Decomposition Of Fallen Logs Of Five Common Subtropical Tree Species And Their Response To Nitrogen Addition

Fallen wood is the most important component of coarse wood residue in forest ecosystem.Its decomposition process is fast and slow,which is very important to the nutrient cycle of forest ecosystem.At present,there are abundant studies on the carbon cycle and dynamics of aboveground living trees and underground soil in subtropical forest,but the research on the decomposition of fallen trees of main tree species and their response to nitrogen deposition（or addition）is very limited.Based on this,this study takes five common tree species in Wuxie National Forest Park in Zhejiang Province-Quercus glauca,Schima superba,Pinus massoniana,Cunninghamia lanceolata and Phyllostachys heterocycla as the research materials,and studies under the conditions of 30 kg·hm^-2·yr^-1（current settlement）and 60kg·hm^-2·yr^-1（double settlement）,The changes of wood density,carbon,nitrogen and phosphorus content,respiration rate in different months and other decomposition characteristics during the decomposition of different tree species are different.The main results include:（1）The decomposition rate constants of five common tree species in subtropical forest are（the faster the reaction rate is,the faster the decomposition is）:Quercus glauca(0.020 yr^-1)>Schima superba(0.018 yr^-1)>Phyllostachys heterocycla(0.014 yr^-1)>Cunninghamia lanceolata(0.007 yr^-1)>Pinus massoniana(0.003 yr^-1).Among them,the decomposition rate of Quercus glauca is significantly higher than Pinus massoniana,and other tree species are slightly different,but not significant.The loss of wood density is an important indicator of the reaction decomposition rate（large absolute value means fast decomposition）.The decomposition rate of different tree species is as follows:Quercus glauca(0.30 g·cm^-3)>Schima superba(0.25 g·cm^-3)>Phyllostachys heterocycla(0.20 g·cm^-3)>Cunninghamia lanceolata(0.10 g·cm^-3)>Pinus massoniana(0.03 g·cm^-3),while the effect of nitrogen addition on the decomposition rate of different tree species（change of wood density）is not significant.（2）In the initial physical and chemical properties,the wood density is as follows:Phyllostachys heterocycla>Quercus glauca>Schima superba>Pinus massoniana>Cunninghamia lanceolata.C content:Pinus massoniana>Cunninghamia lanceolata>Phyllostachys heterocycla>Quercus glauca>Schima superba,N content:Pinus massoniana>Phyllostachys heterocycla>Quercus glauca>Cunninghamia lanceolata>Schima superba,P content:Phyllostachys heterocycla>Pinus massoniana>Quercus glauca>Cunninghamia lanceolata>Schima superba.The initial physical and chemical properties had an effect on the decomposition rate,and the hemicellulose content was positively correlated with the decomposition rate;C.The contents of N and P nutrients,lignin and cellulose were negatively correlated with the decomposition rate.（3）The respiratory rate of fallen trees of different tree species was significantly different.During the monitoring period（October 2021 April 2022）,the overall respiratory rate order was Quercus glauca>Schima superba>Cunninghamia lanceolata>Pinus massoniana>Phyllostachys heterocycla.The monthly respiratory dynamics of the five tree species were significantly correlated with the change trend of air temperature,which showed a significant positive exponential correlation with temperature.The correlation between the respiration rate of fallen trees and the surface humidity of fallen trees was not significant,and the response to nitrogen addition was not significant,indicating that temperature was the most important environmental factor affecting the change of respiration of fallen trees.（4）This study also uses the carbon,nitrogen and phosphorus（CNP）data set of trunk parts before the decomposition of different tree species during the initial construction of the sample plot to simulate and estimate the influence uncertainty of different sampling sizes on the estimation of C,N and P contents at the sample plot level,in order to provide a method reference for relevant research.The results show that the C content of Pinus massoniana trunk varies greatly,about 7-10%,which is significantly higher than that of other tree species.Therefore,when estimating the carbon storage of Pinus massoniana,it is suggested to appropriately increase the sampling amount（n=5～10）to reduce the estimation error,or take the±10%estimation error range into account while still using the small sampling amount of n=3-5.The results showed that the decomposition rates of fallen trees of different tree species were quite different.The decomposition rates of Schima superba and Quercus glauca were higher than those of Pinus massoniana and Cunninghamia lanceolata.In this study,Phyllostachys heterocycla has the lowest respiration rate,but the total decomposition（weight loss rate in response to changes in wood density）is between 2 broad-leaved and 2 coniferous species,indicating that the decomposition of fallen Phyllostachys heterocycla may be affected by other decomposition processes（or its respiratory process is more variable than other species,and its changes cannot be fully monitored in this study）.In general,the response of the decomposition speed of fallen wood to temperature is obvious,and the response to Nsettlement（Naddition）is not significant.The above results are of great significance for the establishment of nutrient cycle model of subtropical forest under the background of climate change,the estimation of carbon,nitrogen and phosphorus reserves,and promoting the sustainable development of subtropical forest from the perspective of forest management.