Abstract Rape is a crop that can be widely used as a green manure. After returning to the field， it has the ability to fertilize and improve the quality of agricultural products. This paper summarized the application model of rape as a green manure and the decomposing characteristics of rape after being returned to the field， and clarified the ecological effects of returning rape to the field， with an attempt to provide scientific basis for the reasonable use of rape and nutrient management in farmland.
　　Key words Green manure; Rape; Application mode; Decomposing characteristic; Field returning effect
　　 Green manure is the essence of Chinas traditional agriculture. As the cleanest organic fertilizer source， it plays an important role in improving soil fertility， preventing soil erosion， and improving crop quality. China has a long history of planting green manure. Since the Western Zhou Dynasty and the Spring and Autumn Period and the Warring States Period， it has experienced the green manure germination stage of hoeing grass and growing grass for fertilizing fields， the green manure cultivation and application stage in the Emperor Hanwu period， the initial green manure discipline construction stage in the Wei， Jin， and Northern and Southern Dynasties， and the rapid development stage in the Tang， Song， Yuan， Ming and Qing Dynasties[1]. In recent years， the adjustment of planting structure， the reduction of agricultural non-point source pollution， the improvement of farmland ecology， the combination of utilizing and increasing fertility and the improvement of agricultural product quality and efficiency have been the main strategic tasks of Chinas agriculture， and green manure has a unique and effective role in these tasks[2]. At present， the state actively promotes the strategy of "prospering agriculture with quality and promoting agriculture with greenness"， and the cultivation and utilization of green manures are conducive to promoting the sustainable and healthy production capacity of cultivated land， which is in line with the countrys strategy of "greening the countryside"[3].
　　Chinas green manure germplasm resources are abundant and diverse， and can be divided into Leguminosae， Compositae， Gramineae and Cruciferae according to botanical classification[4]. Rape belongs to Brassica in Brassicaceae， and is an annual or perennial herb. The annual planting area of rape in China is more than 7 million hm2[5]， its planting area and total output rank first in the world， and it is an important source of vegetable protein and vegetable fat[6]. In recent years， in addition to the traditional oil use methods， rape has been gradually developed for its functions for fertilizer， vegetable， flower， fodder and honey， which has significantly improved rape planting benefits[7]. Rape production mainly uses winter idle fields， which has the advantage of not competing with food crops for land， and the fallen flowers， fallen leaves and straws of rape have a good fertilizer effect， which is better than that of conventional green manure such as Astragalus sinicus L. because of its advantages of good adaptability， strong stress resistance， large material accumulation and excellent landscape effect[8]. Rape has been now widely used in green manure cultivation industry. This paper summarized the application model of rape as a green manure and the decomposing characteristics of rape after being returned to the field， and clarified the ecological effects of returning rape to the field， with an attempt to provide scientific basis for the reasonable use of rape and nutrient management in farmland. 　　Application Modes of Rape as Green Manure
　　Rice field application mode
　　Paddy fields are one of the main farming soil types in China， and they play an important role in ensuring Chinas food safety production[9]. At present， the application mode of rape as a paddy field green manure is mainly the rice-rice-rape mode. Green manure can provide various nutrients such as nitrogen and organic matter to the soil after being buried into the rice field by turning over the soil， thereby achieving the role of fertility improvement[10]. Hu et al.[11] investigated the effects of returning rape straw on the soil enzyme activity and microbial community structure in the cultivated layer of paddy fields， and the results showed that returning straw to the soil can effectively increase the alkali-hydrolyzale nitrogen and available phosphorus contents， improve enzyme activity and microbial biomass and thus promote rice production. Lan et al.[12] screened and optimized different green manure rotation patterns in paddy fields， and the results showed that the green manure rotation， especially the A. sinicus-rice-rice→rape-rice rotation mode， could significantly improve soil organic carbon mass fraction and soil carbon pool management index， which is helpful to improve soil quality. Xiong et al.[13] conducted field trials to select a suitable mixing ratio of A. sinicus and rape in rice fields. The results showed that the mixed ratio of 20% of rape and 80% of A. sinicus in rice fields achieved the highest yield of fresh grass， accompanied with highest nitrogen， phosphorus and potassium accumulation， and the ratio of carbon to nitrogen was closest to the ratio of soil microorganisms demand for carbon and nitrogen.
