Status Of Nitrogen Supply In Oolong Tea Producing Areas And Its Effect On Magnesium Nutrition In Tea Plants

Tea is one of the important economic crops in China and has an important strategic position for China’s economic development.The input of magnesium（Mg）fertilizer plays a key role in the production and quality of tea.However,nitrogen（N）supply will affect soil Mg availability and tea Mg absorption.In this study,a large sample of farmers survey was conducted to determine the status of nitrogen fertilizer application and soil Mg input.Meanwhile,a pot experiment was conducted to determine the effects of nitrogen supply on soil Mg leaching and tea Mg absorption.The results showed that:（1）According to 355 households survey from 2015 to 2017 in Anxi and Wuyishan,the average amount of nitrogen fertilizer in tea plantation is 509.4 kg·hm^-2 in Anxi County,the main nitrogen sources are compound fertilizer and urea,and the annual nitrogen surplus is 479.0 kg·hm^-2.The average amount of nitrogen fertilizer in tea gardens in Wuyishan City is218.1 kg·hm^-2,the main source is compound fertilizer,and the annual nitrogen surplus is 189.8 kg·hm^-2.There is a positive correlation between nitrogen nutrient surplus and nitrogen fertilizer input.The total N content of soil increased with N fertilizer application increased.The total N content of soil in Anxi Tea Garden is higher than that in Wuyishan Tea Garden.Based on the Boundary line method,the optimal utilization efficiency of nitrogen fertilizers in Anxi County and Wuyishan Tea Garden are 411.3 kg·hm^-2 and 157.7 kg·hm^-2,respectively.The reduced space can reach 19.3%and 27.7%,respectively.Based on optimizing nitrogen fertilizer input and management level,the reduction rates of greenhouse gases emission in Anxi County and Wuyishan Tea Garden are62.0%and 68.0%,respectively.（2）297 soil samples were collected from tea gardens in Anxi County.The results showed that the range of soil pH value is from 3.2 to 6.7,with an average of 4.3.More than 68%samples pH are less than 4.5,29%samples are less than 4.0.At the same time,the range of soil exchangeable Mg concentration was from 1.0 to 79.0 mg·kg^-1 with an average of 23.7 mg·kg^-1,and 56%of the samples are in a state of extreme deficiency(<20 mg·kg^-1),26%is in a deficient state(20-40mg·kg^-1),and only 18%of the samples are in the appropriate range(>40mg·kg^-1).Tea garden soil pH and exchangeable Mg content showed a very significant positive correlation,that is,the soil exchangeable Mg content decreased with soil pH decreased.（3）Pot experiment results showed that the amount of Mg leaching increased significantly with the increased of nitrogen fertilizer input.There was a significant positive correlation between the N（NH₄⁺-N and NO₃^--N）concentration and the leaching amount of Mg.The leaching of soil Mg is aggravated with the increase of nitrogen fertilizer application.Meanwhile,the soil pH showed a downward trend and the exchangeable Al increased obviously.A suitable nitrogen supply can promote the tea growth and roots Mg uptake.Excessive nitrogen input will inhibit roots Mg absorption.In summary,the application of nitrogen fertilizer in Anxi County and Wuyishan City is excessive,and the space can reach 19.3%and 27.7%respectively;the soil in Anxi County tea garden is seriously deficient in magnesium,78%is in a state of extreme deficiency and deficiency,and the soil exchangeable magnesium content and pH values showed a positive correlation;soil magnesium leaching is significantly correlated with the amount of nitrogen fertilizer input,and suitable nitrogen application rate could promote the absorption of magnesium by tea trees.