| Magnesium(Mg)is an activator of more than 300 enzymesis,and also the second most abundant cation in living plant cells,which normally requires 1.5-3.5 g kg-1 dry matter of Mg for optimal plant growth.Green plants are autotrophs,which are capable of converting light energy into chemical energy through photosynthesis.One irreplaceable role of Mg in plant cells is that it serves as the central atom of chlorophyll.Normally,one-fifth of the total Mg in plants is bound in chloroplasts,thus that Mg plays an essential role in maintaining the structure of chloroplasts and the function of green leaf cells,and Particular metabolic processes and reactions related to plant growth and biomass accumulation that are influenced by Mg include: chlorophyll formation,photophosphorylation(like ATP formation in chloroplasts),photosynthetic carbon dioxide(CO2)fixation,protein synthesis,phloem loading,partitioning and tilization of photoassimilates,generation of reactive oxygen species,photooxidation in leaf tissues,nitrogen uptake and metabolism,and et al.In addition,the quality of agricultural and horticultural products and its modulation by Mg has increasingly received attention.However,Mg deficiency,which leads to the agricultural products with a significant degradation of Mg concentration in the integrated soil-crop systems,is becoming a serious and worldwide nutritional problem and threat to human health.So,a main cause of Mg deficiency is the low intake of Mg in the daily diet.Considerable soil Mg leaching losses,the “dilution effect” by crop yield improvement and source replenish overlook in Mg fertilizer are three control factors for the continuous decline of Mg nutrition in agricultural products.In China,more than 50% of farmland areas have soils Mg deficiency or potential deficiency with low plant availability of Mg,especially in acid soils in southern China,which resulted in not only a limited crop yield,but also an increased nutritional health risk of Mg deficiency in human body for those who consume mainly plant-based foods.Application of Mg fertilizer represents a rapid,short-term and effective agronomic biofortification approach to close crop yield in Mg deficiency soils and improve nutritional quality of crop and human Mg nutrition.Few reseaches have shown that Mg fertilizer application significantly increases the productivity in corp,but under fieldgrown production condition,detailed information on how Mg application affects the formation of crop yield and nutritional quality,human health impacts,cropping system Mg budget are laching.Therefore,based on the typical open-field pepper(Capsicum annuum L.)-Chinese cabbage(Brassica rape L.ssp.pekinensis)rotation system in yellow soil of southwest China,a farmer survey with soil investigation and two longterm field experiment are conducted to(1)identify the regional vegetable production level and the nutrient status of vegetable fields;(2)determine the effects of Mg fertilization on yield,nutritional quality,Mg leaching loss and human health associated with vegetable consumption in pepper and Chinese cabbage production,respectively,as well as Mg budget in cropping rotation system;(3)assess the effects of application method of Mg fertilizer(such as soil application and foliar application)on the pepper production,soil Mg formationation;furthermore,(4)discuss the control potential of compounded Mg application method on soil Mg leaching.The main conclusions of the present study are as follows:(1)Within the context of sustainable development,soil degradation driven by land-use change is considered a serious global problem but conversion from growing cereals to vegetables is a change that has received little attention,especially in subtropical regions.Soil quality is critical for food security and climate.Therefore,we compared the nutrient status and soil quality parameters(soil organic carbon(SOC),total nitrogen(TN),C/N ratio,p H,phosphorus(P),potassium(K),calcium(Ca),and Mg)between vegetable fields(VF)and land still used for paddy rice-oilseed rape rotation(PRF)that are typical of southwest China.In the VF,fertilizer application were often several times higher than the crop needs or recommended by the local extension service,thus,the crop use efficiency of N,P,K,Ca,Mg were only 26%,8%,56%,23% and 28%,respectively;SOC,C stock,TN,N stock decreased significantly caused by low organic inputs from crop residues and high tillage frequency,and soil C/N ratio decreased slightly;available P(AP)in topsoil increased by 1.92 mg kg-1 for every 100 kg ha-1 of surplus P,and the critical levels of AP and Ca Cl2-soluble P for P leaching were 104 and 0.80 mg P kg-1.Besides,there was a clear trend of soil acidification in the VF.However,increasing concentrations of soil Ca and Mg significantly alleviated topsoil acidification,with the effect increasing over time.Given our findings,we discuss the potential benefits of conservation agricultural practices,integrated soil-crop system management strategies and agricultural technology services for recovering the degraded soil and improving the vegetable productivity in the VF.In conclusion,the current study indicated that there is a significant soil degradation in the VF cropping system in this subtropical region due to excessive fertilizer application and land-use change.Practices such as increasing input of carbon-rich organic amendments,optimizing supply strategies of N and reducing application rate of P have potential to restore soil health and productivity.