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on Soil Nutrients and Microorganism Communities in Double-rice Cropping Field
Abstract [Objectives] This study was conducted to investigate the effects of milk vetch （Astragalus sinicus） on soil nutrients and microorganisms in paddy fields after returning to the field.
[Methods]Three treatments， no fertilization， single application of chemical fertilizers and milk vetch return combined with chemical fertilizers， were set up to determine soil nutrient contents and the quantity of microorganisms， respectively.
[Results] The return of milk vetch combined with chemical fertilizers could significantly increase the contents of organic matter and total nitrogen in the paddy soil， while the contents of available potassium and available phosphorus decreased. Milk vetch return combined with chemical fertilizers could significantly increase the quantity of soil microorganisms in the paddy field.
[Conclusions]The return of milk vetch can be used as a feasible measure to improve soil fertility.
Key words Milk vetch; Returning to field; Soil nutrient; Microorganisms
Received： April 27， 2021 Accepted： June 16， 2021
Supported by National Key R&D Project （2018YFD0800503）; National Natural Science Foundation of China （31860595）.
Rong HU （1995-）， female， P. R. China， master， devoted to research about plant nutrition.
*Corresponding author. E-mail： ngr0329@126.com.
China is rich in green manure resources[1-2]. Milk vetch （Astragalus sinicus）， as a major paddy field green manure， has outstanding fertilization effects， and plays a role in improving soil physical and chemical properties to promote crop growth， increasing the quantity and diversity of soil microorganisms， improving the soil ecological environment， promoting the accumulation of soil organic carbon， and reducing CO2 emissions. Today， when low-carbon agriculture and organic agriculture are promoted， milk vetch green manure will play an irreplaceable role. The United States， Japan， South Korea and other developed countries have vigorously developed the milk vetch industry to improve soil fertility[3-5]. Milk vetch can not only be used as green manure， but also can be used for comprehensive utilization in aspects of nectar production， pasture and vegetables. In this study， in order to explore the impact of milk vetch on the soil during the process of returning to the field， with the soil of paddy field where milk vetch was returned to with the combined application of chemical fertilizers as the research object and no fertilization and single application of chemical fertilizers as controls， the changes in soil nutrient contents and microorganisms were analyzed. This study provides theoretical support for the use of milk vetch in the southern red soil region for improving soil fertility. Materials and Methods
Experiment location and experiment methods
The experiment was carried out at the Long-term Green Manure Experimental Station of Jiangxi Agricultural University in 2016. The test soil was a pigmented paddy soil developed in Quaternary red clay， and double-cropping rice was planted over the years. Before the experiment （1986）， the soil agrochemical properties of the cultivated layer were： pH 6.5， organic matter 33.55 g/kg， total nitrogen 1.79 g/kg， alkali hydrolyzable nitrogen 110.9 mg/kg， available phosphorus （P2O5） 20.8 mg/kg， and available potassium （K2O） 87 mg/kg. The experiment was set with three treatments （Table 1）， in three replicates， arranged in random blocks， and for various fertilization treatments， the application amounts of N， P， and K were ensured to be equal. The nutrient quantity of the organic fertilizer was calculated according to the actual measurement results each time， and supplemented with chemical fertilizers. The nitrogen fertilizer was applied in 3 times according to the ratio of base fertilizer， tiller fertilizer and panicle fertilizer at a ratio of 2∶1∶1. Phosphorus and potassium fertilizers were used as base fertilizer. The chemical fertilizers were urea， calcium magnesium phosphate fertilizer and potassium chloride. The area of each plot was 0.883 m and various plots were separated by cement ridges. Each plot was planted with 25 hills， 2 seedlings per hill for early rice， and 4 seedlings per hill for late rice. Other cultivation management measures were the same for each treatment.
