Differential responses in topsoil and subsoil: N2O, CO2, and CH4 dynamics under extreme rainfall

Abstract

The production and diffusion of greenhouse gases (GHG) in soil layers play a crucial role in surface emissions. However, the impact of subsoil GHG dynamics on surface emissions remains largely unclear. Here, differential dynamics between topsoil and subsoil of N₂O, CO₂, and CH₄ were determined during rainfall events in a soil-column experiment. Soil oxygen (O₂), nitrate nitrogen (NO₃^–-N), ammonium nitrogen (NH₄⁺-N), and dissolved organic carbon (DOC) were measured, and gas diffusivity was calculated during experimental period. The peak N₂O concentration in topsoil was 23.8 % higher than in subsoil. Under short-term hypoxic conditions (15.1–15.3 % O₂ concentration), higher NO₃^–-N concentration in topsoil (53.6–57.8 mg kg⁻¹) was key factor influencing pulsed N₂O concentrations compared to subsoil (31.2–42.2 mg kg⁻¹). N₂O downward diffused from topsoil to subsoil, ranging from 0.0299 to 0.0581 kg ha⁻¹ due to nitrogen topdressing. CO₂ concentration in topsoil was 7.4–25.0 % lower than in subsoil, with extreme rainfall events amplifying these differences by a factor of 3.4. Subsoil contributed 11.1–17.6 % of the cumulative CO₂ emissions during rainfall events. In contrast, CH₄ concentrations showed no significant differences between topsoil and subsoil. These findings underscore the importance of understanding GHG dynamics across soil layers, offering new insights into the role of subsoils in GHG emissions.