Coarse Woody Debris Decomposition And Soil Respiration And Their Influencing Factors In Fragmented Forests

Forest is one of the most important terrestrial ecosystems and plays an extremely important role in the development of human society.However,due to increasing human activities,the global forests have been seriously damaged,and forest fragmentation is serious.Coarse woody debris decomposition and soil respiration are the main paths of carbon entering the atmosphere in forest ecosystems,and play important roles in material circulation and energy flow.The intensification of forest fragmentation is likely to change the intensity of forest carbon release,which may have a profound impact on global climate change.So far,domestic research on forest fragmentation effects mainly focuses on biodiversity,but there is still a lack of research on forest ecosystem functioning,especially on carbon release.Therefore,the study on the decomposition of coarse woody debris and soil respiration and their influencing factors on fragmented forests is of great significance to further reveal the carbon release of regional forest ecosystems.Thousand Island Lake provides an ideal platform for the study of forest fragmentation because of its consistent formation time,uniform isolation matrix（lake water）,clear boundary and similar vegetation types（mostly Pinus massoniana）.In order to find out whether the changes of biotic and abiotic factors caused by forest fragmentation will affect carbon release processes such as coarse woody debris decomposition and soil respiration in the forest,two experiments were carried out in Thousand Island Lake of Chun’an County.We took coarse woody debris of three common tree species:deciduous broad-leaved tree Quercus serrata,evergreen broad-leaved tree Castanopsis eyrei,and coniferous tree Pinus massoniana as experimental objects,and used different litterbags of 0.8 mm（excluding soil macroinvertebrates entering）and 4 mm mesh（allowing soil macroinvertebrates entering）respectively,and then carried out a two-year decomposition experiment on45 plots in fragmented habitats（located in three different habitats:small islands,the edge of large islands and the interior of large islands）,and 15 mainland plots in continuous habitats;at the same time,a one-year soil respiration observation experiment was also carried out in the same experiment plots as above.The main results are as follows:（1）There are interspecific differences of decay rates among coarse woody debris.The deacy rate of Pinus massoniana is the highest,followed by Castanopsis eyrei,and Quercus serrata is the lowest.Wood density,diameter and lignin content have negative effects on the decay rate.（2）Soil macroinvertebrates significantly promoted the decomposition of coarse woody debris.After two years of decomposition,the decay rate in coarse mesh bags was higher than that in fine mesh bags.（3）Two years after decomposition,the decay rate of Pinus massoniana and Quercus serrata decreased with the increase of island area.The decay rate of Pinus massoniana decreased with the increase of island isolation.The decay rate of Pinus massoniana on small islands(0.38±0.02 yr^-1)was significantly higher than on mainland(0.32±0.02 yr^-1).（4）The abundance of termites on small islands was significantly higher than mainland,but there was no significant difference on the fungal fruiting body coverage among four habitats.The termite abundance and fungal fruiting body coverage had significant positive effects on decay rates.After one year of decomposition,the island area had a negative effect on the termite abundance and fungal fruiting body coverage by increasing the woody plant richness.After two years of decomposition,the island area had a positive effect on the fungal fruiting body coverage by increasing the woody plant diversity,while island isolation had a negative effect on the fungal fruiting body cover through a negative effect on the woody plant diversity and biomass.（5）The soil respiration rate varies significantly among different seasons,as summer(3.74±0.11μmol·m^-2·s^-1)>autumn(2.30±0.07μmol·m^-2·s^-1)>spring(1.82±0.05μmol·m^-2·s^-1)>winter(1.40±0.05μmol·m^-2·s^-1).The seasonal difference is mainly driven by the soil temperature.（6）The soil respiration rate was significantly different among different habitats.In general,the soil respiration rate of islands(2.37±0.05μmol·m^-2·s^-1)was significantly higher than mainland(2.08±0.07μmol·m^-2·s^-1),and the soil respiration rate of the edge of big islands(2.46±0.12μmol·m^-2·s^-1)was significantly higher than the interior(2.03±0.06μmol·m^-2·s^-1).（7）The soil temperature and fine root biomass had significant positive effects on the soil respiration rate.The island area and isolation had negative effects on the soil respiration rate through effects on the soil temperature and the fine root biomass.This study shows that forest fragmentation accelerates the decomposition of coarse woody debris and soil respiration rate by changing abiotic and biotic factors,and further reveals the influencing factors of carbon release by fragmented forests,which provides scientific data for future studies of carbon cycle in forest ecosystems.