Collection of rhizosphere soil
The improved soil shaking method was adopted to collect rhizosphere soil[6]， and soil samples were obtained at the mature stage of late rice. Specifically， the whole rice plants were dug up together with the soil， and most of the soil around the roots was removed， leaving only the soil attached to the roots. The attached soil was brushed with a sterile brush to obtain the rhizosphere soil. The rhizosphere soil was put in sterilized triangular flasks in ice boxes and immediately bought back to the laboratory for analysis. If it was too late for analysis on the same day， it would be stored in a refrigerator at 4 ℃ and analyzed the next day.
Determination of soil microorganisms
The quantities of the three types of microorganisms were determined by the plate dilution method; and the quantities of microorganisms in different physiological groups were determined by the most probable number （MPN） method. Determination of soil nutrients
The determination of nutrients in the soil was carried out by conventional determination methods[6]. The total nitrogen was determined by the Kjeldahl method; the organic matter was determined by the potassium dichromate heating oxidation-sulfuric acid volumetric method; the available phosphorus in the soil was determined by the NaHCO3 extraction-molybdenum antimony colorimetric method; the hydrolyzable nitrogen in the soil was determined by alkaline hydrolysis diffusion method; the rapidly available potassium in the soil was determined by ammonium acetate extraction-flame photometer method; and the pH value was measured by a pH meter.
Data processing
EXCEL was used for data sorting， and DPS was adopted for new multiple range test.
Results and Analysis
Paddy soil nutrients
Under long-term different fertilization systems， the results of paddy soil nutrients showed that compared with the non-fertilized control （CK）， the soil organic matter content， total nitrogen content， available phosphorus， and available potassium content under fertilization treatments all increased significantly （P<0.05） （Table 2）. Specifically， compared with single application of chemical fertilizers， the treatment of returning milk vetch to the field combined with the application of chemical fertilizers could significantly increase the soil organic matter and total nitrogen contents， which increased by 7.2% and 10.6%， respectively， while the available phosphorus and available potassium significantly decreased， by 11.6% and 3.2%， respectively.
Culturable microorganisms
Differences between the three types of microorganisms
The dilution plating method was used to determine the quantities of the three types of cultivable microorganisms in paddy soil under different fertilization systems （Table 3）. The results were as follows： the quantity of bacteria in each fertilization treatment was significantly higher than that of the control without fertilization， and the M+F treatment was better than the F treatment in increasing the quantity of bacteria in the soil. The order of the quantities of fungi was： M+F>CK>F， and the differences between the treatments were significant （P<0.05）. Actinomycetes were more in the M+F and F treatments than in the CK. The change law of the total count of microorganisms was consistent with the change law of the bacterial count.
Comparison of the quantities of microorganisms in different physiological groups The MPN method was used to determine the quantities of microorganisms in different physiological groups in the rhizosphere of rice under different fertilization systems. The results （Table 4） showed that compared with the control treatment， the rhizosphere soil of the fertilization treatments exhibited a significant increase in each physiological group. Specifically， the quantities of nitrifying bacteria， denitrifying bacteria， free-living nitrogen-fixing bacteria and ammonifying bacteria in the rhizosphere soil were all in the same order， which was M+F>F>CK. It could be seen that the combination of organic fertilizer and chemical fertilizers could significantly increase the quantity of nitrifying bacteria in the soil compared with the treatments of simply applying chemical fertilizer or no fertilizer.
Correlation analysis of various microbial indexes and soil nutrients
The correlation analysis of the microbial indicators and soil nutrients of different treatments is shown in Table 5. It can be seen that the organic matter content was significantly positively correlated with fungi， and was also positively correlated with other microbial indicators， though not significant. The total nitrogen content of the soil was significantly positively correlated with nitrifying bacteria and denitrifying bacteria， and was also positively correlated with other microbial indicators， but the correlation was not significant. The soil available phosphorus content was positively correlated with the quantities of nitrifying bacteria， denitrifying bacteria， nitrogen-fixing bacteria and ammonifying bacteria， and the quantity of bacteria and the quantity of total microorganism， but the correlation was not significant， and the correlation with the quantities of fungi and actinomycetes was not obvious. The soil available potassium content was positively correlated with various microbial indicators， among which the correlation with nitrifying bacteria and ammonifying bacteria was significant， and that with others was not significant.