　　Dryland application model
　　Dryland agriculture refers to agricultural production based on natural precipitation in semi-arid and dry semi-humid regions[14]. The area of dry farming in China is large， accounting for more than 60% of the countrys cultivated land[15]. The main application modes of rape as upland green manure are wheat-rape rotation， corn-watermelon-rape rotation and so on. Zhao et al.[16] explored the benefits of cultivating rape in dryland during the summer fallow period， and the results showed that sowing rapeseeds at a selected suitable sowing date and ploughing the soil to return the crop to the field before next crop cultivation can achieve the effects of fertilizing the soil and improving water use efficiency. Mo et al.[17] showed that growing green manures ryegrass and rape in winter and then cultivating tobacco can effectively solve the problem of lack of organic fertilizer sources in dryland tobacco areas and improve the yield and quality of flue-cured tobacco. Li et al.[18] selected four green manure varieties （mung bean， Changwu soybean， hairy vetch， rape） and two green manure planting methods （post-wheat sowing， wheat field interplanting） to carry out field experiments. The results showed that the green manure biomass and returned nutrients in the post-wheat sowing method were significantly higher than those of interplanting， and the soil nutrient content and soil enzyme activity were also higher. Sowing rape following wheat cultivation can significantly increase the amounts of phosphorus and potassium returned to the field， and the amounts of nitrogen， phosphorus and potassium returned to the field by interplanting rape were significantly higher than other three varieties. 　　Orchard interplanting mode
　　Orchard green manure cultivation， also known as orchard grass cultivation， is an orchard soil management method introduced from developed countries since the middle of the 20th century[19]， and is one of the important sources of organic fertilizer for "fertilizing gardens by gardens" and "growing trees by land"[20]. Planting green manure in the space between fruit trees can form a "soil-fruit tree+green manure-atmosphere" water and heat exchange mode to capture more light energy for photosynthesis， increase carbon assimilation， promote the accumulation of fruit trees， and improve the quality of fruit trees[21]. In addition， it can also improve soil physical properties， enhance soil fertility， and reduce soil erosion. In 2019， the "orchard green manure soybean-rape rotation fertility-improving technology" is widely used as one of the main agricultural technologies in the country. The technology is to rotationally grow green manure crops such as leguminosae and cruciferous rape in apple orchards， and timely turn the soil over to improve the quality of apples. Liu[22] explored the cultivation technique of interplanting rape in apple orchards， and the results showed that rape can increase soil organic matter and microbial biomass， and meanwhile， special organic acids secreted by rape roots can activate and enrich soil nutrients. Studies by Xu et al.[23] showed that interplanting winter rape in young citrus orchards can not only fertilize the soil and save costs， but also can obtain additional rapeseed income and increase production and income.
　　Tea garden interplanting mode
　　Tea is one of the most important economic crops in China. Interplanting green manures in tea gardens plays an important role in improving the soil conditions of tea gardens， promoting the growth of tea trees， and improving the quality of tea[24]. Liang et al.[25] systematically evaluated and screened rape resources suitable for interplanting in tea gardens. Among them， rape varieties Sunshine 131， Shengguang 86 and Zhongshuang 11 performed relatively well， thus providing fertilizer-use varieties for the construction of high-quality and efficient ecological tea gardens. Lin et al.[26] showed that returning rape to the field in tea gardens can reduce soil acidification， increase soil organic matter， alkali-hydrolyzale nitrogen， available phosphorus and available potassium contents， and improve the drought resistance and cold resistance of tea trees and the yield and quality of tea. Li[27] explored the role of rape straw as an organic fertilizer in tea planting， and the results showed that rape straw can effectively increase tea yield， improve tea garden ecological environment and reduce the occurrence of tea plant diseases and pests. 　　Degradation Characteristics of Rape Returned to the Field
　　Degradation characteristics of rape straw
　　The degradation of rape straw showed different characteristics under the influences of returning time and soil environment. Hu et al.[28] studied the degradation characteristics of rape straw returned to the field and the effect of fertilizing the soil. The results showed that the decomposition percentage of rape straw returned to the field gradually increased with time， and the straw degradation rate was fast early and slow later， and gradually decreased with the amount of straw returned to the field increasing. Wang et al.[29] compared the decomposition speed of rape straw in the soil， open air and blistering methods， and the results showed that the rotting speed of stalks in the soil was faster than in the open air and blistering treatments. Dai et al.[30] studied the degradation characteristics of rape， rice， wheat， corn and broad bean green stalks after being buried into dryland and paddy fields， and found that the cumulative decomposition rates of straws in dryland ranked as rape>rice>corn>wheat>broad bean， and the cumulative decomposition rates of straws in paddy fields were in order of rice>corn>wheat>rape>broad bean. Li et al.[31] studied the decomposing process of rape straw， and the results showed that the destruction of straw tissue structure mainly occurred in the first 10 d of decomposition. The reason for the destruction of tissue structure was closely related to the ratio of phosphorus difficult to hydrolyze to decomposed phosphorus.
