Research On Soil Mg Criteria And Mg Fertilization Effect On Yield And Quality Improvement And Management Strategies For Winter Oilseed Rape

Oilseed rape（Brassica napus L.）,an important oil crop in China,is mainly distributed in the Yangtze River basin.Soil magnesium（Mg）leaching in this region is serious,due to the low soil p H,the climate of the same period of rain and heat and the easy leaching of magnesium ions(Mg²⁺).In recent years,owing to the adjustment of fertilizer structure,such as magnesium-containing calcium superphosphate,calcium-magnesium phosphate fertilizer,etc.gradually replaced by ammonium phosphate and high concentration of compound fertilizer application,the soil Mg are difficult to be effectively supplemented.And the crop harvest takes away a large amount of Mg nutrients,resulting in soil Mg deficiency has gradually become a new limiting factor that hinders the increase of oilseed rape yield.The contradiction between the prominent problem of soil Mg deficiency in oilseed rape production,the significant effect of Mg application to increase yield and the weak basic theoretical knowledge of Mg application to increase yield,and the lack of key technical aspects of Mg fertilizer management and application has been increasing.However,we have not enough knowledge about the soil Mg status and the effect of Mg application to oilseed rape in the main production areas of winter oilseed rape,especially the effect of Mg application in areas with different Mg concentration exhibited a large difference.Therefore,it is urgent to carry out effective research on Mg in oilseed rape,to understand the current status of Mg fertilization in oilseed rape cultivation areas in China and the effect of Mg application in oilseed rape to increase yield and improve quality.To provide theoretical guidance and technical support for the reasonable application of Mg fertilization,and to make efforts to integrate it into the green and high-yielding,light and efficient production of oilseed rape.This study adopted the combination of research sampling test,multi-point field experiment and long-term field positioning experiment.Firstly,we clarified the characteristics of soil Mg balance by understanding the current situation of Mg fertilizer input and quantitative analysis of soil Mg input and output pathways in the main winter oilseed rape producing areas.Then,by establishing the index of soil exchangeable magnesium（ex-Mg）abundance and deficiency in oilseed rape,we knew the degree of soil Mg abundance and deficiency and distribution status in the main winter oilseed rape growing areas.Meanwhile,the effect of Mg application in rape under different soil Mg classification to increase yield and improve quality was studied,and the mechanism of its yield and quality increase was initially explored,and finally the suggestions of Mg fertilization management in winter rape were proposed.The main results are as follows:（1）The apparent balance of annual Mg nutrients（in Mg O）was-70.3 kg/hm² in the rice-oilseed rape rotation system of the main winter oilseed rape producing areas in China,of which the Mg input was about 16.1 kg/hm² and 86.4 kg/hm² was taken away from the crop harvest.In addition to crop harvesting to carry away,leaching and runoff are the main ways of soil Mg loss.The Mg leaching and runoff losses were 26.0 and6.0 kg/hm² in rape season and 29.0 and 7.8 kg/hm² in rice season,respectively.Magnesium application of 45 kg/hm² increased the leaching and runoff losses by 53.8%and 55.0%in rape season and 44.8%and 96.2%in rice season,respectively.Within 60kg/hm² per crop season of Mg fertilizer rate,the apparent balance of Mg nutrient was negative,while Mg application reduced the soil Mg deficit and significantly increased the soil ex-Mg concentration compared to the treatment without Mg.（2）According to the soil ex-Mg and relative yield of<75%、75%-80%、80%-85%、85%-90%and>90%to establish the oilseed rape soil ex-Mg grading standards,extremely deficiency,deficiency,potential deficiency,optimum and abundant corresponding to soil ex-Mg concentration of 0-70,70-120,120-200,200-330 and>330mg/kg,respectively.More than 55%of the soils were deficient in Mg in the main winter oilseed rape producing areas in China,and the areas abundant in ex-Mg account for about 20%.In the case of unchanged rape planting area,Mg application can theoretically increase rape yield by 1.55 million tons.