Contribution Of Soil Fauna To Litter Decomposition Of Michelia Wilsonii And Camptotheca Acuminate Under Nitrogen Additions

Litter decomposition plays an important way for nutrient return of the forest ecosystem.It is the key to connect the above-ground and underground ecological processes of the forest ecosystem.It is also an important link of the material cycle and energy flow of the forest ecosystem.In recent decades,atmospheric nitrogen（N）deposition has been increasing,which may have a direct impact on the process of forest litter decomposition.Soil fauna are important biological factors affecting litter decomposition.N deposition may affect soil fauna,and then indirectly affect litter decomposition.The Yuping District of West China is one of the regions with severe atmospheric N deposition in China.Up to now,the contribution of soil fauna to litter decomposition under the background of N deposition is very limited.In view of this,this study takes the evergreen broad-leaved forest on the western edge of the Sichuan Basin with high N deposition background as the research object.Two litterbags that with or without soil fauna（2 mm and 0.01 mm）were conducted to assess the potential effects of N addition(0 kg·N·ha^-1·year^-1,20kg·N·ha^-1·year^-1 and 40 kg·N·ha^-1·year^-1)on mass remaining,rate of nutrient release of two leaf litter（Michelia wilsonii and Camptotheca acuminata）in this area.The main results are as follows:1)significant difference of mass remaining which between M.wilsonii and C. acuminata under the same treatment.The mass remaining rate of C.acuminata was significantly higher than that of M.wilsonii during the same treatment.The mass loss of M.wilsonii was 47.6%–56.5%,and that of C.acuminata was 58.1%–76.4%.The Olson negative exponential decay model fits the mass loss process well.The k value of the 2 mm litterbag was the highest（1.67）under low-N addition.The k values of the two apertures of the C.acuminata litters showed low-N>Control>High-N.Under the same litterbag treatment,low-N addition promoted the decomposition of C.acuminata and inhibited the decomposition of M.wilsonii,and high-N addition inhibited the decomposition of two types of litter.With the same N addition treatment,removal of soil fauna tended to reduce the rate of litter decomposition.2)C of M.wilsonii and C.acuminate have been released throughout the decomposition process.Decomposition rates of N and P of litters showed release-enrichment-release,especially in M.wilsonii.At the end of the test,the C release rate of M.wilsonii was 65.4–76.6%,and the N and P enrichment rates were enriched to varying degrees;on the contrary,the C,N,and P residual rates of C.acuminate were 10.2–22.5%,23.2–49.9%,and 41.5–66.7%,respectively.Soil fauna had a positive effect on the carbon and nitrogen release rate of M.wilsonii,while the phosphorus release rate was promoted by soil fauna.Soil fauna promoted the release of C,N,and P of C.acuminate litters.And the contribution of soil fauna to the release of C,N,and P from litter of C.acuminate was greater than that of M.wilsonii.Under the same litterbag treatment,low–N treatment usually promoted the release of C,N,and P of C.acuminate,but these of M.wilsonii were inhibited.Under the same N treatment,the C,N,and P release rates of two tree species with soil fauna were higher than those without soil fauna.Soil fauna contributed to C,N,and P release of M.wilsonii were 9.7%,6.6%,and-1.8%,respectively,and contributed 7.3%,18.6%,and 17.7%to C.acuminate.3)The potassium（K）and sodium（Na）element content of M.wilsonii and C.acuminate in each treatment showed a rapid release in general,especially in the early stage of decomposition.At the end of the test,the residual rates of K and Na in the M.wilsonii were 21.1–30.9%and 31.5–53.8%,while these in C.acuminate were10.8–17.4%and 23.1–39.2%,respectively.Soil fauna showed a negative effect on the release of potassium from litters.Soil fauna reduced the release of sodium from C.acuminate,but the effect to M.wilsonii was reversed.With the same litterbag,low–N treatment promoted the release of K and Na of C.acuminate,but it inhibited the M.wilsonii,and high-N addition hindered the decomposition of the K and Na elements in the two litters.With the same N addition,the release rate of K and Na was usually faster when soil fauna in litterbags.4)The cellulose content of the two types of litters gradually decreased,and the residual rate showed a net release pattern,and the cellulose content of M.wilsonii was lower than that of C.acuminate.Lignin content of these had an accumulation process from August to October in each treatment,but the overall performance was a release pattern.The cellulose and lignin residual rates of the two types of litter were 18.5–43.2%and 45.3–71.6%,respectively.Soil fauna showed positive effects on the release of lignin and cellulose from the two litters.With the same litterbag,N addition inhibited the lignin and cellulose degradation of litter.Under the same N addition,the lignin and cellulose release rate was slower when without soil fauna.5)Under different N treatments,the C:N and C:P of M.wilsonii and C.acuminate litters continued to decline during decomposition process,and the ratios of two spcies decreased from 45.8 and 27.2 to 11.6 and 8.2,respectively,While the N:P remains relatively stable.Under the same treatment,the C:N,C:P and lignin:N of C.acuminate was significantly lower than that of M.wilsonii.With the same litterbag,low-N usually increases the C:N and C:P of M.wilsonii,but the opposite was true for C.acuminate.Compared with the control,high-N treatment can promote the littering C:N,C:P and N:P of the two species to a certain extent.With the same N addition,the difference in stoichiometry of litter between litterbags was not significant.In summary,the simulated N deposition has a certain inhibitory effect on the decomposition of the two litters,and the soil fauna have a significant contribution to the litter decomposition.The mass loss of litter,element release pattern and degradation of refractory matter were affected by single N addition or soil fauna.The effects of N addition on litter decomposition and nutrient release were related to tree species and decomposition stages,and the N deposition effect was more pronounced during the later stages of decomposition.These results can provide a scientific basis for further understanding the response and adaptation of soil C sequestration and nutrient turnover to N deposition in subtropical forest ecosystems.