Decades of coffee plantation alters soil methane uptake and soil organic carbon pools in China

The conversion of forest into coffee plantation through deforestation has become one of the main land use changes in tropical region, yet its impact on soil organic carbon (SOC) and methane (CH4) uptake remains unclear, leading to uncertainties in estimating carbon fluxes in tropical area. The main coffee planting areas in China and the adjacent forests were selected to explore the effects of forest-to-coffee conversion and coffee stand ages on SOC and CH4 uptake. We conducted our study by comparing coffee plantations of varying ages to the nearby forests within the same area. We treated the different-aged coffee plantations as our experimental groups and used the forests as our control groups. This paired comparison allowed us to exclude external factors such as climate, soil type, and vegetation differences, ensuring that our analysis focused on the effects of stand age alone. The 25-year, 43-year, and 55-year coffee plantations reduced SOC by 51%, 66%, and 65% compared to nearby forests, while soil microbial biomass carbon decreased by approximately 60%. Coffee stand age influenced ambient CH4 uptake significantly: soils in 43- and 55-year-old coffee plantations and natural forests acted as CH4 sinks, while the 25-year-old stand showed weak CH4 emission. In 25-year, 43-year, and 55-year coffee plantations, the CH4 uptake rates were 87%, 54%, and 65% lower, respectively, compared to the CH4 uptake rates in the natural forests nearby. Soil moisture, inorganic nitrogen content, and CH4 monooxygenase (MMO) activity were the main factors affecting CH4 uptake rates across land uses in the ambient CH4 background. Further CH4 metabolism indicated a close relationship between ambient CH4 uptake, CH4 oxidation, and methanogenesis pathways. Our study highlights the reduction of SOC pools in coffee plantations in China is accompanied with the reduction of CH4 uptake and changed metabolism of CH4-oxidizing microorganisms.