| Global warming is one of the hot spot of the current scientific research; terrestrial ecosystem carbon cycle of human interference is the important research topic in this field. Agriculture eco-economic system is a complicated part in the terrestrial ecosystem carbon cycle and carbon balance, through different forms of human impact, showing the role of uncertainty carbon sources and sinks interaction. O n the one hand, plants fix carbon through photosynthetic accumulation in vegetation and soil, on the other hand,it can cause carbon emissions from a variety of ways,such as soil respiration, fertilizer, livestock, transportation and consumption of fossil fuels and bio-energy and so on, which greatly affected the carbon budget and balance of terrestrial ecosystems.A new problem in the Loess Plateau of the most serious soil erosion has already become a severe challenge that our country has to face on the terrestrial carbon budget uncertainties. How to balance the carbon budget of agriculture eco-economic system and adjust the harmonious development of human interference and natural ecology, there is a great significance for our terrestrial ecosystem carbon cycle and carbon balance. China implemented a large area of Loess Plateau Grain for Green Project in 1999, which is the largest ever man- made vegetation restoration activities, has the past 15 years. It impact on Carbon sequestration capacity and carbon budget, especially lack of systematic research on comparison of the before and after human disturbance.In this paper, a typical watershed in the region of returning farmland on the Hilly Loess Plateau as research subject, the use of experimental analysis, model calculations, historical statistics and household survey data combined method to estimate the carbon storage, carbon emissions, physiological characteristics and carbon sequestration of farmland,forestry and grassland on an annual basis from 1938-2010 years, and then analyzes the annual carbon budget and carbon balance of the entire wa tershed. In order to provide a scientific basis for exploration the ways of mutual adaptation and coordination of the development between ecosystem and economic system, promotion ecological restoration, maintaining the carbon balance and optimization carbon sequestration and emission reduction measures. The main findings are as follows:1. The ecosystem carbon storage shows first decrease slowly after rising trend in Zhifanggou watershed. Annual carbon storage is 18148.33 tC, density is 21.93tC·hm-2a-1, and the per capita carbon storage density is 41.95 t C·a-1, there is a large carbon storage in favor of stable carbon balance of the watershed. Farmland, forest and grassland ecosystem carbon storage is respectively 15%, 58% and 27% of total ecosystem, including vegetation carbon storage and soil carbon storage ratio of 2:3. A larger proportion of soil carbon storage, but the vegetation carbon storage showed an increasing proportion of the annual in farmland ecosystem; there are small differences between the vegetation and soil carbon storage in forest ecosystem; soil carbon storage is the dominant factor in grassland ecosystem. The soil organic carbon richness rate index illustrate that soil carbon storage capacity is forest > farmland > grassland in Zhifanggou Watershed. Grassland degradation has seriously affected the carbon sink function of land; rational fertilization contributes to the accumulation of carbon in farmland, the carbon sequestration capacity of forest ecosystems is stronger than other land use types.2. Black locust is pioneer species of the largest area in the region of returning farmland on the Hilly Loess Plateau. Different stages of black locust unit leaf area of photosynthetic carbon fixation and oxygen release capabilities as follows: young forest> mature forest> middle-aged forest; a single plant of photosynthetic carbon fixation and oxygen release capabilities expressed as middle-aged forest> mature forest> young forest; based on unit of the crown, it is young forest> middle-aged forest> mature forest. Organic carbon of black locust in various organs showed that branches of young and middle-aged are highest, followed by roots, leaves are the least,, while the organic carbon content of mature forest is the highest leaves, branches, followed by root least. N itrogen and soil organic carbon content as a whole showed a decreasing trend in different stages of black locust, with increasing soil depth; especially in the 0-40 cm soil layer thickness is the most obvious changes.3. Carbon emissions of eco-economic system is similar to the "W" type trends in Zhifanggou watershed, which is closely related to changes in human intervention. Annual carbon emissions is 2725.58 tC, density is 3.29 tC · hm-2a-1, per capita carbon emissions density is 6.30 t C·a-1. There is three aspects: Soil respiration(79%)> human carbon emissions(15%) carbon footprint(7%). Carbon emission of soil respiration generated change the significant changes by land use, especially mutual transformation of farmland and forest. Carbon footprint need to generate the corresponding changes, affected by human activities, people own carbon emissions increase year by year from 1960 to 2010 years.4. Carbon budget of ecosystem estimates results are as follows in Zhifanggou watershed: from the ecological perspective analysis, net ecosystem productivity(N EP1) shows that the ecosystem played carbon source function in 1958 and 1975, and the remaining are carbon sink; from an eco-economic system perspective analysis, increased carbon emissions of human impact(NEP2) is shown that the eco-economic system played carbon source function in 1958, 1975 and 1985, the rest of the year study showed carbon sink.5. From eco-economic systems carbon balance perspective of the whole Zhifanggou watershed analysis, carbon deficit accounted for 11% of the actual carbon storage can be in 1958, 8% in 1975 and 4% in 1985. The destruction of the environment has been causing an imbalance in the carbon deficit, given the supplies by carbon storage. Artificial restoration of vegetation can increase carbon storage. From the system point of view, eco-economic system is carbon balance in Zhifanggou watershed. There are factors that affect the carbon balance, such as the climate change, land-use change, carbon sedimentation and human disturbance, etc. The current study shows that there is a big controversy in these factors affecting carbon balance mechanism, and the spatial and temporal variability increased carbon storage and carbon budget uncertainty in terrestrial ecosystems. |
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