Afforestation and deforestation enhanced soil CH4 uptake in a subtropical agricultural landscape: Evidence from multi-year and multi-site field experiments

The impact of afforestation and deforestation on the carbon cycle and carbon sequestration in agricultural landscape has been well studied, while the direction and magnitude of the effects on soil CH₄ fluxes remain uncertain in particular in the subtropical region. Thus, multi-site and multi-year field experiments were conducted to measure soil CH₄ fluxes from an afforestation chronosequence (cropland [wheat-maize rotation], 15-year old forest, 20-year old forest and 30-year forest) and a deforestation chronosequence (secondary forest, grassland, cropland without fertilization and cropland with fertilization [wheat-maize rotation]) in a subtropical agricultural landscape from 2012 to 2017. The soil at all land uses functioned exclusively as a sink for atmospheric CH₄ through the whole experimental years. Soil CH₄ uptakes showed great seasonal and inter-annual variations along with those of temporal patterns of soil environmental variables. At the afforestation chronosequence, annual CH₄ uptake rates averaged 1.37, 1.68, 1.80 and 2.97 kg C ha⁻¹ yr⁻¹ for cropland, 15-year old forest, 20-year old forest and 30-year old forest. Compared to cropland, afforestation increased annual CH₄ uptake by 23 to 117%. Soil CH₄ uptake decreased with increasing soil content, soil NH₄⁺ content and soil NO₃⁻ content but increased with increasing soil DOC content at the afforestation chronosequence (P < 0.05). At the deforestation chronosequence, annual CH₄ uptake rates were 1.37, 1.70, 1.77 and 2.01 kg C ha⁻¹ yr⁻¹ for secondary forest, grassland, cropland without fertilization and cropland with fertilization. Compared to secondary forest, deforestation increased annual CH₄ uptake by 24 to 47%. Soil CH₄ uptakes were negatively correlated with soil water content and positively correlated with soil NO₃⁻ content. We conclude that both afforestation and deforestation have the potential to increase the sink capacities of atmospheric CH₄ in the subtropical agricultural landscape and consequently provide the negative feedbacks to climate system.