Accumulation Of Fe And Mn In Wheat And Its Relationship With Soil Nutrients Under Long-term Application Of Nitrogen And Phosphate Fertilizers

Iron（Fe）and manganese（Mn）are of great significance to human beings,and their concentration in wheat grainsis closely related to human health.In the arid region of northwest China,crop yield is low mainly due to a lack of soil nutrients.Nitrogen（N）and phosphate（P）fertilizer play important roles in increasing wheat yield in the arid region of northwest China,but their unreasonable application will also have a negative impact on wheat trace elements uptake.Most studies focused on the uptake and translocation of trace elements in cereal crops at different N and P application rates,while the change of different soil Fe and Mn forms after N and P application and their relationship with wheat grains Fe and Mn concentrations need to be further clarifiedformsforms,it is of great significance to carry out relevant research to regulate the nutritional level of trace elements in wheat and other crops.Therefore,based on the long-term N and P fertilizer application in the dryland,using Xiaoyan22 as the test material,the wheat grains,glumes and stems and top 0-20 cm soilwere collected for treatments,including no N and P application,N applied at 0,80,160,240 and 320 kg N ha^-1,and P at 0,50,100,150 and 200 kg P₂O₅ ha^-1 from 2019 to 2021.Relationship between the change of soil Fe and Mn forms and wheat grain yield,yield composition,and shoot Fe and Mn accumulation were studied,with the purpose to provide a scientific basis for rationally optimizing the N and P fertilizers application and improving the nutritional quality of wheat grains.The main results obtained are as follows:1.Compared to without N application,long-term N application significantly increased winter wheat yield,shoot Fe accumulation,and grain Fe concentration,but reduced shoot Mn accumulation and grain Mn concentration.Wheat yield and grain Fe concentration did not continue to increase when N application rate exceeded 160 kg N ha^-1,and remaining at5857～6598 kg ha^-1 and 40.2～42.2 mg kg^-1,respectively,while grain Mn concentration remained a lower level,at 30.4～35.3 mg kg^-1.N application significantly decreased soil p H and increased the proportion of loosely bound organic Fe and Mn（Wb O-Fe,Wb O-Mn）and Mn-oxide-bonded Fe（Ox Mn-Fe）in the soil,further enriching soil DTPA-Fe and DTPA-Mn content.The wheat grain Fe concentration had a significantly positive correlation with the yield,biomass,harvest index,spike number（×10⁴）,grain number per panicle,grain Fe accumulation and Fe harvest index,and soil Mn-oxide-bonded Fe（Ox Mn-Fe）,but there was a significantly negative correlation between the grain Mn concentration and soil residual Fe（Res-Fe）.However,the grain Mn concentration was significantly negatively correlated with grain yield,biomass,harvest index,spike number（×10⁴）,grain number per panicle,grain Fe accumulation,and Fe harvest index,and was significantly positively correlated with the Mn accumulation in wheat shoot and grains,but no significant association was found between various soil Mn forms.Therefore,the long-term N application in calcareous soils in the Northwest dryland can increase the bioavailability of soil Fe,promote wheat shoot Fe accumulation,thus increasing the Fe concentration in wheat grains.Although N application increases the soil Mn bioavailability,it significantly inhibits wheat Mn uptake,ultimately leading to a reduction of grain Mn concentration.2.Compared to without P application,long-term P application significantly increased the grain yield,shoot Fe and Mn accumulation,grain Fe and Mn concentration of winter wheat.When the P application rate was 100 kg P₂O₅ ha^-1,the yield was the highest,at 6598 kg ha^-1,and the grain Fe concentration was maximum,at 40.2 mg kg^-1,which increased by 76%and12%,respectively.The grain Mn concentration was 39.0 mg kg^-1.The application of phosphate fertilizer mainly affects the bioavailability of Fe and Mn in soil by improving the slight organic iron（Sb O-Fe）,exchangeable manganese（Ex-Mn）,loosely bound manganese（Wb O-Mn）,and slight organic manganese（Sb O-Mn）in the soil.Under the long-term P application,grain Mn concentration was positively correlated with yield,biomass,spike number（×10⁴）,Shoot Fe accumulation,grain Fe accumulation,and soil DTPA-Fe content,and the wheat grain Mn concentration was positively correlated with Mn accumulation in the shoot and grains,soil exchangeable manganese（Ex-Mn）and loose organic manganese（Wb O-Mn）.In calcareous soils in arid areas of northwest China,long-term P application can not only improve the soil Fe and Mn bioavailability but also promote their translocation from straws to grains.3.Nitrogen and phosphorus combined application（N160P100）can significantly increase wheat grain yield.Compared with monoscopic phosphorus（N0P100）and mono-nitrogen（N160P0）,the yield increased by 85%and 76%.N160P100 significantly reduced soil p H,increased the soil DTPA-Fe content,and promoted shoot Fe accumulation and its transportation to grains.Although the combined N160P100 increased the soil DTPA-Fe content,the Mn concentration in wheat grain decreased significantly.The grain Fe concentration was the largest under the treatment N160P100,which was 40.2 mg kg^-1.The grain Mn concentration was 39.0 mg kg^-1.The grain Mn concentration was 57.9 mg kg^-1,which exceeded the range of human manganese nutrient safety(44.0 mg kg^-1).This result was easy to cause excessive Mn intake and a series of health diseases.Based on the risk of Fe deficiency and excessive Mn intake in human bodies,it is recommended that the dryland in northwest China should not blindly apply phosphorus fertilizer or nitrogen fertilizer,and should pay attention to applicate them reasonably.In summary,long-term N application in calcareous soils in the Northwest dryland can increase the bioavailability of soil Fe,promote wheat shoot Fe uptake and translocate to the grains,thus increasing the grain Fe concentration;although N application increases the soil Mn bioavailability,it significantly inhibits the uptake of Mn by wheat,ultimately leading to a reduction in the wheat grain Mn concentration.The long-term P application,not only which increasing the bioavailability of Fe and Mn in the soil,but also promotes shoot Fe and Mn uptake and their translocation to grains altogether.Taking into account yield,grain Fe and Mn nutritional quality,and environmental and economic benefits,it is recommended to control the N application rate at 160 kg N ha^-1 and the P application at 100 kg P₂O₅ ha^-1 for wheat growth on calcareous soils in the north-western dryland,as excessive N and P application will be not beneficial to improve grains yield and quality.