Contrasting moisture responses of greenhouse-gas fluxes between ridge and furrow positions in a ridge–furrow plastic-mulching system

Ridge–furrow plastic-mulching systems are characterized by coexisting ridge and furrow microenvironments with contrasting soil hydrothermal conditions. However, whether these spatially adjacent positions exhibit different greenhouse gas (GHG) response relationships with environmental drivers remains insufficiently understood. Here, we investigated whether ridge and furrow positions within the same ridge–furrow system represent distinct observation units with different hydrothermal characteristics and gas-specific responses to soil moisture and temperature. We conducted repeated field measurements at 11 tobacco sites and quantified CO₂, CH₄, and N₂O fluxes from fixed ridge and furrow positions, together with soil temperature and volumetric water content at 5 cm depth. Linear mixed-effects models and paired-position analyses were used to evaluate positional contrasts and environmental response relationships. Ridge positions showed higher soil temperature than furrow positions, whereas volumetric water content did not differ significantly between positions. Differences in mean flux magnitude were most evident for CO₂, with higher ridge fluxes than furrow fluxes (model-estimated ridge–furrow difference: 71.44 mg CO₂ m⁻² h⁻¹), while CH₄ and N₂O positional contrasts were weaker and less consistent. In contrast, ridge and furrow positions exhibited contrasting moisture-response relationships for all three gases. CO₂ flux showed a significant positive response to soil moisture on ridges but no significant response in furrows. For CH₄, increasing soil moisture significantly weakened net CH₄ uptake in furrows, whereas no significant moisture response was detected on ridges. For N₂O, furrows showed a significant negative moisture-response relationship, whereas ridges showed a positive but non-significant response. Positional differences in temperature-response slopes were not statistically resolved across gases, whereas moisture-response contrasts were statistically supported. These results indicate that ridge–furrow plastic-mulching systems contain functionally distinct microenvironments in which GHG fluxes exhibit gas-specific moisture-response relationships. Ridge and furrow positions should therefore not be automatically treated as interchangeable observation units when monitoring, interpreting, and scaling GHG fluxes in ridge–furrow systems.