The 2013 Intergovernmental Panel on Climate Change (IPCC) assessment revealed that the CO₂, CH₄ and N₂O concentrations have now reached the highest level during the previous 80 years, possibly owing to anthropogenic activities. A considerable amount of atmospheric greenhouse gas (GHG) is produced and consumed through soil processes.  

Afforestation can greatly affect GHG fluxes from soil due to variability of vegetation by altering the key biogeochemical cycling of C and N and microbial activity, which has been generally regarded as an effective mechanism for reducing the anthropogenic emissions due to its capacity of trees to sequester CO₂ from the atmosphere. However, the impact of afforestation on GHG emissions remains uncertain. 

Dr. DOU Xiaolin, supervised by Prof. CHENG Xiaoli from Wuhan Botanical Garden, conducted a field study in Danjiangkou Reservoir region of central China to investigate the seasonal CO₂, CH₄ and N₂O fluxes as a result of the changes in soil C and N storage and the variable environmental factors following afforestation. 

Afforestation increased the GHG fluxes from the afforested soils in the Danjiangkou Reservoir area of central China. The CO₂, CH₄ and N₂O fluxes from soils were strongly influenced by soil temperature, moisture and soil organic carbon (SOC) content across seasons. Besides, the N₂O flux was also strongly affected by soil organic nitrogen (SON) content.  

Surface litter exclusion reduced soil CO₂ fluxes and N₂O fluxes in the woodland. However, no significant change in CH₄ flux from the afforested soils was found after surface litter exclusion, indicating that CH₄ flux was mainly dependent on the mineral soil rather than the surface litter layer in the subtropical forests of China.  

This study revealed that although afforestation may be of great importance for terrestrial carbon sequestration, it also enhanced the GHG emissions from soil. To predict the long-term effects of afforestation on soil C sequestration, more field studies were need to measure net ecosystem CO₂ exchange to forecast the future changes in ecosystem C cycles in response to global change.  

This research was financially supported by the National Natural Science Foundation of China. Results have been published in Atmospheric Environment entitled “Greenhouse gas (CO_2,CH_4,N₂O) emissions from soils following afforestation in central China”.  

 Temporal variations (a, b) and annual average (c, d) of CO₂ emissions from woodland and shrubland under different treatments over four seasons (Image by DOU Xiaolin)