　　Nutrient release characteristics of rape
　　The release of nitrogen， phosphorus and potassium from rape straw returned to the field will have an impact on subsequent crops. Song et al.[32] studied the decomposition and nutrient release laws of rape straw， A. smicus and their mixed treatments at different ratios. The results showed that the release rates of various nutrients were different， ranking as potassium>phosphorus>nitrogen>carbon， and the degradation degree of rape straw mixed with A. sinicus was accelerated and increased with the amount of A. sinicus. Lu et al.[33] found that with the decomposition time， the contents of active organic carbon and total nitrogen in rape straw increased， and the ratio of active organic carbon to nitrogen decreased. Lyu et al.[34] investigated the effects of different straw return methods on microbial biomass carbon， nitrogen， phosphorus， and soluble organic matter under rape-corn rotation， and the results showed that returning straw+transformation promoter to the soil can significantly increase soil dissolved organic carbon and phosphorus， reduce soil soluble organic nitrogen and effectively increase crop yield. Dai et al.[35] studied the nutrient release characteristics of rice， wheat and rape straw under flooding conditions. The results showed the nutrient release rates followed K>P>C>N， the release amounts were in order of C> K> N>P， and for a short time， the release rate of potassium could reach 98%. Therefore， the amount of potassium fertilizer can be appropriately reduced during the early fertilization of crops， and the use of nitrogen and phosphorus fertilizers should be maintained because the early release rates of nitrogen and phosphorus are not high. 　　Effects of decomposing inoculants on the maturity of rape straw returned to the field
　　Straw decomposing inoculants are a group of complex flora that can degrade the macromolecular materials in straws into simple compounds that can be used by plants， thereby providing carbon， nitrogen， phosphorus， potassium and other elements to the soil， so that straw can return nutrients to the field[36]. Hu et al.[37] showed that returning straw to the field and applying straw decomposing inoculants can increase soil organic matter， total nitrogen， alkaline nitrogen， available phosphorus and available potassium contents， and cation exchange capacity， reduce soil bulk density， and increase the yield of early and late rice. Liu et al.[38] explored the effects of eight kinds of decomposing agents on rape stalks， and the results showed that the eight kinds of decomposing agents had different degrees of promoting effects， and the decomposing effects were different at different stages. Hong[39] showed that returning rape straw to the field with decomposing inoculants can significantly shorten the maturation time of straw.
　　Agricultural Biotechnology2020
　　Ecological Effects of Returning Rape Straw to the Field
　　Improving soil physical and chemical properties， and enhancing soil fertility
　　Rape green manure plays an important role in improving soil organic matter content and fertility. Yuan et al.[40] showed that returning straw to the field with the application of certain fertilizers under the rice-rape rotation mode can effectively increase soil organic carbon， humic acid， humin content and soil nutrients， and has the effect of fertilizing soil and promoting soil stability. Wang et al.[41] found that returning rape at the flowering stage to the field can increase soil organic matter， total nitrogen and total phosphorus contents， increase the total porosity of the paddy soil， and reduce soil bulk density. Gao et al.[42] showed that long-term rice-rice-rape rotation can significantly increase soil organic matter， total nitrogen， and alkali-hydrolyzable nitrogen contents， accelerate soil mineralization， and promote rice absorption of phosphorus and potassium. Guo et al.[43] explored the cultivation technology and benefit analysis of post-wheat forage rape， and the results showed that post-wheat forage rape cultivation can effectively increase soil coverage， reduce wind and water erosion， and increase soil fertility.
　　Increasing the number of soil microorganisms， and improving soil enzyme activity 　　The interaction of soil microorganisms with soil enzymes can promote the transformation and material cycle of various organic matter in the soil， and make the soil show normal metabolic functions[44]. Han et al.[45] used cabbage type rape and mustard type rape with different glucosinolate contents in pot experiments to investigate the effects of rape as green manure on soil microorganisms. The results showed that the number of bacteria and fungi in the soil increased first and then decreased， and the degradation of glucosinolates resulted in an increase in isothiocyanate content， which inhibited the bacteria and fungi in the soil. The mustard type rape had a higher glucosinolate content and a stronger ability to inhibit bacteria and fungi. Li et al.[46] investigated the effects of rape as green manure on soil fertility and bacterial community in the field with wheat as a subsequent crop. The results showed that rape seeding rate could significantly affect soil nutrients and enzyme activities， the return period significantly affected soil organic matter content and sucrase activity， and the interaction between the seeding rate and return period affected the alkaline phosphatase activity. Meanwhile， returning rape to the field can increase the bacterial community related to soil carbon and nitrogen metabolism and improve the diversity of the bacterial community. Liu et al.[47] showed that the returning rape into the soil could increase the activity of protease， sucrase， urease and catalase in tobacco planting soil.
