| To study the spatial distributions of soil carbon and nitrogen and their influencing factors in the tropical rain forest is critical to soil carbon sequestration and to cope with global climate change.More intensive extreme climatic events might be experienced and forests that located between 5-20°N' may experience more frequent or strong typhoon/hurricane.As one of the largest and well-preserved intact tropical rain forests in China,Jianfengling Nature Reserve owns the best development and the complex structure of tropical rainforest,and experiences typhoon frequently.Based on a 60 ha tropical mountainous rainforest dynamic monitoring site in Jianfengling Nature Reserve,the characteristics of forest gap and the heterogeneity of environmental factors inside and outside the gaps were investigated,as well as the spatial distribution of soil organic carbon,stability of soil physical and chemical carbon,soil total nitrogen and available nitrogen,and soil respiration.Stepwise regression analysis and RDA were performed to explore the influencing mechanisms.This research aims to: 1)find out whether the typhoon could increase the number of gaps and the heterogeneity of the environmental factors,and then affecting the stability and spatial distribution of soil carbon and nitrogen content in the tropical forest,and 2)explore the factors affecting the spatial distribution of soil carbon and nitrogen content in this region.This research can provide basic data for further understanding on the structure and function of tropical montane rain forest ecosystem.The results are as follows:?1?The average soil organic carbon content in the study area was 34.36 g/kg with a coefficient of variation value of 27.53%.The average light fraction organic carbon content was 7.94 g/kg,accounting for 23.1% of the total soil organic carbon,while those of heavy fraction organic carbon were 26.43 g/kg and76.9% respectively.The fractions of soil chemical carbon component were following alkyl C?41.61%?> O-alkyl C?33.00%?> carbonyl C?23.78%?> aromatic C?2.60%?.Soil inertia index??alkyl C + aromatic C?/?O-alkyl C + carbonyl C??was 79%,while SOC decomposition degree coefficient?alkyl C/ O-alkyl C?was 1.26 on average.?2?Semi-variance analysis showed that spatial structure ratio of C1/?C0+C1?of soil organic carbon,light fraction and heavy fraction organic carbon,microbial biomass carbon was 0.555-0.713,indicating a moderately spatial dependence,while that of soil alkyl carbon and oxygen alkyl carbon was 0.758-0.986 which indicated a strongly spatial dependence.Ordinary kriging interpolation and the spatial distribution of contour showed similar spatial distribution of soil organic carbon,light fraction organic carbon and heavy fraction organic carbon which were increased along ridge line,and generally distributed as combining patch with binding.There was also an increased trend for soil alkyl carbon and oxygen alkyl carbon along the ridge line.However,distribution of soil alkyl carbon was mainly patch and that of soil oxygen alkyl carbon was mainly binding.Sixty 20m×20m sampling sites were divided into three types based on MRT.The result of RDA on soil organic carbon and physical components showed that the impact of abiotic factors on soil organic carbon and physical components was more significantly than that of biotic factors,and these factors exerts covariantly.Such results were similar to that of RDA on soil carbon chemical components.Gradient change on the first two axises with biotic and abiotic factors for soil organic carbon and heavy fraction organic carbon was consistent.They were increased with the increasing of tree basal area and soil catalase activity.Light fraction o rganic carbon increased with the increasing number and richness of plant,number of leguminous plants,and biomass of line root?0-10 cm?.Soil oxygen alkyl carbon increased with the increasing of plant biomass,while decreased with the increasing of soil pH value.?3?The spatial structure ratio of soil total nitrogen was 0.485,indicating a moderately spatial dependence.However,those of soil nitrate nitrogen and ammonium nitrogen ranged from 0.775 to 0.842 indicating strongly spatial dependence.The spatial distribution of soil total nitrogen showed increased trend along ridge line,and were generally distributed combining patch with binding.There was a similar spatial distribution pattern for soil nitrate nitrogen and ammonium nitrogen,which were increased from west to east,but the distribution pattern was mainly binding.?4?The spatial structure ratio of soil total respiration,root respiration and litter quantity ranged from 0.289 to 0.62,indicated that they were moderately spatial depended.Soil respiration nitrogen content was higher in the southwest flat area,and decreased along ridge line.It was generally distributed combining patch with binding.?5?The relationship of soil carbon,soil nitrogen and environmental factors showed that along with the increase of elevation,average tree height,soil ammonium nitrogen and soil urease activity on the first and second principal components axis,the overlay gradients of soil carbon-water,nitrogen-water,and carbon-nitrogen-water also increased gradually.With the gradual increasing of tree basal area,soil alkyl carbon content,plant biomass and soil oxygen alkyl carbon,the gradient of overlay distribution for soil carbon and nitrogen was also increased gradually?6?The intensity of typhoons has been increased while the frequency has been decreased since 1949.The gaps were formed more in northeast areas due to the direction of typhoon.The area of most canopy gaps?85%?was less than 200 m2.The gaps were mainly distributed in slopes of 20° and 35° with a maximum age of 23.5.The wood density of gap-makers was 0.6 g·cm-3-0.7 g·cm-3.The emissions of N2O fluxes were higher inside the gaps and in wet season.There were significant relationships between N2O fluxes and soil temperature and water content.There was about 18 % emission would be underestimated in this region if ignored the effects of forest gaps. |
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