Effects of afforestation on soil nitrous oxide emissions in a subtropical montane agricultural landscape: A 3-year field experiment

Afforestation, through the conversion of upland cropland to forest, is of great significance to land use change in montane (hilly) agricultural landscapes worldwide. Such afforestation is implemented to improve soil and water conservation and facilitate terrestrial carbon sequestration. However, particularly for subtropical and tropical regions, the effects of afforestation on soil N2O emissions have not been well investigated. Therefore, a three-year field experiment was conducted to simultaneously monitor N2O emissions from three paired sites of afforestation with cypress (Cypressus funebris) and adjacent cropland under a wheat-maize rotation system. The experiment was carried out in a subtropical montane agricultural landscape in southwest China. In both forest and cropland ecosystems, the N2O emissions exhibited a pronounced spatial and temporal variability. These variations in N2O emissions can be well explained by the spatiotemporal dynamics of environmental variables, such as soil temperature, WFPS, soil NH4+ or NO3− availability, because these environmental variables correlated significantly with the N2O emissions across different experimental sites and years. The average annual N2O fluxes were 2.69 kg N ha-1 for cropland and 0.13 kg N ha-1 for afforestation. It is noteworthy that the annual N2O fluxes for afforestation found in the present study constitute one of the lowest recorded N2O fluxes for unfertilized forest ecosystems globally. Overall, across all experimental sites and years, afforestation with cypress (Cypressus funebris) stands significantly decreased N2O fluxes by over twenty times relative to the adjacent cropland. This outcome suggests that afforestation could be an effective mitigation strategy for soil N2O emissions in a subtropical montane agricultural landscape.