The Effect Of Nitrogen And Phosphorus Addition On The Plant Stoichiometric Ratio And Soil Carbon And Nitrogen Processes In The Understory Of Chinese Fir Plantation

Nitrogen（N）and phosphorus（P）are for two elements that limit the productivity of forest ecosystems.In order to obtain more wood production,N and P are commonly used in plantation management.With the global change,N deposition intensification in south China has changed the limiting factors of forest productivity,especially among Chinese fir（Cunninghamia lanceolata（Lamb.）Hook.）plantations in the red soil region of southern China where P is scarce.At present,people pay greater attention to the plantation which can produce economic benefits,but less attention to the understory.Understory play an irreplaceable role in the forest ecosystem,not only helping to maintain biodiversity,but also conserving water and preventing forest soil erosion.Ecological stoichiometry can better help us understand the relationship between understory and soil material circulation under global changes.In addition,greenhouse gases（including nitrous oxide（N₂O）and carbon dioxide（CO₂））are one of the factors affecting global change.Forest soil,as an important source of greenhouse gas generation,is easily changed by environmental factors.Therefore,by studying the understory stoichiometric characteristics,soil carbon and N process,understand the southern red soil hilly region of Chinese fir plantation forests plant survival and development status,help us understand the understory succession law,in the role of soil carbon,and understory ecosystem response to climate change policy.In this paper,a continuous N and P addition experiment were designed by selecting representative subtropical Chinese fir plantation.Seven years later,collected four places in the typical plant（refer to appendix）,including Lophatherum gracile Brongn.,Woodwardia japonica（L.f.）Sm.,Dryopteris atrata（Kunze）Ching and Dicranopteris dichotoma（Thunb.）Berhn.as well as the corresponding soil samples.Four kinds of undergrowth plants were divided into different functional types（Gramineae and Ferns）,and the effects of nitrogen and phosphorus addition on the stoichiometric characteristics and C and N processes of undergrowth plants and soil were studied by measuring the total nutrients of plants C,N and P,the processes of soil C and N,and greenhouse gas emissions.The main results show that:（1）the concentrations of C,N and P and the stoichiometric ratio range of different functional plants and organs in the understory were different,and the concentrations of C,N and P and the stoichiometric ratio of different functional plant organs were different in the response to the addition of N and P.The effect of N and P addition on C,N and P content and stoichiometric ratio of understory depends more on different functional plants and different species.There was a significant correlation between the stoichiometric ratios of C,N and P in different plant organs.（2）the concentrations of C,N,P and stoichiometric ratios in the soil of different functional plants in the understory were different,and the concentrations of N and P in the soil of gramineous plants were significantly higher than that of Ferns,and the soil nutrient accumulation of gramineous plants was higher than that of Ferns.Concentrations of C/N and C/P in soil of different functional plants were different.The effect of N and P addition on the content of C,N and P in soil depends more on N addition,different functional plants and different species,and more on stoichiometric ratio depends on different species.The correlation between C,N and P elements in soil of different functional plants was poor,but the correlation between stoichiometry was better.（3）according to the correlation analysis between the concentrations of C,N,P and stoichiometric ratio of different plant organs and soil,the correlation between the concentrations of C,N,and P of different functional plant organs and soil is significantly different,while the correlation between the concentrations of different functional plant organs and soil is less significant.（4）the concentrations of soil availability C and N of understory were affected by the interaction of N and P.The concentrations of NO₃^--N in soil of different functional plants were significantly different from those of AP,and there was a close correlation between the concentrations of soil availability C and N.The response of N mineralization rate of plant soil to N and P addition is different.The N mineralization rate of Ferns is not only affected by N addition,but also depends on the interaction of N and P.The N mineralization rate of Gramineae depends on the addition of P.There was a significant correlation between soil N mineralization rate and soil nutrients.The soil N₂O emission rate of different functional plants was the highest among N2-P,and the addition of N and P had an interactive effect on the soil N₂O emission rate.The soil N₂O emission rate also been dependent on the interaction between N addition and different functional plants and the interaction between P addition and different functional plants.Addition of N and P had no significant effect on soil CO₂emission rate.The difference of soil CO₂ emission rate is dependent on the interaction between different functional plants and N addition,P addition and different species.Distinct functional plants and different N and P addition treatments showed diversity in the cumulative N₂O emission law,and the N2-P treatment of different functional plants had the highest emission trend and the largest emissions.Soil CO₂ of the four understories showed a high emission rate at the initial stage of emission,and the increasing rate of cumulative emission slowed down the change of time.In summary,the fixation of C in different organs and soils,and the utilization efficiency of N and P in different functional plants are different,and they are consistent in the restriction factors of soil N and P.The concentrations of C,N and P in soil of different species vary greatly,and the response mechanism to the addition of N and P is different.The correlation between plant elements C,N and P was poor,and that between stoichiometry was better.Different organs had different effects on the element circulation between understory and soil.The mechanism of soil absorption of element P is different in the process of soil nitrification.Organic compounds composed of elements C and N in soil are closely related to the source of N mineralization.Different functional understory also showed different N₂O and CO₂emission rates and cumulative emission patterns.Therefore,when considering the influence of environmental factors on the growth and development rules of understory and the rules of soil emission of N₂O and CO₂,differences between different functional plants should be distinguished.