Effects Of Simulated Warming On Soil Inorganic Nitrogen In A Young And A Mature Cunninghamia Lanceolata Stand In Humid Subtropical Region

As an important feature of global climate change,global warming has varying degrees of impact on the cycle of soil elements in forest ecosystems.According to the forecast of Intergovernmental Panel on Climate Change(IPCC)and Climate Action Tracker(CAT)'s assessment of current climate policies in different countries,it is estimated that global temperature will rise by 2.3 to 4.1?by 2100.Warming will affect soil inorganic nitrogen content by affecting the nitrogen cycle of forest soil,thus affecting the productivity of forest ecosystem.At the same time,plants play an important role in regulating the nitrogen cycle of forest soils,but traditional experiments have mainly focused on laboratory cultivation to study the effect of increasing temperature on nitrogen cycle.Warming may affects the soils of a young and a mature forest in different degree.Taking the soils of a young and a mature Cunninghamia lanceolata stand as a research object,the responses of soil inorganic nitrogen content to stimulated warming were observed by means of soil cable warming which simulates a future climate change scenario of+4?warming.After a period of 3-year warming,by studying soil-plant-microbial system in the sample plot,the results are different from the conclusions of most warming experiments:Due to warming,total dissolved inorganic nitrogen(DIN)and ammonium nitrogen(NH₄⁺-N)contents in different soil layers of the young and mature stand was significantly reduced,while nitrate nitrogen(NO₃^?-N)content decreased or increased not significantly,the DIN and NH₄⁺-N contents of the young stand declined more sharply.According to the results,this research studies the organic nitrogen substrate input of inorganic nitrogen,the microbial mineralization process of organic nitrogen substrate,and the loss of inorganic nitrogen.The results are as follows:(1)Judging from the organic nitrogen substrate input of inorganic nitrogen,after warming,dissolved organic carbon(DOC)and dissolved organic nitrogen(DON)in different soil layers of a young and a mature Cunninghamia lanceolata stand decreased due to the reduction of soil organic matter decomposition.Besides,the new photosynthetic products exuding from root decreased,which led to the decrease of organic nitrogen substrate.By warming the mature stand,it was found that DIN and NH₄⁺-N contents were reduced,which proved that what mentioned above was caused by the decrease of organic nitrogen substrate input.(2)From the perspective of the microbial mineralization process of organic nitrogen substrate,in the in-situ mineralization experiment,warming caused water stress,which reduced the rate of nitrogen mineralization.Meanwhile,warming also resulted in the decrease of soil microorganisms and enzyme activity,which indirectly reduced the rate of soil mineralization,and that in turn reduced inorganic nitrogen content in the soil;Compared with the in-situ mineralization experiment,the indoor mineralized soil was cultivated under the condition of 60%saturation moisture capacity,which showed that warming promoted nitrogen mineralization.According to the study of soil microbial biomass,the main reason affecting the activity of microorganisms and enzymes was environmental and nutritional stress caused by warming,and in the young Cunninghamia lanceolata stand,soil microorganisms and enzyme activity were more sensitive to increasing temperature.(3)From the view of the loss of inorganic nitrogen,another two reasons that led to the decrease of soil inorganic nitrogen content were nitrogen leaching and loss of gaseous nitrogen.The sharp increase of?¹³C and?¹⁵N isotope abundance in the soil and those in the leaves of the soil of young stand proved the decrease of soil moisture content and the severe loss of inorganic nitrogen.Indoor and outdoor culture experiments also found that after warming,the emission rate of nitrous oxide(N₂O)in the soil showed an increasing trend compared with the control,and the young stand increased greatly with severe gaseous loss.Therefore,the 3-year warming changed the physical and chemical properties of soil in the young and mature stand.The moisture content and microbial organic nitrogen substrate were reduced.After warming,the number of microorganisms and enzyme activity in soil showed a tendency to reduce due to environmental stress,and meanwhile,the increase of rainfall leaching and gaseous loss were the other two reasons for the decrease of soil inorganic nitrogen content.The three aspects indicated that soils of the young stand were more sensitive to increasing temperature.