METHANE FLUX DYNAMICS IN AGRICULTURAL ECOSYSTEMS: ROLE OF EXTREME WEATHER EVENTS AND MOLECULAR MARKERS

Abstract

Methane (CH₄) is the second most potent anthropogenic greenhouse gas after CO₂, with an atmospheric concentration exceeding 1,923 ppb in 2024 — a 160% increase above pre-industrial levels (~722 ppb) — and a global warming potential 27–30 times greater than CO₂ over a 100-year horizon. Agriculture contributes approximately 40% of total anthropogenic CH₄ emissions (~145–155 Tg CH₄ yr⁻¹), with rice paddies alone accounting for 28–32 Tg CH₄ yr⁻¹ (8–10% of global totals) through prolonged anaerobic methanogenesis. Net CH₄ flux from agricultural soils is governed by the balance between anaerobic methanogenesis and aerobic methane oxidation (methanotrophy). Under continuously flooded conditions, soil redox potential (Eh) falls below −150 mV, driving daily emissions of 150–350 mg CH₄ m⁻² day⁻¹, while well-aerated upland soils act as net CH₄ sinks (−0.5 to −3.5 mg CH₄ m⁻² day⁻¹). Extreme weather events — including flooding, drought, and heatwaves — alter these dynamics non-linearly: flooding can accelerate methanogenesis by 200 500% while concurrently reducing methane oxidation efficiency by 40–80%, whereas drought reduces CH₄ production by 50–90% through increased soil aeration. Temperature sensitivity of methanogenesis (Q₁₀ = 2.0–4.2) further amplifies emissions under warming scenarios. At the molecular level, the functional genes mcrA and pmoA are frequently utilized as molecular markers to assess methanogenic and methanotrophic community abundance (10⁶–10⁸ and 10⁵–10⁷ copies g⁻¹ dry soil, respectively) and their functional activity. The mcrA/pmoA ratio serves as a reliable proxy for the source–sink balance, with ratios of 2.0–2.5 under continuous flooding versus 1.2–1.6 under alternate wetting and drying (AWD) — the latter reducing seasonal emissions by ~30%. This review synthesizes current quantitative understanding of how extreme weather events modulate CH₄ flux in agricultural ecosystems and highlights the utility of molecular markers in developing targeted mitigation strategies.