(2)A two-year field experiment was conducted to investigate the effects of varying Mg fertilization rates on pepper leaf chlorophyll and net photosynthetic rate,formation of fruit yield and plant biomass.The results indicated that,with the increase of the application rate of Mg fertilizer(0-67.5 kg ha-1),the responses of pepper yield to Mg application were consistent with a linear-plateau model,i.e.,yield increased as the Mg application rate increased to 67.5 kg ha-1.Compared to control,Mg fertilization significantly increased the 2-yr average pepper yield and net economic income by 25.6% and 40.1%,respectively.In views of yield composition,the increase of yield depended on the increase of fruit number per plant and fruit weight.However,in views of plant biomass,because no obvious effect was observed in harvest index(HI)of pepper as affected by Mg fertilization,and the contribution of yield increase can be completely ascribed to the increase of plant biomass.Also,it was found that the average contribution rate of plant biomass accumulation before and after flowering and fruit-setting stage were 9.85%-28.4% and 71.6%-90.1%,respectively.What's more,the plant nutritional mechanism of pepper yield formation is as follows: at the critical growth stage(i.e.,flowering and fruit-setting stage),Mg application increased the shoot Mg concentration,chlorophyll concentration and net photosynthetic rate in leaf,then the positive bio-chemical reactions and formation feed-back among the above three parameters collectively promoted the accumulation of plant biomass and the allocation of carbohydrates to fruit.However,it is currently unable to establish the critical shoot and soil Mg concentration to achieve high biomass accumulation,because the relationship between the plant shoot/soil Mg concentration and yield are characterized by significant linear positive correlation.These results further indicated that there is still a considerable gap for improving the pepper yield to plateau value,and the deficiency of soil exchangeable Mg and the imbalance of soil K/Mg ratio are two main limiting factors.(3)Magnesium fertilization is a promising practice to improve vegetable yield.However,its impacts on vegetable quality and human health have not been revealed.A field experiment was conducted to investigate the effects of varying Mg fertilization rates on quality of pepper fruit.Further,result of the field experiment was linked to pepper consumption data from the China Health and Nutrition Survey(CHNS)in the “disability-adjusted life years”(DALYs)framework to evaluate the potential health impact of Mg fertilization for the first time.Compared to control,Mg application increased concentrations of Mg and capsaicinoids,decreased those of Ca,zinc(Zn)and vitamin C(Vc),and had no effect on K and iron(Fe)in pepper fruit.As a result,Mg fertilization decreased the comprehensive nutrition level of pepper by 16.6%.Furthermore,the current health burden of Chinese adult population associated with pepper consumption is estimated at 21.3 million DALYs per year,with the risk being increased by 5.40 DALYs for per megagram of Mg fertilizer application.Increasing health risk was mainly attributed to decreasing concentrations of Ca and Vc in pepper fruit,though the increased Mg intakes offset the impact of 1.74% to 14.4%.In conclusion,Mg fertilization significantly reduced nutritional quality of pepper fruit,and increased human health risks associated with consumption of pepper fruit.Therefore,to ensure food safety and human health,combined application strategy of Mg fertilizer and other microelement fertilizers(especially Ca,Fe and Zn)should be vigorously promoted to the current intensive production agriculture.(4)Magnesium is one of essential plant nutrients needed for optimal growth,yield and quality formation.Also,soil application of Mg fertilizer has been shown to be an effective approach to improve vegetable Mg nutrition.Leafy vegetables can accumulate relatively high levels of heavy metals in the aboveground plant parts.However,it remains unclear as to whether soil-applied Mg affects the nutritional quality and human health risk of heavy metals from field-grown Chinese cabbage.Here we conducted a two-year,two-crop cycle field experiment in southwest China to evaluate crop yield,vegetable nutrition and heavy metal accumulation in Chinese cabbage supplied with varying levels of Mg(0-90 kg ha-1).Soil application of Mg did not increase the cabbage yield.However,it did increase the vegetable vitamin C and water-soluble protein content by 20.0% and 57.9% with 45 and 22.5 kg Mg ha-1 application,respectively,compared to control.The nitrate content of Mgsupplied(45 kg ha-1)cabbages was significantly lower,by about 14%,than