Conclusions and Discussion
Fertilization is one of the most profound agricultural measures affecting soil quality and its sustainable use. Different fertilization systems lead to different soil microbial populations， quantities and activity， which further lead to different soil biological fertility， while this difference will have an important impact on soil structure， fertility and productivity.
Effects of returning milk vetch to field on soil nutrient contents in paddy field Agricultural management measures are an important factor affecting soil fertility. Factors such as fertilization system， planting system and irrigation mode will affect the nutrient contents and structure of soil. The results of this study showed that compared with the control treatment without fertilization， fertilization measures could significantly increase the content of organic matter， total nitrogen， available potassium， and available phosphorus in the soil， which is consistent with the results of Chen et al.[7] and Zhang et al.[8]. On the one hand， with long-term fertilization， N， P， K and other nutrients in the fertilizers enter the soil with the fertilization， increasing the contents of soil nutrients. On the other hand， fertilizers provide abundant nutrients for soil microorganisms， which increase the biological activity of soil microbes. Since soil microorganisms contain nutrients such as C， N， P， etc.， they are a nutrient reservoir， so the increase of their quantity will inevitably lead to the increase of soil nutrients. The increase of soil microbial activity will accelerate the cyclic transformation of nutrients in the soil， especially the increase in the quantity and activity of some nitrogen-fixing bacteria， cellulose-decomposing bacteria， and phosphorus- and potassium-solubilizing bacteria， which can significantly increase the effective nutrients in the soil.
This study showed that the combination of milk vetch return and chemical fertilizers had a higher impact on soil nutrient contents than the application of chemical fertilizers alone， which is consistent with the research results of Zhan et al.[9]， Yang et al. [10] and Qin et al.[11]. The reason for these differences is that， on the one hand， the return of milk vetch provides abundant nutrients for microorganisms， and on the other hand， microorganisms can convert non-bioavailable nutrients into bioavailable nutrients by fixing nitrogen， dissolving phosphorus， and dissolving potassium， ultimately improving the fertility of the soil.
Effect of returning milk vetch to field on microorganisms
The results of this study showed that long-term return of milk vetch with the supplement of chemical fertilizers could significantly increase the quantity of soil microorganisms， which is consistent with the results of most studies[12-13]. The reason for the application of chemical fertilizers to increase the quantity of soil microorganisms is that although chemical fertilizers do not directly provide nutrients such as C sources to microorganisms， on the one hand， fertilization promotes plant growth， and plant root exudates are more vigorous， which increases the carbon source in the soil; and on the other hand， the N and P elements in chemical fertilizers provide good nutrients for the growth of microorganisms. The reason why the application of organic fertilizer increases the number of soil microorganisms is that it provides a large amount of nutrients for the growth of microorganisms. In addition， organic fertilizer can promote the growth of plant roots and increase root exudates， which also increases the quantity of microorganisms to a certain extent. Conclusions
Fertilization measures could significantly increase soil nutrient contents and the total quantity of soil microorganisms. Among them， the effect of returning milk vetch to the field combined with chemical fertilizers was more obvious than that of the application of chemical fertilizers alone. Therefore， returning milk vetch to the field combined with the application of chemical fertilizers is an ideal fertilization measure.
References
[1] WANG CJ， ZHOU SB， CHEN ND， et al. Diversity of forage resources in Anhui Province[J]. Pratacultural Science， 2006， 23（5）： 12-16. （in Chinese）
[2] CHEN ZH， LI XF， YUN XJ， et al. Diversity and conservation of forage germplasm resources in China[J]. Pratacultural Science， 2009， 26（5）： 1-6. （in Chinese）
[3] DAE JK， DAE SC， SUNGCHUL CB， et al. Effects of soil selenium supplementation level on selenium contents of green tea leaves and milk vetch[J]. Journal of Food Science and Nutrition， 2007， 12（1）： 35-39.
[4] NAOMI A， HIDETO U. Nitrogen dynamics in paddy soil applied with various 15N-labelled green manures[J]. Plant and Soil， 2009（10）： 3-4.
