Study On Characteristic Of Carbon Balance In Middle Arid Region Of Ningxia

Land is the material basis for human survival. Different land use types attach different vegetation, bearing different human activities, also containing different carbon transfer process. The changes of land use patterns will break carbon balance formed for a long time and lead to changes in regional carbon budgets. Middle arid region of Ningxia has experienced a long period of deforestation, especially in recent years the large scale of the returning farmland to forest (grass), ecological migration engineering, resulted in great changes in land use types, which will have a great effect on carbon cycle. Therefore, it is necessary to construct an appropriate method suitable for studying the characteristic of carbon balance in the arid and semi arid area, which is on the basis of establishing corresponding relations between the carbon absorption/emission and land use types. The method and data support can be provided for regional land use planning, assessment of ecosystem service value and establishment of ecological compensation mechanism. Meanwhile, these can improve carbon cycle research system.The representative counties, Yanchi, Tongxin and Haiyuan, were chosen as study areas, which are in the middle arid region of Ningxia. Data sources include meteorological data, TM images, MODIS product, soil data and statistical data. On the base of analyzing the meteorological conditions, the potential evapotranspiration and land use patterns of the study areas, starting from carbon absorption and emission, the characteristics of carbon balance of different land use types were calculated and analyzed by using remote sensing, geographical information system, field investigation etc. The main conclusions are as follows:(1) The result of CAS A accuracy test shows that, the linear correlation has higher value between simulated and measured values. Correlation coefficient (R2) is 0.6002, so the CASA model used to simulate the carbon absorption in the study areas has certain credibility.(2) The carbon absorption intensity(CAI) sequence analysis shows that, in the monthly level, CAI and months show inverted U type relationship. CAI is the maximum in August. CAIs of Yanchi, Tongxin and Haiyuan counties are 35.80,35.75 and 46.93 gC/(m2-a), respectively. In the annual level, the counties’ annual CAIs (mean) in 2000 are low, less than 70 gC/(m2·a). Although CAIs increase in 2006, but rates are relatively low. In 2012, CAIs have a significant growth. Compared with 2000, CAIs’increases are271.23%,208.01% and 139.80%, respectively. The CAI spatial analysis shows that, with the vegetation growth improved, spatial heterogeneity of CAIs are more obvious.(3) CAIs of different land use types exist differences. There is higher biomass accumulation in the woodland, so its CAI is the maximum. Mean values of the three years (2000,2006,2012) are 120.7,210.0 and 243.7 gC/(m2-a) in Yanchi, Tongxin and Haiyuan counties, respectively. The next is cropland, mean values are 85.7,101.3 and 101.0 gC/(m2·a). Grassland is the third, mean values are 73.0,68.0 and 87.3 gC/(m2-a). Because of green vegetation, and limit of the image spatial resolution, it is difficult to distinguish exactly between houses and roads with cropland and grassland in counties, so industrial, mining and residential area (IMRA) also has CAIs, mean values of CAIs are 46.0,73.0 and 72.7 gC/(m2-a). With the passage of time, CAIs of different land use types increase, but there are differences in different periods of change amplitude. Considering of the different areas of the different land types, by further analysis of contribution to the overall regional carbon absorption of all types of land, we can get the orders from high to low, that is grassland, cropland, woodland and IMRA.(4) Topographical factors can affect carbon absorption capability. Analysis about CAI respond to altitude and slope shows that CAIs in the low altitude (slope) and high altitude (slope) are greater than other ranges, because of there are a lot of cropland in the low altitude (slope) and woodland in the high altitude (slope). In terms of aspect, the change of CAI is not obviouse with the variation of aspect in Yanchi. CAI of Tongxin and Haiyuan counties show the same change law that is CAI in the shady slope is maximum, the next is semi shady slope and sunny slope is minimum.(5) The order of carbon emission intensity from high to low is that:industrial and lining fossil energy consumption, human and animal respiration, soil microbial respiration, livestock and poultry breeding, straw burning, agriculture fossil energy consumption. Carbon emissions of different land use types in three counties show the same law, that is the order from high to low is grassland, cropland, IMRA, woodland and unutilized land.(6) In the study period, net carbon values of three counties are negative, it means that total carbon absorption is less than total carbon emission; the study areas are carbon output, but the value of output decrease. From 2000 to 2012, the decrement rates are 48.87%,26.89% and 38.42% in Yanchi, Tongxin and Haiyuan counties, respectively. It may be attributed to the gradual increases of carbon absorption ability, which indicates that with the implementation of the returning cropland to forest (grass), ecological migration engineering, the kinds and quantity of vegetation in study areas have a certain degree of recovery. The analysis of different land use types shows that woodland is carbon input and other kinds of land use types are carbon output. Among them, outputs of grassland, cropland and unutilized land decreased with time going by, while the output of IMRA increased and became uppermost contributor of carbon output in 2012.