the control.Further,it also significantly decreased the accumulation of cadmium and nickel in the above-ground tissues by reducing their uptake from soil to root or their translocation from root to shoot.Magnesium application,however,increased chromium uptake.A human health risks assessment nonetheless showed that the contribution of chromium to target hazard quotient and target carcinogenic risk from Mg-supplied plants were indeed much lower than that of cadmium and nickel,proving the value of crop Mg supplementation for ameliorating non-carcinogenic and carcinogenic risks to humans with the consumption of Chinese cabbage.Here we show that soil application Mg in the range of 22.5-45 kg ha-1 to Chinese cabbage will significantly improve its nutritional qualities and alleviates the potential human health risks of heavy metals with Chinese cabbage consumption.(5)Considerable amount of soil Mg leaching loss is one of the most factors that lead to Mg deficiency in the current agro-ecosystems.Over the world,the average Mg leaching loss in cropland and orchard cropping systems were 44.6 kg ha-1 season-1 and 103 kg ha-1 yr-1,respectively.Even so,the vast majority of farmers in the actual agricultural production did not pay attention to the application of Mg fertilizer,and absolute deficiency of soil Mg can be got aggravation with a consequence of those long-term unbalanced crop fertilization practice neglecting Mg depletion of soils through crop Mg removal.In this field experiment,the average Mg leaching in the open-field pepper-Chinese cabbage rotation system was 33.9-74.2 kg ha-1 yr-1,which was similar to the accumulation of plant Mg,and linearly increased with the increase of Mg application.The intensity of Mg leaching was synchronized with the concentrated rainfall over the year,so that the range of contribution rate of annual Mg leaching in the pepper cropping season was 65.4-74.4%,while the remaining contribution value was evenly divided between Chinese cabbage cropping season and fallow period.Fortunately,these Mg leaching leachates under the current experimental cropping system,did not lead to groundwater pollution.In addition to water input,soil texture,application rate of Mg fertilizer and plant Mg accumulation are all important control factors affecting Mg leaching.Besides,the optimal application rate of Mg fertilizer in pepper and Chinese cabbage seasons were 62.1 and 21.8 kg ha-1,respectively,on basis of the fertilization management strategy for optimizing crop yield and maintaining zero Mg budget over the cropping system.Theoretically,under the field-grown vegetable production in southern China,synergistically improving the type and rate of Mg fertilizer(controlled-release/slow-release)and application methods(soil application + foliar spray),as well as modifying soil properties(SOC/p H)in vegetable fields and agronomic practices,which are effective potential approachs to further optimize crop yield,and reduce Mg input and leaching loss,but are also urgent needs to realize the sustainable development of green agriculture in the whole region.(6)Both soil application(45 kg Mg ha-1 and foliar application with 0.5% concentration(mass concentration,wMgSO4 7 H2O)/vwater)of Mg fertilizer significantly increased pepper yield and Mg concentration in commercial fruit,and these increase effects were consistent;while the concentration of Ca,Vc,nitrate and water-soluble protein were significantly decreased due to the ‘dilution effect' caused by pepper yield increase and the antagonism of Mg.In terms of comprehensive quality impacts calculated via standardized radar area ratio approach,the order of above two Mg treatments was as follows: 0.5% foliar application ? soil application.Moreover,compared with soil application,0.5% foliar application practice improved the transfer efficiency of Mg nutrition from plant shoot to fruit and commercial fruit rate of pepper.In addition,compared with the control,soil application significantly improved the exchangeable Mg concentration in surface soil(0-20 cm)among the harvest stage,while that of change corresponded a depletion trend in the foliar application treatment,and the loss rate of soil exchangeable Mg in 0.5% foliar treatment was 4 mg kg-1 season-1.Under the premise of maintaining the base soil exchangeable Mg concentration,there at least an additional 10.8 kg ha-1 of Mg fertilizer should be inputed to the current pepper production by soil application approach.Accordingly,compared with soil application,this new combined Mg application strategy,which reduced the application of Mg fertilizer by 68.4%,may obtain the same pepper quality and yield,or even higher yield,while reduce Mg leaching by 33.7%.Therefore,current agricultural management practices need to comprehensively consider the synergism all of crop nutritional requirment,quality and yield,soil productivity,and environmental costs.In short,it is necessary to deepen understanding of the application rate and method of Mg fertilizer in the agriculture production on basis of multi-objective optimization. |
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