[5] NAKAYAMA H. Characteristics of rice （Oryza sativa） growth， yield and soil nitrogen by cultivating Chinese milk vetch （Astragalus sinicus L.） as green manure[J]. Tohoku Agricultural Research， 2005（58）： 35-36 .
[6] YAO HY， HUANG CY. Soil microbial ecology and its experimental technology[A]. 2006. （in Chinese）
[7] CHEN YA， CHEN DH， YOU YW， et al. Variations of red earth fertility and available techniques of fertilization[J]. Journal of Plant Nutrition and Fertilizer， 1999， 5（2）： 115-121. （in Chinese）
[8] ZHANG AJ， ZHANG MP. Study on regularity of growth and decline of soil organic matter under long-term fertilization for yellow fluvo-aquic soil[J]. Journal of Anhui Agricultural University， 200 29（1）： 60-63. （in Chinese）
[9] ZHAN QH， CHEN J. Continual nutrient supplying capacity and crop responses based on long-term fertilizer experiment in vertisol[J]. Acta Pedologica Sinica， 2006， 43（1）： 124-132. （in Chinese）
[10] QIN SW， GU YC， ZHU ZL. A Preliminary report on the long-term location experiment of the fertility evolution of fluvo-aquic soil and the effect of fertilization[J]. Acta Pedologica Sinica， 1998， 35（3）： 367-375. （in Chinese）
[11] ZHAO XQ， LU RK. Effect of organic fertilizer on soil phosphorus adsorption[J]. Acta Pedologica Sinica， 199 28（1）： 7-13. （in Chinese）
[12] WANG BR， XU MG， WEN SL. The effects of combined application organic manure and chemical fertilizers on fertility of red soil[J]. Chinese Agricultural Science Bulletin， 2005， 21（2）： 160-163. （in Chinese）
[13] LI XY， ZHAO BQ， LI XH， et al. Effects of different fertilization systems on soil microbe and its relation to soil fertility[J]. Scientia Agricultura Sinica， 2005， 38（8）： 1591-1599. （in Chinese）
Abstract [Objectives] This study was conducted to investigate the effects of milk vetch （Astragalus sinicus） on soil nutrients and microorganisms in paddy fields after returning to the field.
[Methods]Three treatments， no fertilization， single application of chemical fertilizers and milk vetch return combined with chemical fertilizers， were set up to determine soil nutrient contents and the quantity of microorganisms， respectively.
[Results] The return of milk vetch combined with chemical fertilizers could significantly increase the contents of organic matter and total nitrogen in the paddy soil， while the contents of available potassium and available phosphorus decreased. Milk vetch return combined with chemical fertilizers could significantly increase the quantity of soil microorganisms in the paddy field.
[Conclusions]The return of milk vetch can be used as a feasible measure to improve soil fertility.
Key words Milk vetch; Returning to field; Soil nutrient; Microorganisms
Received： April 27， 2021 Accepted： June 16， 2021
Supported by National Key R&D Project （2018YFD0800503）; National Natural Science Foundation of China （31860595）.
Rong HU （1995-）， female， P. R. China， master， devoted to research about plant nutrition.
*Corresponding author. E-mail： ngr0329@126.com.
