Physiological Mechanism Of Humic Acid Regulates Apple Growth And Nitrogen Absorption And Utilization

The phenomenon of excessive application of nitrogen fertilizer in apple production is common.Under the current situation of low soil organic matter content in apple orchard,nitrogen fertilizer applied to the soil is easy to be lost through a variety of ways,which not only leads to the waste of resources,but also caused environmental problems that cannot be ignored.Therefore,it is of great significance to reduce nitrogen loss and improve nitrogen utilization efficiency for sustainable development of apple industry.In this study,apple dwarf stock M9T337 seedlings,four-year-old and eight-year-old�Yanfu 3'/M26/Malus pumila and fifteen-year-old�Gala'/Malus pumila,as well as ¹⁵N isotope tracer technology were used to explore the effects of different humic acid dosage on the plant growth and nitrogen absorption and assimilation characteristics of apple,the effects of humic acid on physiological and nitrogen utilization characteristics of apple to different nitrogen forms,and the effects of split application of nitrogen fertilizer and humic acid on the efficient nitrogen utilization of apple,and the effects of humic acid on nitrogen reduction and efficiency enhancement in apple production.The main results were as follows:1.Application of humic acid significantly increased the root tip number,total root length and root surface area,and significantly increased root activity.With the increase of humic acid dosage,the root biomass and activity were increased firstly and then decreased,and the maximum values were H3 treatment.The relative expression of ANR1 gene,which is a specific transcription factor for lateral root development,in humic acid treatments were up-regulated in different degrees.Humic acid application significantly increased the total N content and the activities of key N assimilation enzymes(NR,NiR,GS and GOGAT)in leaves and roots of M9T337 seedlings to different degrees,promoted the assimilation of nitrogen in plant.The relative expression of nitrate transporter gene in M9T337 seedling roots showed that the relative expression levels of NRT1.1?NRT1.2?NRT2.1?NRT2.2 and NRT2.4in humic acid treatments were up-regulated in different degrees,which promoted the absorption and transport of nitrogen in roots.2.The biomass,root architecture parameters(total root length,total root surface area,and root tip number)and root activity in HA treatments were improved significantly,which showed the trend of increasing firstly and then decreasing with the increase of humic acid dosage,and the highest value occurred in HA3 treatment(6 g�kg^-1).The shoot biomass,chlorophyll content,and photosynthetic rate(Pn)were significantly increased with the increase of HA dosage.HA significantly increased the nitrogen modulation capacity of each organ,which was conducive to promote the absorption and utilization of fertilizer N by the plant.The ¹⁵N utilization rate of different treatments displayed with an order of HA3>HA4>HA2>HA1>CK.Compared with CK,the ¹⁵N utilization rate increased by 7.99,6.26,5.11,and 2.08 percentage points,respectively.The soil ¹⁵N residual rate increased with the increase of humic acid dosage,and the ¹⁵N residual rate of humic acid treatment increased by 4.37,10.69,14.95 and 19.34 percentage points,respectively.The ¹⁵N loss rate decreased significantly with the increase of humic acid dosage.3.The growth of M9T337 seedlings had different responses to different nitrogen forms.CO(CH₂)₂-N treatment had the highest shoot growth and Pn of apple seedlings,while NO₃^--N treatment had the highest morphological development and root activity,and ammonium sulfate had the lowest index of both above ground and root.Compared with nitrogen application alone,Pn and root activity of CO(CH₂)₂-N,NO₃^--N and NH₄⁺-N treatments combined with humic acid were increased by 33.75%,33.88%,36.92%and 22.31%,20.94%,27.08,respectively,and other indexes were increased to varying degrees.The application of humic acid significantly promoted the absorption and regulation ability of M9T337 seedlings root,stem and leaf to the three forms of nitrogen.The Ndff value of the shoot was CO(CH₂)₂-N>NO₃^--N>NH₄⁺-N,while the Ndff value of the root was NO₃^--N>CO(CH₂)₂-N>NH₄⁺-N.The combined application of humic acid significantly increased the total nitrogen content of each treatment,increased the utilization rate of NO₃^--N,CO(CH₂)₂-N and NH₄⁺-N by 9.22,4.89 and 7.77 percentage points,and