Methane (CH₄), the second most important greenhouse gas, plays an important role in the chemical and radiative balances in the Earth’s atmosphere. Soil acting as a CH₄ source or sink is sensitive to land use change, such as deforestation and conventional tillage, however, little is known about how afforestation affects the role of soil acting as CH₄ sink in subtropical China. 

Supervised by Prof. CHENG Xiaoli, WU Junjun, a research assistant of Wuhan Botanical Garden, conducted a year-long measurement of CH₄ in afforested lands (woodland and shrubland) and the adjacent cropland to assess the effect of afforestation on CH₄ uptake rate and its carbon-isotopic signature in the Danjiangkou Reservoir of central China. 

Afforestation significantly enhanced CH₄ uptake capacity by altering the biotic and abiotic properties of soil, with the CH₄ uptake rate in the land use following the order shrubland > woodland > cropland. Compared to the cropland, higher CH₄ uptake rates in afforestation land were related to higher quantity and quality of soil organic matter (i.e., high soil organic carbon and labile carbon(C) content, and low C : N (nitrogen) ration), and larger microbial biomass. Meanwhile, the CH₄ uptake rate was negatively correlated with inorganic N concentration, suggesting the high inorganic N concentration in the cropland possibly inhibited the CH₄ uptake rate. The isotope fractionation factor further provided evidence for the CH₄ uptake, as a lower fractionation factor corresponded to a higher oxidation rate in afforested land. 

Results suggest that change in soil properties (reducing N inputs and increasing C inputs simultaneously) through afforestation is critical for enhancing CH₄ uptake capacity. 

This research was financially supported by the National Natural Science Foundation of China and the "Strategic Priority Research Program B of the Chinese Academy of Sciences”. Results have been published in Soil Biology and Biochemistry entitled “Afforestation enhancedsoilCH₄uptake rate in subtropical China: evidence from carbon stable isotopeexperiments”. 

Temporal variations (a, c) and annual average (b, d) in CH₄ uptake rates and carbon isotope fractionation factor under different land use types (Image by CHENG’s group)