China is rich in green manure resources[1-2]. Milk vetch （Astragalus sinicus）， as a major paddy field green manure， has outstanding fertilization effects， and plays a role in improving soil physical and chemical properties to promote crop growth， increasing the quantity and diversity of soil microorganisms， improving the soil ecological environment， promoting the accumulation of soil organic carbon， and reducing CO2 emissions. Today， when low-carbon agriculture and organic agriculture are promoted， milk vetch green manure will play an irreplaceable role. The United States， Japan， South Korea and other developed countries have vigorously developed the milk vetch industry to improve soil fertility[3-5]. Milk vetch can not only be used as green manure， but also can be used for comprehensive utilization in aspects of nectar production， pasture and vegetables. In this study， in order to explore the impact of milk vetch on the soil during the process of returning to the field， with the soil of paddy field where milk vetch was returned to with the combined application of chemical fertilizers as the research object and no fertilization and single application of chemical fertilizers as controls， the changes in soil nutrient contents and microorganisms were analyzed. This study provides theoretical support for the use of milk vetch in the southern red soil region for improving soil fertility. Materials and Methods
Experiment location and experiment methods
The experiment was carried out at the Long-term Green Manure Experimental Station of Jiangxi Agricultural University in 2016. The test soil was a pigmented paddy soil developed in Quaternary red clay， and double-cropping rice was planted over the years. Before the experiment （1986）， the soil agrochemical properties of the cultivated layer were： pH 6.5， organic matter 33.55 g/kg， total nitrogen 1.79 g/kg， alkali hydrolyzable nitrogen 110.9 mg/kg， available phosphorus （P2O5） 20.8 mg/kg， and available potassium （K2O） 87 mg/kg. The experiment was set with three treatments （Table 1）， in three replicates， arranged in random blocks， and for various fertilization treatments， the application amounts of N， P， and K were ensured to be equal. The nutrient quantity of the organic fertilizer was calculated according to the actual measurement results each time， and supplemented with chemical fertilizers. The nitrogen fertilizer was applied in 3 times according to the ratio of base fertilizer， tiller fertilizer and panicle fertilizer at a ratio of 2∶1∶1. Phosphorus and potassium fertilizers were used as base fertilizer. The chemical fertilizers were urea， calcium magnesium phosphate fertilizer and potassium chloride. The area of each plot was 0.883 m and various plots were separated by cement ridges. Each plot was planted with 25 hills， 2 seedlings per hill for early rice， and 4 seedlings per hill for late rice. Other cultivation management measures were the same for each treatment.
Collection of rhizosphere soil
The improved soil shaking method was adopted to collect rhizosphere soil[6]， and soil samples were obtained at the mature stage of late rice. Specifically， the whole rice plants were dug up together with the soil， and most of the soil around the roots was removed， leaving only the soil attached to the roots. The attached soil was brushed with a sterile brush to obtain the rhizosphere soil. The rhizosphere soil was put in sterilized triangular flasks in ice boxes and immediately bought back to the laboratory for analysis. If it was too late for analysis on the same day， it would be stored in a refrigerator at 4 ℃ and analyzed the next day.
Determination of soil microorganisms
The quantities of the three types of microorganisms were determined by the plate dilution method; and the quantities of microorganisms in different physiological groups were determined by the most probable number （MPN） method. Determination of soil nutrients
The determination of nutrients in the soil was carried out by conventional determination methods[6]. The total nitrogen was determined by the Kjeldahl method; the organic matter was determined by the potassium dichromate heating oxidation-sulfuric acid volumetric method; the available phosphorus in the soil was determined by the NaHCO3 extraction-molybdenum antimony colorimetric method; the hydrolyzable nitrogen in the soil was determined by alkaline hydrolysis diffusion method; the rapidly available potassium in the soil was determined by ammonium acetate extraction-flame photometer method; and the pH value was measured by a pH meter.
Data processing
EXCEL was used for data sorting， and DPS was adopted for new multiple range test.
Results and Analysis
Paddy soil nutrients
Under long-term different fertilization systems， the results of paddy soil nutrients showed that compared with the non-fertilized control （CK）， the soil organic matter content， total nitrogen content， available phosphorus， and available potassium content under fertilization treatments all increased significantly （P<0.05） （Table 2）. Specifically， compared with single application of chemical fertilizers， the treatment of returning milk vetch to the field combined with the application of chemical fertilizers could significantly increase the soil organic matter and total nitrogen contents， which increased by 7.2% and 10.6%， respectively， while the available phosphorus and available potassium significantly decreased， by 11.6% and 3.2%， respectively.
