| A city is a large and densely populated urban area and is where human activities intensively taken place. With the growth of urban population and urbanization, many environmental problems are associated with urban development. Problems such as air pollution, biological and environmental degradation, and fresh-water supply shortage have become a growing concern increasing over the years. The Urban forests, called the "lungs of the city", play an important role in improving urban environment quality, regulating the relations between nature and human, and supporting sustainable development in urbanization. As a consequence, a great deal of efforts has been made to investigate structure, function and ecosystem services of urban forests around the world. In this study, physical and chemical properties of soil, patterns of stand biomass and productivity, nutrient biological cycle process, and carbon storage capacity in Camphor tree (Cinnamomum camphora (L.) Presl) and Masson pine(Pinus massoniana Lamb) mix-forest ecosystems were investigated in Changsha city, Hunan province. The results showed as following:1. The soils in the studied forests were acidic with a mean pH scale of 4.7. Soil water content averaged 21.8% and total soil porosity was 44.6%. Soil organic matter content was 17.59 g·kg-1. The concentration of total nitrogen (N) and phosphate (P) in soils was 0.99 and 1.13 g·kg-1, respectively. Both available-N and available-P contents were relatively low and accounted for around 2% of the total N and P in the study sites. The concentration of potassium (K) was low (6.13 g·kg-1) but available-K content was in adequate level (150.15 mg·kg-1). The concentration of Calcium (Ca) and Magnesium (Mg) in soils was 0.79 and 3.00 g·kg-1, which was relative low.The concentrations of micronutrients in soils were in an order as Fe> Mn> Zn> Cu> Pb> Ni> Co> Cd, with the highest in Fe (60797 mg·kg-1) and lowest in Cd (1.18 mg·kg-1). It was found that Mn was obviously transferred down along soil profiled due to leaching process. No apparent patterns were found for micronutrient elements, Cu, Fe, Zn, Cd, Ni, Pb and Co in terms of accumulation and leaching in soils.2. Mean biomass of an individual tree was 191.1,292.0,29.4, and 18.0 kg in the four tree species, Camphor tree, Masson pine, Symplocos sumuntia and Symplocos anomala in the studied mix-forests. Biomass was highest in stem organ, and was significantly higher than that in bark, branch, leaves and root (p<0.05). Biomass was in an order:stem> root> branch> leaves> bark in tree species Camphor tree, but was in an order as stem> branch> root> leaves> bark in Masson pine, Symplocos sumuntia and Symplocos anomala. Under the similar habitat, there was a significant difference of biomass production for different tree species even they were in the same age. For example, mean biomass of an individual tree in a 22-year old Masson pine was 1.5 time as high as that in a 22-year old Camphor tree, and mean tree biomass of a 20-year old Symplocos sumuntia was 1.6 times higher than that in a 20-year old Symplocos anomala. The results indicated that biomass production of an individual tree was primarily determined by biological features of the tree species, and environmental variables were the second most important factors in controlling biomass production.Stand biomass of overstorey in the forests was 76.14 t-hm-2, of which Camphor tree was 28.67 t·hm-2, accounting for 37.7%; Masson pine was 17.51 t·hm-2, accounting for 23.0%; Symplocos sumuntia was 22.91 t·hm-2, accounting for 30.0%, and Symplocos anomala was 7.05 t·hm-2, accounting for 9.3% of the total biomass in overstorey.Biomass in shrub layer was 16.15 t·hm-2, of which Neolitsea aurata was 1.47 t·hm-2, Gardenla Jasminoides was 0.64 t·hm-2, and other seedlings was 14.04 t·hm-2.Biomass in herbivorous layer was 0.22 t·hm-2, of which Woodwardia japonica was 0.15 t·hm-2, Spider brake (Pteris multifida Poir) was 0.03 t·hm-2 and Thysanolaena maxima was 0.04 t·hm-2.Biomass in lianas layer was 0.05 t·hm-2, of which Smilax china was 0.02 t·hm-2 and Paederia scandens was 0.03 t·hm-2. Biomass in dead floor layer was 5.00 t·hm-2, of which non-decomposed part (L) was 0.53 t·hm-2, semi-decomposed part (F) was 2.57 