Accurate estimation of land--atmosphere CO2 flux is essential to understanding the feedback between the terrestrial biosphere and the atmosphere. However, the observation network is too sparse for terrestrial carbon cycle studies of regions, continents, or the globe. In August 2012, a field experimental observatory was established in Ningxiang (eastern Hunan Province, southern China), where a set of micrometeorological and eddy covariance instruments were installed. This site is surrounded by a mixed evergreen forest over a hilly zone and is located in the subtropical East Asian monsoon region.
"Our work is intended to aid understanding of the carbon budget in the East Asian subtropical forest ecosystem and improve the terrestrial carbon cycle model using the observed CO2 flux data from this site," said XIE Zhenghui, a scientist of Institute of Atmospheric Physics (IAP).
XIE’ team established quality control and gap-filling procedures to provide complete datasets from this site for terrestrial carbon cycle research. Then, they quantified diurnal and seasonal variations in the net ecosystem CO2 exchange (NEE) and its component fluxes, gross ecosystem productivity (GEP) and total ecosystem respiration (RE). Moreover, the relationship between the CO2 fluxes and climate factors was also analyzed.
Results showed that the target ecosystem appeared to be a clear carbon sink in 2013; and the net carbon uptake (i.e. the ？NEE), RE and GEP showed obvious seasonal variability, being lower in winter and under drought conditions and higher in the growing season. Furthermore, the daytime NEE was mainly limited by the water-stress effect under dry and warm atmospheric conditions, rather than by the direct temperature-stress effect.
References:
Jia, B., Z. Xie*, Y. Zeng, L. Wang, Y. Wang, J. Xie, and Z. Xie, 2014: Diurnal and seasonal variations of CO2 fluxes and their climate controlling factors for a subtropical forest in Ningxiang, Advances in Atmospheric Sciences, 32(4), doi: 10.1007/s00376-014-4069-4
Download [Early Online]: http://159.226.119.58/aas/EN/abstract/abstract2536.shtml
Fig 1. Seasonal variations of (a) REref (red dashed line), air temperature (Ta, black solid line), and (b) NEE (blue dots), GEP (green squares), RE (red triangles) for 2013. REref is the base respiration at the reference temperature [10°C]; negative values for NEE indicate that the ecosystem is acting as a carbon sink.
Fig 2. (a) Relationship between the daytime NEE and global radiation in 2013 (the solid line represents a nonlinear regression using the modified Michaelis–Menten model [Eq. (4) in Jia et al. (2014)]; (b) residuals of observed NEE data and the regression curve in panel (a) versus air temperature (squares indicate the mean for each 1°C class); and (c) NEE residuals versus vapor pressure deficit (squares indicate the mean for each 2 hPa).
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