reduced the ¹⁵N loss rate by 23.44,23.22 and 18.25 percentage points,respectively.4.Split application of nitrogen fertilizer significantly increased leaf area,chlorophyll content(SPAD)and net photosynthetic rate(Pn)of�Fuji'apple,increased leaf protective enzyme activity and delayed leaf senescence at fruit ripening stage.The Ndff value of each organ under N split application treatments were significantly higher than that of N application at one time and N application at two times,which was conducive to the absorption and utilization of N.The ¹⁵N utilization rate of N split application treatments was 33.6%,which was significantly higher than that at one time and two times.Split application of nitrogen significantly increased the single fruit weight and improved the fruit quality,and the soluble solids,soluble sugar and sugar-acid ratio were significantly higher than the other treatments.5.Compared to CK treatment,the single fruit weight of the three humic acid treatments were significantly increased by 4.1%,8.8%and 13.6%,and the fruit yield per tree were increased by 5.4%,11.9%,and 17.8%,respectively.The fruit quality was also significantly improved.The effect of humic acid application at three times(HA-3)was better than humic acid application at two times(HA-2)and humic acid application at one time(HA-1).These three humic acid treatments improved the Ndff values,and the Ndff values in all the organs showed the same order as HA-3>HA-2>HA-1>CK.Compared with CK,the ¹⁵N utilization rates of three humic acid treatments were increased by 5.08%?13.34%,while the ¹⁵N loss rates were decreased by 10.27%?20.17%,and the HA-3 treatment showed the best effect.The ¹⁵N residual amount in the 0?60 cm soil layers of the three humic acid treatments were significantly higher than CK,while those in 60?120 cm soil layers showed opposite.6.Compared with no application of organic fertilizer,the combined application of organic-inorganic fertilizers significantly promoted the growth of apple,and the effect of combined application of organic-inorganic fertilizer was better than that of one-time application of organic-inorganic fertilizer.The combined application of organic-inorganic fertilizer increased the ability of nitrogen absorption and regulation(Ndff value)of each organ,which was beneficial to the absorption of fertilizer nitrogen by trees.The total nitrogen content of plant and the ¹⁵N utilization rate were significantly higher than those of one-time combined application of organic-inorganic fertilizer and no application of organic fertilizer.7.Compared with recommended amount of nitrogen fertilizer(N₁₀₀),25%nitrogen reduction(N₇₅)slowed down the growth of M9T337 seedlings,while 25%nitrogen reduction combined with humic acid(N₇₅+HA)significantly promoted the growth of M9T337 seedlings,but the difference between the two treatments was not significant.N₇₅+HA significantly increased the leaf area,chlorophyll content,Pn and the activities of key nitrogen assimilation enzymes(NR,NIR,GS and GOGAT)in M9T337 seedlings to different degrees.Ndff value of M9T337 seedling organs in N₇₅+HA treatment was significantly higher than that in N₇₅treatment,but still lower than that in N₁₀₀ treatment.Compared with N₇₅ treatment,the total nitrogen content,¹⁵N uptake and ¹⁵N utilization in N₇₅+HA treatment were increased by68.46%,102.53%and 102.62%,respectively,and the differences were significant.Compared with N₁₀₀ treatment,it was improved,but the difference was not significant.Compared with N₇₅ and N₁₀₀ treatments,N₇₅+HA treatments significantly increased the soil ¹⁵N residual rate and decreased the ¹⁵N loss rate,the differences reached significant levels.8.25%reduction of nitrogen fertilizer combined with humic acid(N₂₂₅+HA)significantly increased leaf area,chlorophyll content(SPAD),leaf nitrogen content,net photosynthetic rate(Pn),fruit weight,and yield per plant.Compared with recommended amount of nitrogen fertilizer(N₃₀₀)and nitrogen fertilizer reduction of 25%(N₂₂₅),the differences reached a significant level.N₂₂₅+HA significantly increased the content of soluble sugar and the ratio of sugar to acid,and improved the flavor of fruit.N₂₂₅+HA significantly increased the absorption and utilization of fertilizer nitrogen,and the utilization rate of ¹⁵N was 7.18 and 3.41 percents higher than that of N₂₂₅ and N₃₀₀,respectively.N₂₂₅+HA significantly increased the residue of fertilizer nitrogen in soil,especially in 0?60cm soil layer,and reduced the loss of fertilizer nitrogen.