Effects Of Nitrogen Deposition On Soil Carbon And Nitrogen Pools And Soil Respiration In An Urban-Rural Gradient Forest

Soil carbon and nitrogen cycle process is the core of energy exchange and material cycling in forest ecosystem,and soil carbon and nitrogen storage and cycling are coupled with each other.Soil respiration,in turn,is the second most important carbon flux in terrestrial ecosystems after plant photosynthesis,and is a key process regulating the ecosystem carbon cycle.In the last 150 years,with the rapid development of industrialisation,the burning of fossil fuels and the rapid increase in emissions of nitrogenous compounds due to agricultural and livestock production,this nitrogen has landed in forest ecosystems in the form of dry deposition and wet deposition,with far-reaching effects on forest soil carbon and nitrogen dynamics,making nitrogen deposition another important factor affecting forest soil carbon pool and nitrogen pool in recent times.In recent years,due to rapid urbanisation,nitrogen deposition in urban forests has been much higher than in natural forests,and higher nitrogen deposition may lead to imbalances in soil nutrient stoichiometry and’nitrogen limitation’in most urban soils,so urban forest soils may be more sensitive to increased nitrogen deposition.To date,research and understanding of the effects of nitrogen deposition on soil carbon pool and nitrogen pool in urban-rural gradient forests at home and abroad are still very limited,and there is an urgent need to strengthen research on the effects of nitrogen deposition on soil carbon pooland nitrogen pool and carbon cycle in urban and rural forest ecosystems during urbanization,which is of great theoretical research significance to improve the theory of carbon cycling in forest ecosystems.Therefore,in this study,we propose to use the established typical urban-rural gradient forest sample plots as a research platform to measure the changes of soil carbon,nitrogen content（Total soil carbon,total nitrogen and its active fraction）and carbon output（Soil respiration rate and heterotrophic respiration rate）after nitrogen deposition through simulated nitrogen deposition(Control-CK:0 kg·N·ha^-1·yr^-1;Low nitrogen added-LN:50kg·N·ha^-1·yr^-1;High nitrogen added-HN:100 kg-N·ha^-1·yr^-1)experiments,and to explore how the sustained four-year nitrogen addition affects the carbon pool and nitrogen pool and soil respiration of urban-rural gradient forest ecosystems.The main findings are as follows:（1）In the fourth year of nitrogen addition,soil organic carbon and its active carbon fraction（dissolved organic carbon（DOC）;microbial biomass carbon（MBC））showed a clear urban-rural gradient pattern and a downward trend as the soil layer deepens.Soil carbon storage in forests along the urban-rural gradient showed that urban forest>suburban forest>rural forest.Nitrogen addition significantly increased the content of organic carbon and its active fraction in rural forest.（2）In the fourth year of nitrogen addition,the total nitrogen and organic carbon content of forest soils on the urban-rural gradient followed similar trends and increased with time of nitrogen addition,while their active nitrogen fraction（dissolved organic nitrogen（DON）;microbial biomass nitrogen（MBN））and C/N decreased with soil deepened.Soil nitrogen storage and active nitrogen fractions in forests on an urban-rural gradient showed that urban forest>suburban forest>rural forest.Nitrogen addition increased the content of total nitrogen and its active nitrogen fraction in rural forest soil.（3）The effect of nitrogen addition on soil respiration and its components on forests on urban and rural gradients differed in the fourth year of nitrogen addition.Nitrogen addition inhibited total soil respiration respiration and heterotrophic respiration in urban and suburban forests;it significantly promoted heterotrophic respiration in rural forest.（4）The organic carbon and total nitrogen content of forest soils on the urban-rural gradient increased after four years of continuous nitrogen addition compared to the initial nitrogen addition,indicated that nitrogen addition facilitated the accumulation of carbon and nitrogen storage.Continuous nitrogen addition significantly increased DOC content and decreased soil MBC and MBN content in rural forest soil.The study also showed that continuous nitrogen addition for four years reduced soil p H,which in turn inhibited total forest soil respiration on the urban-rural gradient.Nitrogen deposition inhibited heterotrophic respiration in urban and suburban forests,and inhibited autotrophic respiration in rural forest.In summary,the results of this study showed that forest soil carbon,nitrogen and their active fractions show significant urban-rural gradients,and that nitrogen deposition has a significant impact on forest soil carbon and nitrogen pools and soil respiration,which in turn affects nutrient cycling in forest ecosystems.This study provided a scientific basis for elucidating the process of carbon and nitrogen nutrient cycling in forest soils along the urban-rural gradient in the context of increasing nitrogen deposition,and provided theoretical support for ecological risk assessment of urbanization processes in the context of increasing nitrogen deposition.