Initial Response Of Ecological Stoichiometric Characteristics Of Cunninghamia Lanceolatae Plantation To Nitrogen Deposition

Cunninghamia lanceolate is a fast-growing evergreen conifer mainly distributed in subtropical areas of China.It is a very important wood and fiber resource in south China.However,due to the influence of atmospheric nitrogen deposition in recent years,the problems of quality decline and growth retardation of Chinese fir plantation have gradually become prominent,which have affected the long-term maintenance of forest productivity.In this context,it is of great significance to study the response mechanism of forest ecosystem to nitrogen deposition in China.To explore the specific influence to the Chinese fir plantation nutrient nitrogen deposition,this paper takes the C.lanceolata plantation of 11 and 18 years old in the Bamian mountain of Hongya county in Sichuan province as research object,and sprays ammonium nitrate in the middle of every month from April,2019 to the end of November,2019 to simulate nitrogen deposition.Three levels of nitrogen addition were set:control(CK,0 g N·m^-2·a^-1),low nitrogen(LN,5 g N·m^-2·a^-1),high nitrogen(HN,10 g N·m^-2·a^-1).This test uses ecological metrology method to research the effect of short-term nitrogen deposition on the leaf stoichiometry of Chinese fir plantation and its seasonal dynamic change,and to discuss the correlation between leaf and root stoichiometry and its difference between growth season and non-growth season,for providing a certain theoretical basis on the the future scientific managing methods and sustainable development.The main results of this experiment are as follows:（1）Short-term nitrogen addition had a significant effect on C,N and P contents of Chinese fir leaves,and seasonal changes and different concentrations were observed under the influence of time and nitrogen addition.The C content in the leaves of Chinese fir forest at different ages was 11a>18a.Compared with CK,the C content of11a and 18a fir forests increased by 0.9%and 3%under LN treatment,and decreased by 3.05 and 3.9%under HN treatment,respectively.N content in leaves of 18a Chinese fir forest was significantly affected by nitrogen addition.Compared with CK treatment,N content under LN and HN treatment increased by 8.8%and 15%,respectively.P content of 11a fir forest was significantly affected by nitrogen addition.Compared with CK treatment,P content increased by 11%under HN treatment.Both11a and 18a fir forests showed higher leaf P availability in the growing season.The C:N ratio of two forest ages is relatively stable.Both C:P ratio and N:P ratio showed similar variation trends in different seasons,and both of them decreased significantly in non-growing seasons.C:P ratio and N:P ratio are mainly limited by P element,indicating that the effectiveness of P element is low and the ecosystem is mainly limited by P element.（2）The research results of leaves and roots in growing season and non-growing season showed that the C and N contents and measurement ratios of leaves born in the current year and the next year of the growing season of 11a fir forest were higher than that of leaves in the non-growing season and their stoichiometric ratios.However,P content showed the opposite trend,and the change between different nitrogen treatments was not significant.The content of annual and annual leaves of 18a fir forest in non-growing season was greater than that in growing season.11a the content and ratio of grade 3 roots in growing season and non-growing season were more stable,and were less affected by N,P elements and seasons.The content and ratio of roots of grade 1-2 in 18a Chinese fir forest are relatively high,which are less affected by seasons.（3）In the growth season,the content of C and P,the ratio of C:N and the ratio of C:P were negatively correlated with the ratio of C:P and the ratio of N:P,and the content of P was negatively correlated with the content of N and P.The correlation between roots and leaves of 18a C lanceolata was not significant.In non-growing season,the C content,C:N ratio and C:P ratio of the roots of 11a fir were positively correlated with the C:N ratio and C:P ratio of the leaves.The P content and N:P ratio of the roots of 18a fir were significantly correlated with the CNP content,C:N ratio and C:P of the leaves.The content and measurement ratio of CNP in two forest age leaves were greatly affected by NP of roots.