Observation Study Of Ecosystem Carbon Exchange In Grassland Over Semi-arid Loess Plateau

The ecosystem carbon exchange, as a key factor influencing the carbon cycles, has gradually became one of the core content in the study of global warming. Because of the special geographical position and climate conditions at semi-arid Loess Plateau, the fragile ecosystem is very sensitive to the effect of climate change and human activity. The long-term continuous measured CO2 flux, water flux, radiation and various meteorological elements at the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) site, provide the powerful data foundation for us to further study the characteristics of ecosystem carbon exchange over semi-arid Loess Plateau. In this study, we first provided an assessment of gross ecosystem productivity, the use of available water and light by the semi-arid grassland under seasonally varying soil moisture conditions, due to the uneven distribution of precipitation; Then, we discussed the effect of clouds and aerosols on net ecosystem carbon exchange; Finally, we compared the basic characteristics of CO2 flux between SACOL, Xinglong mountain and Tongyu sites.The examination of vegetation productivity and use of light and water resources is important for understanding the carbon and water cycles in semiarid and arid environments. We made continuous measurements of carbon dioxide and water vapor fluxes over the semi-arid Loess Plateau by using the eddy covariance (EC) system. These measurements allow an examination of EC-estimated gross ecosystem productivity (GEP), light use efficiency (LUE), and water use efficiency (WUE) of the steppe. Daily variations of GEP, LUE, and WUE were associated with daily variations of incident photosynthetically active radiation (PAR), ambient temperature (Ta), and vapor pressure deficit (VPD). The magnitudes of these variations were also dependent on canopy development. On the daily basis, GEP linearly correlated with evapotranspiration rate and PAR. LUE correlated positively with leaf area index, Ta, and soil moisture availability but negatively with the surface reflectivity. Throughout the growing season, both GEP and LUE responded strongly to precipitation-fed soil moisture in the top 40cm of the soil. An examination of the responses of LUE and WUE to PAR under different soil moisture conditions shows that when soil water availability exceeded VPD, the steppe was more efficient in light use, whereas it was not in water use.The impacts of clouds and atmospheric aerosols on the terrestrial carbon cycle at semi-arid Loess Plateau in Northwest China are investigated, by using the observation data obtained at the SACOL site. Daytime (solar elevation angles of larger than 50°) net ecosystem exchange (NEE) of CO2 obtained during the midgrowing season (July-August) are analyzed with respect to variations in the diffuse radiation, cloud cover and aerosol optical depth (AOD). Results show a significant impact by clouds on the CO2 uptake by the grassland (with smaller LAI values) located in a semi-arid region, quite different from areas covered by forests and crops. The light saturation levels in the canopy are low, with a value of about 434.8 W m-2. Thus, under overcast conditions of optically thick clouds, the CO2 uptake increases with increasing clearness index (the ratio of global solar radiation received at the Earth surface to the extraterrestrial irradiance at a plane parallel to the Earth surface), and a maximum CO2 uptake and light use efficiency of vegetation occur with the clearness index of about 0.37 and lower air temperature. Under other sky conditions, CO2 uptake decreases with cloudiness but light use efficiency is enhanced, due to increased diffuse fraction of PAR. Additionally, under cloudy conditions, changes in the NEE of CO2 also result from the interactions of many environmental factors, especially the air temperature. In contrast to its response to changes in solar radiation, the carbon uptake shows a slightly negative response to increased AOD. The reason for the difference in the response of the semi-arid grassland from that of the forest and crop lands may be due to the difference in the canopy's architectural structure.Finally, based on the eddy covariance (EC) observation at the SACOL and Xinglong Mountain and Tongyu degraded sites, the CO2 flux and its relationships with meteorological elements at these sites are compared and investigated. The result shows that both the SACOL and Xinglong Mountain sites are under the same climate system, but there are significant differences between the monthly mean variations of the CO2 fluxes. The carbon exchange of the crops at the Xinglong Mountain site is mainly due to vegetation phenology, while it mostly dependent on the precipitation at the SACOL site. Furthermore, the critical values of meteorological elements which have influences on the CO2 fluxes at the both sites are low, and it will have some negative effect on the CO2 exchange if the meteorological element exceeds its critical value. The CO2 uptake in daytime and respiration in night at the Tongyu degraded grass site are obviously greater than that at the SACOL site during the period from January to August in 2007. Although the less precipitation at Tongyu in 2007, the CO2 uptake is not less than that at SACOL site as expected, which indicates that the fluxes of CO2 is not only rely on vegetation but also on geographic position as well as environmental factors. Additionally, the critical value of each meteorological element (e.g. air temperature, soil temperature, soil moisture) is significantly greater at the SACOL site than that at Tongyu, when the CO2 uptake reach the peak. This indicates that the regulating capacity of the degraded grassland to the climate change (e.g. global warming) at the Tongyu site is stronger than that for the sparse vegetation at the SACOL site.