Study On Soil Orgnic Carbon And Its Influencing Factors Of Abies Fargesii Forest In Shennongjia

Abies fargesii is the most important edificators in subalpine area of Hubei Shennongjia, and Abies fargesii forest is the most extensive distribution area of coniferous forest types in this area. Research on carbon density and distribution of typical forest types in Shennongjia area contribute to enrich the understanding on soil carbon density characteristics of typical forest region in China. This study collected inventory data of Abies fargesii forest in Shennongjia, and based on the field investigation, sampling and determination, the soil carbon density of Abies fargesii forest was estimated, and the soil carbon density, activated carbon component characteristics and its influencing factors were analyzed, and the relationship between soil organic carbon and the influence factors was discussed. The main results were as follows:(1) Four structures of Abies fargesii forest for Abies fargesii and Betula albo-sinensis forest(ABF), Abies fargesii and Pinus amandii forest(APF), Abies fargesii and Fargesia spathacea forest(AFF), Abies fargesii and Rhododendron simsii forest(ARF) were selected. Soil organic carbon density of different community structure characteristics for Abies fargesii forest was studied, and the effect of stand structure, ground vegetation biomass and soil physical and chemical properties to soil carbon density were discussed. The results indicate that total soil carbon density in 0~60 cm was ARF(225.70 t·hm-2), AFF(181.87 t·hm-2), ABF(156.54 t·hm-2), APF(108.65 t·hm-2) in order, the difference of soil carbon density was extremely significant. Correlation analysis showed that soil carbon density was high negatively correlated with vegetation species diversity index and soil sand content, and high positively correlated with litter storage, the content of soil N, soil clay content and silt content, and positively correlated with average DBH and biomass of shrub and grass layer, while the soil carbon density was not correlated with forest canopy and soil pH value. Litter storage, soil total N content and soil clay content were the main factors affecting soil carbon density of Abies fargesii forest in Shennongjia.(2) Based on inventory data of Abies fargesii forest in Shennongjia, the soil carbon density was estimated by establishing model between soil carbon density and forest average DBH, and then the spatial distribution of soil carbon density was discussed. The results showed that soil carbon density was 75.25 t·hm-2 in 0~20 cm layer, while which was 130.89 t·hm-2 in 20~40 cm layer. The minimum soil carbon density occurred in an altitude of 2200 m, and maximum appeared in an altitude between 2800~3000 m. The soil carbon density on the North Slope was biggest, followed by the northeast, northwest, southwest, west, south, east slope, and soil carbon density on the southeast slope was least. With the increase of age, the soil carbon density increased gradually. Compared with the young forest, soil carbon density of over mature forest in 0~20cm and 0~40cm increased 15.66% and 16.47% respectively.(3) Dynamics of litter production and composition, nutrient return and decomposition of natural forest of Abies fargesii were studied in Shennongjia. The results showed that the total litter production of natural forest was 6217.44 kg·hm-2. The main litter fractions were leaves, followed by the twigs, bark and fruit were less. The litter production appeared two peak during September to Octomber and May to June, and the litter production was least from January to February. The annual nutrient returns of natural forest was 77.84 kg·hm-2, and the order of macro-element returns was N(47.13 kg·hm-2)>K(18.15 kg·hm-2)>Ca(7.77 kg·hm-2)>P(4.60 kg·hm-2)>Mg(0.19 kg·hm-2). The litter decomposition coefficient was 0.303, half declining and total declining was 1.70 and 9.30 years respectively. During the process of litter decomposition, the SOC content(0~10cm) increased by 3.95%~27.03% at different decomposition time.(4) Three soil organic carbon fractions of microbial biomass carbon(MBC), water soluble organic carbon(WSOC) and easily oxidized carbon(EOC) were selected, and soil active organic carbon content characteristics and its influence factor of Abies fargesii forest in different altitude gradients were analyzed. The results showed the MBC, WSOC and EOC content increased with the altitude increase, and decreased with the deepening of soil. The MBC ranges from 0.30 to 1.22 g·kg-1 in different altitude, and the ratio of total organic carbon ranged from 1.05% to 2.09%. The WSOC ranges from 0.16 to 0.69 g·kg-1, and the ratio from 0.57% to 1.20%. The EOC ranges from 5.93 to 17.20 g·kg-1, and the ratio from 21.19% to 29.23%. the three soil active carbon fractions content are all showed high positively correlation with total soil organic carbon content, total N content and soil moisture, while appeared high negatively correlated with soil temprature. Total soil organic carbon content is the decisive factor affecting active organic carbon fractions content.(5) The secondary forest of Abies fargesii, Fargesia spathacea forest and grassland which natural recovery after clearcutting were selected in Shennongjia, and the existing virgin forest was taken as contrast, and the change of soil carbon density was discusssed. The results showed that soil carbon density of secondary forest, Fargesia spathacea forest and grassland decreased 29.43%, 55.66% and 65.18% respectively after virgin forest was clearcut. The secondary forest was the best recovery type for soil carbon density; the next was Fargesia spathacea forest, then was grassland. In the process of recovering, Fargesia spathacea forest and grassland had better recovery effect to soil carbon density in 0~20cm layer, while vegetation restoration had no effect on soil organic carbon below 20 cm. In the case of no other interference, soil carbon density of secondary forest restored to pre-harvest levels need about 82 years.In conclusion, soil carbon density was influenced by the factors of stand structure, origin, soil physical and chemical properties, litter quantity and decomposition rate, and harvesting of human. And its distribution in different altitude, slope direction and ages showed significant differences. Through the study on soil carbon density characteristics and its influencing factors of Abies fargesii forest in Shennongjia, the forest soil carbon density situation and carbon sequestration potential can be understood more, and it can provide scientific basis for forest sustainable management and improvement of soil carbon pool management in the area.