t·hm-2 and decomposed part (H) was 1.90 t·hm-2.Annual amount of litter-fall in the forests was 7601.66 kg·hm-2·a-1, of which litter-needle was 3541.22 kg·hm-2·a-1, litter-broad-leaves was 1878.20 kg·hm-2·a-1, litter-fruit was 796.67 kg·hm-2·a-1 and litter-organic debris was 1385.57 kg·hm-2·a-1.Stand productivity in overstorey in the mix-forests was 4.85 t·hm-2·a-of which Camphor tree was 2.62 t·hm-2·a-1, Masson pine 0.73 t·hm-2·a-Symplocos sumuntia 1.14 t·hm-2·a-1 and Symplocos anomala 0.36 t·hm-2·a-1.3. Mean concentration of macronutrient elements in the four tree species was:N was 5.92-8.25 g·kg-1, P 0.43-0.58 g·kg-1, K 1.50-2.53 g·kg-1, Ca 4.36-10.21 g·kg-1 and Mg 0.84-1.10 g·kg-1. Concentration of micronutrient elements ranged as:Fe 165.90-344.56 mg·kg-1, Cu 7.53-9.00 mg·kg-1, Zn 21.63-41.49 mg·kg-1, Mn 138.78-619.27 mg·kg-Cd 0.46-2.25 mg·kg-1, Ni 2.88-3.83 mg·kg-1 and Pb 5.70-13.65 mg·kg-1.The total storage of nutrients in the mix-forest ecosystems was 1423.44 kg·hm-2. Annual retention amount was 132.07 kg·hm-2·a-1, annual return amount was 238.97 kg·hm-2·a-1 and annual uptake amount was 371.04 kg·hm-2·a-1. The utilization coefficient (retention/uptake) was 0.36, cycling coefficient (return/uptake) was 0.64, and turnover time was about 6 years. The results suggested that the Camphor tree and Masson pine mix-forest ecosystems were characterized by fast return rate, high cycling intensity, high utilization efficiency and relative short turnover time in terms of nutrient biological cycle. These features provide benefits for the forest ecosystems in maintaining site fertility.4. Average carbon content in overstorey in the mix-forests was: Camphor tree 539.73 g·kg-1, Masson pine 458.40 g·kg-1, Symplocos sumuntia 470.13 g·kg-1 and Symplocos anomala 463.50 g·kg-1. Carbon content in different organs did not significantly differ in Camphor tree (p>0.05), but was considerably different in Masson pine (p<0.05). Carbon content was statistically lower in leaves than in stem and branch in Symplocos sumuntia (p<0.05), and was considerably higher in stem and bark than in leaves in Symplocos anomala (p<0.05).It was found that Neolitsea aurata had the highest carbon concentration (481.47 g-kg-1) in shrub layer, the next was Gardenla Jasminoides (452.81 g·kg-1) and other seedlings (466.67 g·kg-1).Mean carbon concentration in the herbivorous layer was:Woodwardia japonica 475.55 g·kg-1, Spider brake (Pteris multifida Poir) 422.71 g·kg-1 and Thysanolaena maxima 396.04 g·kg-1Carbon content in lianas layer was:Smilax china 470.36 g·kg-1 and Paederia scandens 437.69 g·kg-1. Carbon content was higher in shoot than in root in both herbivorous and lianas layers.Carbon content in dead floor layer was:non-decomposed part (L) 401.80 g·kg-1, semi-decomposed part (F) 436.59 g·kg-1 and decomposed part (H) 352.72 g·kg-Carbon content in litter-fall layer was:litter-needle 325.03 g·kg-1, litter-broad-leaves 360.75 g·kg-1 litter-fruit 425.18 g·kg-1 and litter-organic debris 461.80 g·kg-1.Carbon content in soils (60 cm depth) was 10.20 g·kg-1, and was decreased with increasing of soil depth.5. Total carbon storage in the forest ecosystems was 132.68 t·hm-2, of which overstorey layer was 38.52 t·hm-2, shrub layer was 7.27 t·hm-2, herbivorous layer was 0.09 t·hm-2, lianas layer was 0.02 t·hm-2, dead floor layer was 2.00 t·hm-2 and soil layer (0-60cm) was 84.78 t·hm-2. Carbon storage in different compartments of the ecosystems was in an order:soil vegetation> dead floor layer.6. Annual carbon sequestration in overstory layer was 2.44 t·hm-2·a-1, of which Camphor tree was 1.37 t·hm-2·a-1, Masson pine 0.35 t·hm-2·a-1, Symplocos sumuntia 0.54 t·hm-2·a-1 and Symplocos anomala 0.18 t·hm-2·a-1.Annual carbon stored in litter-fall was 2.81 t·hm-2·a-1, of which litter-needle was 1.15 t·hm-2·a-1, litter-broad-leaves 0.68 t·hm-2·a-1, litter-fruit 0.34 t·hm-2·a-1 and litter-organic debris 0.64 t·hm-2·a-1. Soil respiration (exclude root respiration) was 3.58 t·hm-2·a-1. Net ecosystem productivity was 1.67 t·hm-2·a-1 in the Camphor tree and Masson pine mix-forest ecosystems in Changsha city. Our results indicated that the Camphor tree and Masson pine urban forests was a carbon sink.
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