Culturable microorganisms
Differences between the three types of microorganisms
The dilution plating method was used to determine the quantities of the three types of cultivable microorganisms in paddy soil under different fertilization systems （Table 3）. The results were as follows： the quantity of bacteria in each fertilization treatment was significantly higher than that of the control without fertilization， and the M+F treatment was better than the F treatment in increasing the quantity of bacteria in the soil. The order of the quantities of fungi was： M+F>CK>F， and the differences between the treatments were significant （P<0.05）. Actinomycetes were more in the M+F and F treatments than in the CK. The change law of the total count of microorganisms was consistent with the change law of the bacterial count.
Comparison of the quantities of microorganisms in different physiological groups The MPN method was used to determine the quantities of microorganisms in different physiological groups in the rhizosphere of rice under different fertilization systems. The results （Table 4） showed that compared with the control treatment， the rhizosphere soil of the fertilization treatments exhibited a significant increase in each physiological group. Specifically， the quantities of nitrifying bacteria， denitrifying bacteria， free-living nitrogen-fixing bacteria and ammonifying bacteria in the rhizosphere soil were all in the same order， which was M+F>F>CK. It could be seen that the combination of organic fertilizer and chemical fertilizers could significantly increase the quantity of nitrifying bacteria in the soil compared with the treatments of simply applying chemical fertilizer or no fertilizer.
Correlation analysis of various microbial indexes and soil nutrients
The correlation analysis of the microbial indicators and soil nutrients of different treatments is shown in Table 5. It can be seen that the organic matter content was significantly positively correlated with fungi， and was also positively correlated with other microbial indicators， though not significant. The total nitrogen content of the soil was significantly positively correlated with nitrifying bacteria and denitrifying bacteria， and was also positively correlated with other microbial indicators， but the correlation was not significant. The soil available phosphorus content was positively correlated with the quantities of nitrifying bacteria， denitrifying bacteria， nitrogen-fixing bacteria and ammonifying bacteria， and the quantity of bacteria and the quantity of total microorganism， but the correlation was not significant， and the correlation with the quantities of fungi and actinomycetes was not obvious. The soil available potassium content was positively correlated with various microbial indicators， among which the correlation with nitrifying bacteria and ammonifying bacteria was significant， and that with others was not significant.
Conclusions and Discussion
Fertilization is one of the most profound agricultural measures affecting soil quality and its sustainable use. Different fertilization systems lead to different soil microbial populations， quantities and activity， which further lead to different soil biological fertility， while this difference will have an important impact on soil structure， fertility and productivity.
Effects of returning milk vetch to field on soil nutrient contents in paddy field Agricultural management measures are an important factor affecting soil fertility. Factors such as fertilization system， planting system and irrigation mode will affect the nutrient contents and structure of soil. The results of this study showed that compared with the control treatment without fertilization， fertilization measures could significantly increase the content of organic matter， total nitrogen， available potassium， and available phosphorus in the soil， which is consistent with the results of Chen et al.[7] and Zhang et al.[8]. On the one hand， with long-term fertilization， N， P， K and other nutrients in the fertilizers enter the soil with the fertilization， increasing the contents of soil nutrients. On the other hand， fertilizers provide abundant nutrients for soil microorganisms， which increase the biological activity of soil microbes. Since soil microorganisms contain nutrients such as C， N， P， etc.， they are a nutrient reservoir， so the increase of their quantity will inevitably lead to the increase of soil nutrients. The increase of soil microbial activity will accelerate the cyclic transformation of nutrients in the soil， especially the increase in the quantity and activity of some nitrogen-fixing bacteria， cellulose-decomposing bacteria， and phosphorus- and potassium-solubilizing bacteria， which can significantly increase the effective nutrients in the soil.
This study showed that the combination of milk vetch return and chemical fertilizers had a higher impact on soil nutrient contents than the application of chemical fertilizers alone， which is consistent with the research results of Zhan et al.[9]， Yang et al. [10] and Qin et al.[11]. The reason for these differences is that， on the one hand， the return of milk vetch provides abundant nutrients for microorganisms， and on the other hand， microorganisms can convert non-bioavailable nutrients into bioavailable nutrients by fixing nitrogen， dissolving phosphorus， and dissolving potassium， ultimately improving the fertility of the soil.
