Effects Of Long-term Canopy Simulated Nitrogen Deposition On Soil Nitrogen Mineralization Rate In A Warm Temperate Forest

Since the industrial revolution,atmospheric nitrogen deposition has been increasing due to the intensification of human activities such as the application of agricultural fertilizers and the burning of fossil fuels,and China has become one of the three major nitrogen deposition hotspots in the world.Forest ecosystems are generally nitrogen-limited,and nitrogen deposition has become an important factor in regulating the effectiveness of nitrogen in forest ecosystems and has an important impact on the structure and function of forest ecosystems.Nitrogen mineralization is one of the key processes in the nitrogen cycle of forest ecosystems and usually plays a decisive role in forest productivity.Soil nitrogen mineralization is a process in which microorganisms in soil continuously transform organic nitrogen into inorganic nitrogen that can be directly absorbed and utilized by plants through decomposition,and is one of the key processes of nitrogen cycling in forest ecosystem.Nitrogen mineralization rate determines the availability of nitrogen in the soil,which in turn determines forest productivity.Therefore,it is of great significance to study the change rule and regulation mechanism of soil nitrogen mineralization in forest ecosystem under the background of nitrogen deposition for predicting future forest productivity and sustainable development of forest ecosystem.However,previous studies have sprayed nitrogen solutions directly on the forest floor,ignoring the uptake and retention of atmospheric nitrogen deposition by the forest canopy,which may not accurately reflect the effect of atmospheric nitrogen deposition on the rate of nitrogen mineralization in forest soils.Our study was conducted on the long-term field control platform of simulated nitrogen deposition in the forest canopy established by the Henan Dabieshan National Field Observation&Research Station of Forest Ecosystem,at the Jigong Mountain site in Xinyang.Since the implementation of treatment in April2013,the experimental platform has been continued for up to 9 years by simulating nitrogen deposition by different methods in the forest canopy and understory.Five treatments were conducted:Control（no addition of experimental N）,CAN25(canopy addition of N at 25 kg N ha^-1yr^-1),CAN50(canopy addition of N at 50 kg N ha^-1yr^-1),UAN25(understory addition of N at 25 kg N ha^-1yr^-1),UAN50(understory addition of N at 50 kg N ha^-1yr^-1).Four replicates were conducted for each treatment,with a total of 20experimental plots.By comparing soil N mineralization rates at different sampling times under different N addition methods（canopy and understory N addition）and different N addition levels（low and high concentration N solutions）,two main scientific questions were investigated:1)What are the differences in the effects of different N addition methods on forest soil N mineralization rates between canopy and understory,and 2)What are the effects of different N addition levels on forest soil N mineralization rates?What are the results of the study?The results of the study were as follows:（1）Compared with the Control,canopy nitrogen addition（CAN）and understory nitrogen addition（UAN）treatments both increased the annual average ammoniation,nitrification and mineralization rates of warm temperate forest soil.CAN increased by 185.71%,43.10%and 42.40%,respectively.UAN increased by 110.71%,30.57%and 32.82%,respectively.Contrast analysis of different N additions methods showed that the annual average rates of ammonification,nitrification and mineralization of soil N were higher under the CAN than the UAN,increasing by 35.59%,9.60%and 7.21%,respectively.（2）The ammoniation rate of soil increased with the increase of nitrogen addition level in CAN.CAN25 and CAN50 increased significantly by 114.29%and 257.14%compared with the average annual ammoniation rate under Control.In addition to significantly increasing the ammoniation rate of soil directly,CAN also promoted ammoniation by increasing the concentration of SOC.The response of soil nitrification rate to nitrogen addition level was different at different sampling times.In September 2021 and April 2022,the nitrification rate of CAN25 was higher than that of CAN50,and in December 2021 and July 2022,the nitrification rate of CAN50 was higher than that of CAN25.Compared with the annual mean nitrification rate of Control,CAN25 showed no significant difference,while CAN50 significantly increased by 60.57%.Soil nitrification rate was directly affected by CAN.The difference of mineralization rate was consistent with nitrification rate among different treatments of CAN.（3）Under the UAN treatment,the effects of nitrogen addition level on ammoniation rate,nitrification rate and mineralization rate were different under different sampling time.In July 2022,the ammoniation rate of UAN50 was significantly higher than that of Control,but there was no significant difference between UAN25 and Control.At other sampling times,there was no significant difference between the ammoniation rate of UAN and Control.Compared with the average annual ammonification rate of Control,UAN25 showed no significant difference,while UAN50 significantly increased by121.43%.Nitrogen addition directly increased soil ammoniation rate.In December 2021 and July 2022,the nitrification rate of UAN25 and UAN50 was significantly higher than that of Control,while at September2021 and April 2022,there was no significant difference between UAN25 and UAN50 and Control.Compared with the average annual nitrification rate of Control,there was no significant difference in UAN25,while CAN50 significantly increased by 47.13%.UAN indirectly affects nitrification rate mainly by changing soil p H value.The difference of mineralization rate among different treatments of understory nitrogen addition was consistent with the nitrification rate.（4）The ammoniation rate,nitrification rate and mineralization rate of warm temperate forest soil in Jigong Mountain were significantly different at different sampling times,showing a time dynamic change trend of high in July and September,and low in April and December.In summary,both CAN and UAN promoted the rates of ammonification,nitrification and mineralization of soil nitrogen.However,different N addition methods（CAN and UAN）had different effects on N transformation rates,and soil N transformation rates were higher under CAN than UAN.CAN directly and significantly increased the rates of ammonification and nitrification.In the UAN treatment,N addition directly and significantly increased the ammonification rate and indirectly influenced the nitrification rate by adjusting soil p H.Mineralization rate was also mainly influenced by nitrification rate.