Diffusion and transformation of methane within the soil profile and surface uptake in dryland spring maize fields under different fertilizer application depths

It is important to assess the potential uptake of methane (CH4) in dryland farming and its contribution to global warming mitigation by elucidating the migration characteristics of CH4 in the soil profile. Therefore, before planting spring maize in the Loess Plateau region of China in 2019–2020, we applied fertilizer at soil depths of 5, 15, 25, and 35 cm (D5, D15, D25, and D35, respectively). The uptake of CH4 and diffusion flux were determined simultaneously in the soil surface and profile. We found that the dryland soil was a sink for CH4 and the cumulative CH4 uptake amount by soil without fertilization during the maize growth stage was 1.54 kg ha–1. Fertilization inhibited the uptake of CH4 by the soil, but deep fertilization promoted its uptake. The cumulative surface CH4 uptake amounts under D15, D25, and D35 were significantly higher than that under D5, i.e., by 42.4%, 105.6%, and 169.1%, respectively. The CH4 concentration and diffusion flux decreased as the soil depth increased, and CH4 diffused downward in the soil profile. The CH4 consumption rate was greater than the production rate in the 0–20 cm soil layer, whereas the opposite was found in the 20–30 cm layer. The surface CH4 uptake flux was significantly positively correlated with the diffusion flux in the 0–30 cm soil layer. Inorganic nitrogen (N) inhibited the diffusion of CH4 from the surface through the soil profile and its diffusion within the profile. The highest maize yield and biomass were obtained under D25. Therefore, increasing the fertilization depth can promote the uptake of CH4 by the soil in maize fields to alleviate climate change, and a fertilization depth of 25 cm is suitable for balancing the requirements for environmental protection and agricultural production. Our findings provide important insights into the production, consumption, diffusion, and uptake of CH4 in dryland farming areas.