| Maize served as an important food source and industrial materials,which is widely planted around the world.Maize is used as silage or edible grain and processed into a variety of food and industrial products,including starch,sweeteners,oil,beverages,glue,industrial alcohol,and fuel ethanol.China is one of the three largest maize producers in the world.From 1995 to 2019,the annual average production area of maize in China was2.8×107 hectares,and the total yield was about 1.5×109 tons.As an essencial element of crop growth,nitrogen fertilizer determines grain yield.When the yield reaches 60%-70%of the yield potential,16.4 kg N is required for 1000 kg of grain.In China,the high yield of maize depends on high nitrogen input.Nitrogen fertilizer is uptaken by plant for growth following a large amount of nitrogen loss in the farmland.N loss through leaching was nonnegligible.Agricultural production not only enhances Cheses food security,but also brings environmental pollution.A clean and sustainable maize production system can ensure the yield of corn biomass and minimize environmental pollution which is crucial to the development of green agriculture.Urea served as a traditional N source that is easily hydrolyzed.With abundant precipitation,alpine landforms,unique soil types and steep slope cultivation,nitrogen leaching losses is serous in sloping farmland of Three Gorges Reservoir Area（TGRA）,which deteriorates the eutrophication and agricultural non-point source pollution.Slow-release fertilizer can effectively reduce nitrogen leaching loss and improve the absorption of nitrogen by crops and increse the nitrogen utilization rate.Traditional slow-release fertilizer is expensive with complicated manufacturing technique,increasing the cost.The storage time and environment will also influence its fertilizer efficiency.ME is the main component of ME formaldehyde,which is the raw material of decorative building materials,furniture,daily necessities,plastics,pigments,leather,and matrix-based materials.Disposing melamine contained products leads to environmental pollution.The WHO recommend the highest concentration of melamine in infant food and normal food was 1mg kg-1 and 2.5 mg kg-1 respectively or had the potential to cause renal and urinary metabolic disorders.In addition,the nitrogen content of ME is as high as 66.7%,which has the potential to be a N source.ME is stable in the soil and difficultly to be hydrolyzed can decrease the N leaching loss from the slop farmland in the TGRA.Microbial decomposition can promote nitrogen release from ME.Melamine methylated resin can promote the growth of radish（Raphanus sativus）,cucumber（Cucumis sativus）and lettuce（Lactuca sativa）by compost.potato（Solanum tuberosum L）,cabbage（Brassica pekinensis）and paddy（Oryza sativa L）can uptake the N from melamine for production.Whether the N release from melamine accorded with the crop N demand and ensuring the melamine residue was limited to the allowed concentration promoted by WHO plays a key role in melamine research.However,field systematic observation on nitrogen supply from ME and plant feedback to ME application are unknown,and therefore estimating melamine in the field is important.Based on the above,the N release rule of ME was highlighted and maize was chosen to learn the migration of N from ME in the plant-soil system by pot and filed experiments.The effects of ME alone application and ME/urea mixture on maize growth and NUE was also studied to find the optimum ratio of ME to urea.The influence of ME/urea mixtures on net returns of maize production was also concerned.The residual amount of ME in grains was also measured to conducting food safety evaluation.This study provided a new proposal for melamine used in agriculture by combining the N slow release from ME with the clean production of maize production systems.The main findings of the study are as follows:（1）We learned the release rules from ME in the field.The cumulative release rates of N from different N sources in all sampling times were in the following order:urea treatment（U）>Sulfur-coated（S-coated）urea treatment（SU）>ME treatment（M）.Compared with urea and S-coated urea,the N release rate from ME was slow in the field.Only 38%of the total nitrogen from ME was released into the soil after it been applied144 days later,however,the ratio was 72%for S-coated urea.Urea was hydrolyzed totally within 14 days after it was ploughed in the soil.According to the linear fitting plot of the cumulative N release rate versus time,The N release of ME was slower than urea and S-coated urea by 97%and 41%,respectively.N from ME would been kept in the soil after one year and therefore once appllication for one year.（2）The nitrogen leaching rule from ME and effect of ME to decrease the N loss in soil were studied by soil column leaching simulator.After 21 times irrigation,the total nitrogen loss from ME was negligible,accounting for only 7.12%of the total 1884 mg N rate in each column,while the ratio was 72%and 39%of urea and S-coated urea respectively.ME remained in the topsoil（0-20 cm）that rarely migrate to the deep soil.After 21 times irrigation,the total nitrogen concentration of the ME in topsoil（0-20 cm）was 1.28 g kg-1,being 48.4%and 44.2%higher than that of the urea and the S-coated urea at the same N rate,respectively.After two-year field experiment,it concluded that ME also improved the total nitrogen content in the soil.（3）The maize pot experiment studied the ability of ME convert into nitrogen available for maize growth in the soil-plant system,as well as its absorption rules by maize.The results showed that ME was mineralized into NO3--N and NH4+-N by soil microorganisms that was available for plants in the soil.The bases were as follows:the soil NO3--N concentration melamine treatments were higher than that of CK treatment.The amount of ME and soil NO3--N concentration showed a significant positive correlation with correlation coefficients are 0.98,0.92,0.98 in T1,T2,T3 period,respectively.Maize plants would effectively absorb N from ME which was supported by the following bases:the amount of ME was positively correlated with the total nitrogen concentration in maize roots,stems and leaves with correlation coefficients are 0.98,0.99,0.96,respectively.Lower melamine application would improve the plant growth while higher inhibited.Application of high amount（8MPK）of ME would inhibit the growth of maize.ME did not affect the micronutrient content of maize.Compared with ME directly used as a nitrogen source for crops,ME mixed with other nitrogen fertilizers to form a multi-component compound fertilizer or used as a fertilizer additive is more effective way for ME use.（4）We learned the effects of ME alone and ME and urea mixtures on maize production,growth,nitrogen uptake,nitrogen use efficiency,the concentration of various forms of nitrogen in the soil with prolonged ME application and the optimal ratio of ME to urea.The results showed that ME alone failed to meet the nitrogen demand of maize,as the release rate of nitrogen from ME in the soil was slow.Plant was short as the insufficient N supply at the early stage which would not be make up in the later.ME/urea mixture（1:1）achieved excellent performance in field experiments.The yield and the aboveground dry weight of ME and urea mixture（1:1）were comparable to those of urea and S-coated.At the same time,ME and urea mixture（1:1）obtained high nitrogen use efficiency（NUE）and high aboveground nitrogen uptake.It is found that ME was a buried fertilization,no top-dressing is required,which reduced the labor cost through income analysis.In 2019,the net income of the ME and urea mixture（1:1）was greater than that of S-coated urea and urea treatments.The net income of the ME and urea mixture（1:1）was the highest in 2020.（5）The range of ME residues in maize grains of different nitrogen treatments was0.24–1.8 mg kg-1,and the melamine intake of people according to the ME residue ranged from 2–23.9（μg/kg bw/d）within the recommendation promoted by WHO.According to WHO,the maximum content of ME in food and animal feed is limited to 2.5 mg kg-1,and the tolerable daily intake is 63000 ng/kg bw/day.ME applied in the field as an N source increased the concentration of ME in grains.The grain melamine concentration of M2U8,M5U5,M7U3 and M higher than CK by 61%,894%,33%and 167%,respectively.The melamine residue in grain will not pose a threat to the health humans through the evaluation of HQ,which would be helpful to achieve the clean production of maize and the ME agricultural use. |
