 53（6）： 1343-1356. （in Chinese）
[8]MEI JA， MEI SH， CHEN XG， et al. Effect of Sowing Time and Density on Yield and Yield Components of Directseeding Rapeseed[J]. Hubei Agricultural Sciences， 2011， 50（21）： 4337-4340. （in Chinese）
[9]ZHANG M， YAO XD， ZHANG YH. Effects of seeding date and seeding density on the plant growth and seed yield of directseeded rape in notillage paddy field[J]. Acta Agriculturae Shanghai， 2010， 26（3）： 48-51. （in Chinese）
Editor： Yingzhi GUANG Proofreader： Xinxiu ZHU
[Methods]A field experiment was carried out by setting five N fertilizer treatments and three density levels.
[Results]Increasing seeding density and N application rate could improve the seed yield of rape. The suitable N application rate for the three densities were calculated using the fitted fertilizer efficiency models， respectively， to be 186.77， 221.35 and 236.14 kg/hm2， at which the yields were the highest. The results showed that in this area， the seeding density of directseeding rape could be selected in the range of 6.0×105-7.5×105 plants/hm2， and the suitable N application rate could be in the range of 221.35-236.14 kg/hm2.
[Conclusions]This study provides a theoretical basis for the maximization of spatial resource and efficient utilization of fertilizer.
Key words N application rate； Seeding density； Directseeding rape； Fertilizer response model
China is a major producer of rapeseed with a planting area of rape over 7 500 000 hm2 and an annual average yield over 14 000 000 t[1]. The planting area and total average yield both account for about 30% of those in the world. The main producing areas of rape are mainly distributed in the middle and lower reaches of Changjiang River， the Yunnan-Guizhou Plateau and Sichuan Basin[1]. In recent years， with the loss of rural labor force and the increase of labor cost， paddy field notillage directseeding rape has been developed rapidly. Compared with traditional seedling raising and transplanting method， directseeding rape has the advantages of labor saving， deeper root， difficult lodging and relatively lighter diseases[2]. However， in different areas and under different weather conditions， the optimal density and N application rate differ for directseeding rape in the highyield highefficiency cultivation mode. Therefore， the study on the effects of directseeding density and N application rate on the growth and development of directseeding rape and its yield is of great significance， and the exploration of the optimal combination of directseeding density and N application rate for paddyfield notillage directseeding rape will provide a scientific basis for the extension of the cultivation technique.
Materials and Methods
Experimental locations and materials An experiment about different N levels and seeding densities on paddysoil directseeding rape was carried out in Liuhe Village， Jiuzhen Town， Tianmen City. The tested cultivar was Huayouza 62， and the tested soil had medium fertility. The soil had a pH of 6.7， and contained organic matter 18.2 g/kg， alkalihydrolyzale nitrogen 115.6 mg/kg， rapidly available phosphorus 15.8 mg/kg and rapidly available potassium 131.2 mg/kg.
Experimental design
The experiment was designed with five N treatments and three density levels， with three replicates， so there were 45 treatments in total. The N treatments were pure N 0 kg/hm2， pure N 90 kg/hm2， pure N 180 kg/hm2， pure N 270 kg/hm2 and pure N 360 kg/hm2. For all the N fertilizer， 50% was applied as the base fertilizer， 10% was used as the hole applied fertilizer applied within 7-10 d after final singling， and the remaining 40% was applied as shooting fertilizer in early February. The three density levels were 4.5×105， 6.0×105 and 7.5×105 plants/hm2. The N treatments were in randomized block arrangement， and each plot had an area of 20 m2. K， P， and B fertilizers were all applied as the base fertilizer， according to K， P and B application rates of 180， 120 and 15 kg/hm2， respectively. The experimental field was selected from paddy soil with high elevations and lower underground water levels. The height of rice stubble was controlled lower than 10 cm， and weeding should be performed. In this experiment， seeding was performed on October 15， and thinning and final singling were performed after full stand and at the threeleaf stage according to experimental requirements. The growth stages of the tested rape were as follows： date of seeding： October 15， 2012， emergence stage： October 19， 2012， squaring stage： February 1， 2013， bolting stage： February 14， 2013， initial flowering stage： March 17， 2013， fullbloom stage： March 22， 2013， final flowering stage： March 30， 2013， and the maturation stage： May 7， 2013. The whole growth period was 204 d.
