In-soil CH4 partial pressure and diffusive emissions in flooded soils: effects of labile carbon and microbial inoculation

Background and Aims: Rice paddies are a major source of atmospheric methane (CH ₄ ), yet screening root traits for low-emission breeding requires systems in which soil CH ₄ partial pressure ( p CH ₄ ) can be measured and manipulated independently of root exudate supply. We introduce a plant-free pot system using silicone coils to sample in-soil p CH ₄ and deliver O ₂ to the soil, and evaluate whether p CH ₄ predicts diffusive CH ₄ emission and whether methanogen abundance predicts p CH ₄ . Methods Flooded sand-loam soil was amended with starch or inoculated with methanogen-rich lake sediment to span a tenfold range of mcrA gene copy numbers. In-soil p CH ₄ was monitored over 117 days using permanently installed silicone coils. Diffusive CH ₄ emission was measured by static chamber, and oxygenation was imposed by purging coils with air or pure O ₂ for 72 h. Results In-soil p CH ₄ predicted diffusive emission only below approximately 16 kPa, above which bubble formation decoupled the two variables. Methanogen abundance did not predict p CH ₄ or diffusive emission beyond a low baseline; labile carbon supply was the controlling factor. Oxygenation reduced p CH ₄ to one third regardless of whether air or pure O ₂ was supplied. Conclusion The silicone coil system may isolate root gas exchange traits from substrate effects of root exudates, identifying root surface area and barrier strength as priority screening traits. Measurements must be made before the soil gas phase saturates, as free gas formation decouples diffusive chamber flux from CH ₄ production, and ebullition must be quantified to close the CH ₄ mass balance.