Effects Of Simulated Nitrogen Deposition On Growth Physiological Characteristics And Soil Characteristics Of Melaleuca Alternifolia Seedlings

The study aimed to investigate the growth and physiological response of Melaleuca alternifolia seedlings to simulated nitrogen deposition,in the context of increased global nitrogen deposition.Annual Melaleuca alternifolia seedlings were planted in pots at an on-campus nursery from April to September 2022 for the experiment,with urea used as the nitrogen source donor.Four nitrogen addition treatments were set with different concentration gradients:control（CK）,low nitrogen（LN）,medium nitrogen（MN）,and high nitrogen（HN）,representing nitrogen deposition levels of 0,60,120,and 240(kg·hm^-2·a^-1),respectively.By comparing growth,physiological,oil content,and soil characteristics of seedlings at different levels of nitrogen addition,this study aimed to predict the threshold of nitrogen deposition that Melaleuca alternifolia seedlings can tolerate.The results provide insights into the physiological and ecological adaptations of Melaleuca alternifolia to nitrogen deposition stress,as well as a theoretical basis for breeding and cultivation of the species in nitrogen deposition stress areas.The study results show that:（1）Plant height,ground diameter,and biomass of seedlings in each treatment group increased with the increase of nitrogen application time,with the MN group achieving the maximum values at the end of nitrogen application,showing significant increases of 60.4%,11.9%,and 186.6%,respectively,compared to the CK group.Leaf weight ratio of seedlings in each N addition group was significantly higher than that of the control group,while the root weight ratio and root crown ratio were lower than that of the control group,indicating that N application prompted seedlings to allocate more biomass to leaves and caused inhibition of the growth of underground parts.The seedling quality index peaked at medium concentration of N addition.（2）Chlorophyll content increased significantly after 90 days of nitrogen application treatment.Total chlorophyll and chlorophyll a content peaked in the MN group,while chlorophyll b and carotenoid content achieved maximum values in the HN group.Apparent quantum efficiency,maximum net photosynthetic rate,average daily net photosynthetic rate,and average daily stomatal conductance were significantly higher than the control at different levels of N addition,peaking at the medium N treatment.The light saturation point and light compensation point,as well as the dark respiration rate of each N addition group,showed a trend of MN>HN>LN>CK,indicating that excessive N weakened the photosynthetic ability of Melaleuca alternifolia and reduced its adaptation to light.（3）Soil total nitrogen and ammonium nitrogen contents of Melaleuca alternifolia seedlings showed a trend of first increasing and then decreasing with the increase of nitrogen concentration,with the highest value under the medium nitrogen treatment,while soil nitrate nitrogen content reached its peak in the high nitrogen group.Among mineral nutrients,the contents of aluminum and manganese decreased with increasing N application concentration,while the contents of sodium and phosphorus increased and reached their peak values in the high N treatment.Potassium,calcium,and magnesium contents reached their maximum value in the low N group.（4）The oil content of Melaleuca alternifolia seedlings showed a trend of first increasing and then decreasing with the increase of nitrogen concentration,with the maximum value achieved under the low nitrogen treatment.The conclusion of the study showed that different concentrations of N addition promoted the growth of height,ground diameter and total biomass of Melaleuca alternifolia,especially the accumulation of leaf biomass,and the best growth condition of Melaleuca alternifolia seedlings was achieved under medium N addition treatment.The chlorophyll content was highest in the medium N treatment,which led to the highest photosynthetic capacity in the medium N treatment,as evidenced by the peak net photosynthetic rate in the medium N treatment,and decreased in the high N treatment compared to the medium N treatment,so the high level of N addition reduced the photosynthetic capacity of Melaleuca alternifolia seedlings.The malondialdehyde content was greatest in the high N treatment,indicating that high levels of N addition caused nutrient imbalance and increased membrane lipid peroxidation in plants,which disturbed plant metabolic processes and thus affected plant growth and development.It indicates that N addition within a certain threshold can promote the growth and development of Melaleuca alternifolia seedlings,but excessive N deposition will inhibit the growth and development of Melaleuca alternifolia seedlings and even reduce the oil content of Melaleuca alternifolia.