Determination indices and methods
Soil sample acquisition and analysis： Soil samples were collected after the harvest of rice about half a month before the application of the base fertilizer. The whole experimental field was the sampling unit， where 15 sampling points were distributed. The 0-20 cm arable layer soil was collected， and airdried， grinded and sieved with a 20mesh sieve， for the determination of the basic five elements of samples[3].
Investigation of growth stages： The growth and development of directseeding rape were investigated at wintering stage and maturation stage， and the investigated items include plant height， rhizome diameter， number of primary effective branches， number of secondary effective branches， number of pots per plant （number of all pods on single plant）， number of pods on the main inflorescence， number of seeds per pod （30 pods were randomly selected from each plant， and the average value was calculated） and 1 000seed weight （the weight of 1 000 seeds was determined randomly）. For the investigation， two replicates were selected from each treatment， and 10 reprehensive plants were selected from each plot. The average value of each item was calculated as the investigation result. Comparison of yield and yield components： At maturation stage of rape， five representative plants were selected from each plot， for the investigation of yield and yield components according to the method of laboratory test of standard field plot experiment.
Data reduction and statistical analysis were performed with EXCEL and SAS 9.4， as well as variance analysis. The fitting analysis of linear plus plateau fertilizer efficiency model was performed with SPSS 19.
Results and Analysis
Effects of different treatments on growth of directseeding rape at wintering stage
It could be seen from Table 1 that at wintering stage， increasing N application rate could promote the growth and development of rape， and under different seeding density levels， the plant height， rhizome diameter and maximum leaf length all increased with the increase of N application rate. Under the N application rate lower than 270 kg/hm2， there were very significant differences between different N application levels， while there were no significant differences between the 270 and 360 kg/hm2 levels， indicating that the N application rate of 270 kg/hm2 had already satisfied the demand for N element by the growth and development of plant. Though the leaf number， maximum leaf width and expansion all increased with the N application rate increasing under the different planting densities， but the variation trends were not accordant. Furthermore， under the same N application rate， the low seeding density treatment grew better， and the high seeding density treatment showed the poorest growth. The main reason might be that the increase of seeding density caused the decrease of N element acquired by individual plant and the reduction of growth space， and finally affected plant growth.
Effects of different treatments on yield of directseeding rape
It could be seen from the data in Table 2 that under the three different densities， the increase of N application rate could promote the growth and development of rape. The plant height， rhizome diameter and height of the branching part all increased with the increase of N application rate， and the variation trends were basically the same as those at wintering stage. Under the same N application rate， the increase of density would cause the decreases in plant height and rhizome diameter， because the increase of the density would reduce the fertilizer amount absorbed by individual plant， and the growth space would be reduced correspondingly. The number of primary branches increased with the N application rate increasing， and tended to be stable at a certain rate. Among the yield components， with the increase of N application rate， the effective pods per plant， the number of seeds per pod and 1 000seed weight all increased， and when the N application rate reached a certain value， the effective pods per plant and number of seeds per pod tended to be stable， while the 1 000seed weight was on the decrease. At the seeding density of 4.5×105 plants/hm2， the effective pods per plant， number of seeds per pod and 1 000grain weight were the highest， of 73 pods/plant， 22 seeds/pod and 3.69 g， respectively. As to yield， the application of nitrogen could significantly improve yield， and when the N application rate reached a certain level， the yield tended to a steady state， which accords with the previous research results[4]. The suitable N application rate differed according to seeding density， and could be deduced through the fertilizer efficiency model. It could be seen from Table 3 that when no nitrogen was applied， the yield would be reduced with the seeding density increasing. At the seeding density of 4.5×105 plants/hm2， the yield of rape was 2 049.5 kg/hm2 under no nitrogen application， and when the N application rate reached 360 kg/hm2， the yield was the highest of 2 659.1 kg/hm2. However， the yield at the N application rate of 180 kg/hm2 tended to be stable. The rape planted at the densities of 6×105 plants/hm2 and 7.5×105 plants/hm2 showed the yields of 2 028.4 and 2 007.1 kg/hm2 under no nitrogen application， respectively， and exhibited the highest yields of 3 336.3 and 3 781.9 kg/hm2 at the N application rate of 270 kg/hm2， respectively. The yield tended to a steady state around the N application rate of 270 kg/hm2. Further analysis showed that increasing the seeding density of rape could effectively improve yield. The yield at the density of 7.5×105 plants/hm2 was significantly higher than those at other two densities， and when the N application rate was 270 kg/hm2， the yield was the highest of 3 781.9 kg/hm2. Therefore， in production， in order to improve yield， the seeding density could be properly improved， and N fertilizer could be applied at a proper amount.