（3）Magnesium application was beneficial to promote vegetative growth and pod development in rapeseed,thus increasing rapeseed yield.Rapeseed yield increased with the increase of Mg fertilizer rate in a trend of first increase and then stable,and the maximum yield level was reached at 45 kg/hm² of Mg fertilizer rate,with an average yield increase of 9.4%.The effects of Mg fertilizer rate on yield,yield components and Mg uptake varied in different Mg classifications.In areas with extremely Mg deficiency（0-70 mg/kg）,the maximum yield of rapeseed could be increased by 34.3%with Mg application,with 45 kg/hm² of Mg fertilizer to reach the plateau yield level,and Mg application significantly increased the pods per plant and seeds per pod of rapeseed,and the Mg concentration and uptake in seeds by 14.1%and 64.4%,respectively.With the increase of soil ex-Mg,the yield increment decreased and pods per plant increased as the main factor of yield increase,the effect of Mg application in ex-Mg abundant areas（>330 mg/kg）to improve rapeseed and yield was not significant.Pod was the main contributing organ of seed biomass and Mg application could increase the contribution of pod by 38.2%.This was associated with larger leaf Mg concentration,biomass,and leaf assimilate transfer efficiency in the pre-reproductive period in Mg-sufficient environments,which enhanced pod number（6.3%-21.4%）.In addition,Mg application significantly increased the surface area and biomass of pod（by 22.3%and23.1%,respectively）and increased the soluble sugar concentration of pods and transferred it to the seeds,with the highest increase of 20.3%and 17.0%in soluble sugar and starch concentration in the seeds,respectively.（4）Magnesium application significantly improved the quality of rapeseed.Magnesium increased oil,oleic acid and linoleic acid concentration,oil and protein yield of rapeseed by 3.0%,9.1%,9.4%,15.8%and 11.7%,and decreased erucic acid,linolenic acid,stearic acid and palmitic acid concentration by 42.7%,4.5%,23.0%and14.8%,respectively.The effect of Mg application to improve the quality of rapeseed was closely related to the soil ex-Mg,and the oil,protein,oleic acid concentration,oil and protein yield increased gradually with the increase of ex-Mg.The magnitude of reduction of linoleic,linolenic,stearic and palmitic acid contents with Mg application increased with the increase of soil Mg.The erucic acid concentration showed an opposite trend,with a reduced rate of Mg application in the high Mg trials.Magnesium application affected the quality of rapeseed mainly by promoting the transfer of assimilates to the seeds,providing sufficient substrate for lipid synthesis and thus increasing the oil concentration.Moreover,Mg contributed to the conversion of saturated fatty acids to unsaturated fatty acids,and the percentage of saturated fatty acids decreased by 17.4%and the percentage of unsaturated fatty acids increased by9.7%with Mg application compared to the treatment without Mg application.（5）Recommendations for Mg management in winter oilseed rape in China were presented,integrating the recommended Mg fertilizer rate,Mg varieties and Mg application methods.In the area of soil ex-Mg less than 200 mg/kg,Mg fertilizer is recommended and should be applied into the soil through base fertilizer,if the target yield is higher than 3000 kg/hm²,30-45 kg/hm² Mg fertilizer should be applied to the base,if the target yield is less than 2000 kg/hm²,15-40 kg/hm² Mg fertilizer should be applied to the base.Besides,the Mg supplement should be sprayed when Mg deficiency symptoms occurred during the growth of oilseed rape.Magnesium fertilizer is not necessary if the target yield is less than 2000 kg/hm² in the area of soil ex-Mg more than 200 mg/kg,if the target yield is higher than 2000 kg/hm²,10-20 kg/hm² of base Mg fertilizer or spraying Mg fertilizer at the flowering stage as appropriate.Magnesium sulfate and magnesium chloride significantly increased the yield of rapeseed both by increasing the pod per plant and seed per pod of rapeseed,and the application of magnesium sulfate was slightly more effective than magnesium chloride,with yield increases of 14.1%and 11.8%,respectively.In this study,the main loss pathways and Mg balance characteristics of soil Mg in the rice-oilseed rape crop rotation system were determined.The soil ex-Mg abundance and deficiency index for oilseed rape was established to understand the distribution of soil Mg abundance and deficiency in the main production areas of winter oilseed rape in China,and the yield increase potential was evaluated.And the effect of Mg application on rapeseed yield and quality in the main winter rapeseed producing areas of China was clarified,and the mechanism of Mg application to improve rapeseed yield and quality was initially elucidated.Finally,combined with the recommended Mg fertilizer rate,Mg fertilizer species and application method,the Mg nutrient management of winter oilseed rape was presented.It provides theoretical basis and guidance for Mg fertilization to improve rapeseed yield and quality.