　　Absorbing heavy metals， and treating contaminated soil
　　Rape is a plant rich in cadmium， which can be used to purify the soil when it is used in heavy metal pollution areas. Liu et al.[48] investigated the response of different types of rape （mustard type rape Lvsheng 1， cabbage type rape Zhibowang and Xiangzayou 6） to cadmium pollution， and the results showed that the mustard type rape Lvsheng 1 was more suitable for the remediation of cadmium-contaminated soil. Gao et al.[49] studied the differences in cadmium absorption and accumulation between different rape varieties using soil culture methods. The results showed that under both acidic and alkaline soil conditions， the cadmium concentration in the aboveground part was highest in Jinghua Guanqing and lowest in Xiawang. The cadmium concentrations of aboveground and underground parts were significantly different between different varieties， and for the same rape variety， acidic soil was more conducive to the absorption and accumulation of cadmium by rape than alkaline soil. Yu et al.[50] studied the effects of rape-rice rotation on crop uptake and accumulation of cadmium. The results showed that different cadmium uptake characteristics of rape varieties exhibited differences in cadmium accumulation， and the rhizosphere of different rape varieties activated soil available cadmium to different degrees， which affected the cadmium content in grains of subsequent rice. 　　Reducing the amount of fertilizer applied， and promoting the growth of subsequent crops
　　Green manure can play a role of replacing partial chemical fertilizers. It can provides elements required by crops， and realize the use of organic and inorganic fertilizers in farmland[51]. Li et al.[52] found that the return of straw to the rice-rape rotation area could reduce the amount of potassium fertilizer application， increase the available potassium content in the soil， and obtain a good economic yield of grain and oil. Hu et al.[53] explored the effects of returning rape straw to the field and phosphorus fertilizer regulation on rice growth and yield through field experiments. The results showed that returning rape straw to the field could promote rice tillering and yield increase， but excessive phosphorus application would reduce panicle bearing tiller rate， and phosphate fertilizer should be applied in an appropriate amount. Duan et al.[54] found that after rape stalks were returned to the field， the yield of rice under low or no phosphorus treatment was not significantly different from that under high phosphorus treatment， and even higher than that of high phosphorus treatment， so reduced phosphorus application after returning straw to the field had a positive effect on reducing environmental pollution and promoting rice production.
　　Problems and Outlook
　　In 2010， the Central Document No. 1 pointed out that the reduction of pesticides and fertilizers should be carried out to promote the comprehensive utilization of straw. The Ministry of Agriculture and Rural Affairs has formulated the "Zero Growth Action Plan for Fertilizer Use by 2020" in 2015. The use of rape as fertilizer is an important measure to reduce the application of chemical fertilizers and protect the ecological environment of farmland. Therefore， the rational development and utilization of rape as is in line with the countrys "rural greening" development concept.
　　Innovating varieties， and improving supporting technologies
　　Rape fertilizer has the advantages that other green manures do not have， but most of the rapeseeds sold on the market are purposed for oil and vegetable， and the varieties have large biomass， tall plant type， thick and hard stems， and the disadvantages of slow decomposition and strong difficulty in burying into the soil when used as green manure. In the future， we should actively explore the development and selection of high-quality special rape green manure germplasm resources in different regions， soil types and cultivation modes， and establish a supporting high-yield and efficient cultivation technology system. 　　Actively guiding farmers， and increasing government support
　　The cultivation and harvesting of rape in China mainly takes rural households as production units， and the degree of mechanization is low. Most of the rape varieties cultivated by farmers are purposed for oil and vegetable. Government departments should strengthen the technical development and promotion of green manure rape varieties also purposed for vegetable and oil and improve the mechanization degree of rape industry， to comprehensively improve the efficiency of rape production and increase the farmers enthusiasm for planting rape.
　　Comprehensively utilizing， practicing the "rape+" development concept
　　We should give full play to the reproductive advantage of rape that can be long and short in growth period. To practice the "rape+" development concept， its oil， vegetable， honey， feed， fertilizer and ornamental functions can be comprehensively developed， and measures should be taken according to local conditions to realize regional characteristic farming model， to thereby realize the deep integration of primary， secondary and tertiary industries and comprehensively improve the economic， social and ecological benefits of rape cultivation.
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