The optimal N application rate for the highest yield under different densities
Fertilizer efficiency fitting analysis was performed on N application rate and yield of rape using linear plus plateau model[5-7]， and the optimal N application rate was determined.
Linear plus plateau model： y=a+bx （x≤C）
y=P （x>C）
wherein y is seed yield （kg/hm2）， x is fertilizer application rate （kg/hm2）， and a is intercept； and b is regression coefficient， C is the point of intersection of the linear plus plateau， and P is plateau yield （kg/hm2）. When b>Px/Py， C is the optimal fertilizer application rate， and when b≤Px/Py， the recommended fertilizer application rate is 0 kg/hm2.
Fertilizer efficiency fitting analysis was performed on N application rate and yield of rape planted according to different seeding densities using linear plus plateau model， obtaining the fertilizer efficiency models of the two （Fig. 1）.
Seeding density 4.5×105 plants/hm2：
y=3.08x+2 067.22（x<186.77）
y=2 642.86（x≥186.77）
Seeding density 6×105 plants/hm2：
y=5.93x+2 021.57（x<221.35）
y=3 334.2（x≥221.35）
Seeding density 7.5×105 plants/hm2：
y=7.52x+2 000.46（x<236.14）
y=3 776.25（x≥236.14）
According to the fitted fertilizer efficiency models， the correlation coefficients of the three models were 0.991， 0.995 and 0.999， respectively， all reaching the very significant level. The N application rate suitable for the directseeding rape at the three planting densities was calculated， respectively. The calculated suitable N application rates for the three planting densities （4.5×105， 6×105 and 7.5×105 plants/hm2） were， respectively， 186.77， 221.35 and 236.14 kg/hm2， at which the theoretical yields reached the maximum values， which were 2 642.86， 3 334.20 and 3 776.25 kg/hm2， respectively. It could be seen from Fig. 1 that the lower the seeding density， the less the suitable nitrogen the rape required. The yields all increased with the N application rate increasing， and after the N application rate reached a certain level， the yield kept a steady state with the N application rate increasing. Conclusions and Discussion
The results of this study showed that the growth and development of rape differed to certain degrees under different densities. Under the same N application level， the rape grew better under low density than under high density. The rape grown under low density showed plant height， rhizome diameter， effective pods per plant， number of seeds per pod and 1 000seed weight and yield index better than the rape grown under high density. These differences between different densities were mainly due to the growing space for individual plants under low density was larger than that for plants under high density； the spacing between rape plants under high density was small， resulting in restriction on lateral growth； and under the highest density， the nutrients absorbed by individual plants were less than those absorbed by those under low density when the total amounts of the nutrients were fixed. Comprehensively， the traits including plant height， rhizome diameter， effective pods per plant， number of seeds per plant， and 1 000seed weight and the yield index of lowdensity rape were all better than those of highdensity rape.