Effect of returning milk vetch to field on microorganisms
The results of this study showed that long-term return of milk vetch with the supplement of chemical fertilizers could significantly increase the quantity of soil microorganisms， which is consistent with the results of most studies[12-13]. The reason for the application of chemical fertilizers to increase the quantity of soil microorganisms is that although chemical fertilizers do not directly provide nutrients such as C sources to microorganisms， on the one hand， fertilization promotes plant growth， and plant root exudates are more vigorous， which increases the carbon source in the soil; and on the other hand， the N and P elements in chemical fertilizers provide good nutrients for the growth of microorganisms. The reason why the application of organic fertilizer increases the number of soil microorganisms is that it provides a large amount of nutrients for the growth of microorganisms. In addition， organic fertilizer can promote the growth of plant roots and increase root exudates， which also increases the quantity of microorganisms to a certain extent. Conclusions
Fertilization measures could significantly increase soil nutrient contents and the total quantity of soil microorganisms. Among them， the effect of returning milk vetch to the field combined with chemical fertilizers was more obvious than that of the application of chemical fertilizers alone. Therefore， returning milk vetch to the field combined with the application of chemical fertilizers is an ideal fertilization measure.
References
[1] WANG CJ， ZHOU SB， CHEN ND， et al. Diversity of forage resources in Anhui Province[J]. Pratacultural Science， 2006， 23（5）： 12-16. （in Chinese）
[2] CHEN ZH， LI XF， YUN XJ， et al. Diversity and conservation of forage germplasm resources in China[J]. Pratacultural Science， 2009， 26（5）： 1-6. （in Chinese）
[3] DAE JK， DAE SC， SUNGCHUL CB， et al. Effects of soil selenium supplementation level on selenium contents of green tea leaves and milk vetch[J]. Journal of Food Science and Nutrition， 2007， 12（1）： 35-39.
[4] NAOMI A， HIDETO U. Nitrogen dynamics in paddy soil applied with various 15N-labelled green manures[J]. Plant and Soil， 2009（10）： 3-4.
[5] NAKAYAMA H. Characteristics of rice （Oryza sativa） growth， yield and soil nitrogen by cultivating Chinese milk vetch （Astragalus sinicus L.） as green manure[J]. Tohoku Agricultural Research， 2005（58）： 35-36 .
[6] YAO HY， HUANG CY. Soil microbial ecology and its experimental technology[A]. 2006. （in Chinese）
[7] CHEN YA， CHEN DH， YOU YW， et al. Variations of red earth fertility and available techniques of fertilization[J]. Journal of Plant Nutrition and Fertilizer， 1999， 5（2）： 115-121. （in Chinese）
[8] ZHANG AJ， ZHANG MP. Study on regularity of growth and decline of soil organic matter under long-term fertilization for yellow fluvo-aquic soil[J]. Journal of Anhui Agricultural University， 200 29（1）： 60-63. （in Chinese）
[9] ZHAN QH， CHEN J. Continual nutrient supplying capacity and crop responses based on long-term fertilizer experiment in vertisol[J]. Acta Pedologica Sinica， 2006， 43（1）： 124-132. （in Chinese）
[10] QIN SW， GU YC， ZHU ZL. A Preliminary report on the long-term location experiment of the fertility evolution of fluvo-aquic soil and the effect of fertilization[J]. Acta Pedologica Sinica， 1998， 35（3）： 367-375. （in Chinese）
[11] ZHAO XQ， LU RK. Effect of organic fertilizer on soil phosphorus adsorption[J]. Acta Pedologica Sinica， 199 28（1）： 7-13. （in Chinese）
[12] WANG BR， XU MG， WEN SL. The effects of combined application organic manure and chemical fertilizers on fertility of red soil[J]. Chinese Agricultural Science Bulletin， 2005， 21（2）： 160-163. （in Chinese）
[13] LI XY， ZHAO BQ， LI XH， et al. Effects of different fertilization systems on soil microbe and its relation to soil fertility[J]. Scientia Agricultura Sinica， 2005， 38（8）： 1591-1599. （in Chinese）