Increasing N application rate could significantly improve the yield of rape， and increasing seeding density also could significantly improve yield. The results of this study showed that at the seeding density of 4.5×105 plants/hm2， the highest yield of rape was 2 659.1 kg/hm2， and when the seeding density was increased to 6×105 plants/hm2， the highest yield was 3 336.3 kg/hm2， which increased by 25.5%； and when the seeding density was increased to 7.5×105 plants/hm2， the highest yield was 3 781.9 kg/hm2， which increased by 13.4%. It could be seen that with the further increase of seeding density， the gain in yield caused by the increase of seeding density decreased gradually. In production， yield could be improved by properly improving seeding density. When the density reaches a certain value， the increase of seeding density would affect the growth of rape， and the effective pods per plant， number of seeds per pod and 1 000seed weight would decrease， thereby resulting in decrease of yield[8-9 ].
When the seeding density is increased， the required suitable N application rate also increases gradually， and when the seeding density reaches a certain value， the increase of the suitable N application rate decreases gradually. In this study， the suitable N application rate was 186.77 kg/hm2 at the density of 4.5×105 plants/hm2， 221.35 kg/hm2 at 6×105 plants/hm2， and 236.14 kg/hm2 at 7.5×105 plants/hm2， which only increased by 14.79 kg/hm2 compared with that at 6.0×105 plants/hm2. The main reason was that the increase of density affected the growth of rape， which correspondingly restricted the improvement of rape biomass and yield. The results of this study showed that in this area， the seeding density of directseeding rape could be selected in the range of 6.0×105-7.5×105 plants/hm2， and the suitable N application rate could be in the range of 221.35-236.14 kg/hm2， so that maximization of spatial resource and efficient utilization of fertilizer could be effectively realized. References
[1]China statistical yearbook[M]. Beijing： China Statistics Press， 2016. （in Chinese）
[2]HU WX， LI ZX， XU BQ， et al. Cultivation Techniques of High Efficiency of Directed Sowing Rapeseed in Rice Fields with Tillage Free[J]. Journal of Anhui Agricultural Sciences， 2007， 35（10）： 2883-2883. （in Chinese）
[3]BAO SD. Soil agrochemistry analysis[M]. Beijing： China Agriculture Press， 2000. （in Chinese）
[4]WANG Y， LU JW， LI XK， et al. Study on Nitrogen Fertilizer Effect and Optimum Fertilizer Ratefor Transplanting and DirectSeeding Rapeseed[J]. Scientia Agricultura Sinica， 2011， 44（21）： 4406-4414. （in Chinese）
[5]WANG XR， CHENG XP， ZHANG FS， et al. Application of fertilization models in recommended fertilization in China[J]. Journal of Plant Nutrition and Fertilizer， 1998， 4（1）： 67-74. （in Chinese）
[6]WANG SR， CHEN XP， GAO XZ， et al. Study on simulation of "3414" fertilizer experiments[J]. Journal of Plant Nutrition and Fertilizer， 2002， 8（4）：409-413. （in Chinese）
[7]ZHANG MQ， LI J， KONG QB， et al. Progress and Prospect of the Study on CropresponsetoFertilization Function Model[J]. Acta Pedologica Sinica， 2016， 53（6）： 1343-1356. （in Chinese）
[8]MEI JA， MEI SH， CHEN XG， et al. Effect of Sowing Time and Density on Yield and Yield Components of Directseeding Rapeseed[J]. Hubei Agricultural Sciences， 2011， 50（21）： 4337-4340. （in Chinese）
[9]ZHANG M， YAO XD， ZHANG YH. Effects of seeding date and seeding density on the plant growth and seed yield of directseeded rape in notillage paddy field[J]. Acta Agriculturae Shanghai， 2010， 26（3）： 48-51. （in Chinese）
Editor： Yingzhi GUANG Proofreader： Xinxiu ZHU