Enhanced efficiency nitrogen fertilizers suppress soil methane uptake from a mulch drip-irrigation system in arid...

Limited information is available on the impact of Enhanced Efficiency Nitrogen Fertilizers (EENFs) on soil methane (CH₄) uptake and its relationship with diffusion in dryland farming. To address this, we conducted a two-year cotton (Gossypium hirsutum L.) field experiment under drip irrigation in arid northwestern China. The study assessed the effects of two EENFs (ESN and SuperU) and conventional urea, applied at 120 and 240 kg N ha^-1, on surface CH₄ uptake and profile diffusion. ESN is polymer-coated urea, and SuperU is a stabilized N fertilizer containing urease and nitrification inhibitors. ESN and SuperU were fully side-banded at pre-plant due to their low solubility and tendency to float, while urea, with its higher solubility, was applied via fertigation. We measured surface CH₄ uptake using static chambers and CH₄ concentrations at 5, 15, 30, and 60 cm soil depths using in-situ diffusive sampling. CH₄ diffusion rates were calculated using the concentration gradient method. Soil ammonium, (NH₄⁺) and nitrate (NO₃^-) nitrogen concentrations, moisture, and temperature were determined. The results showed that, compared to urea, applying ESN and SuperU reduced cumulative surface CH₄ uptake (30.1 %-42.7 %) and increased soil NH₄⁺ (30.1 %-33.5 %) and NO₃^- (20.6 %-46.0 %) concentration. Soil CH₄ concentration decreased with depth, with the highest CH₄ oxidation in the 15-30 cm layer. CH₄ concentrations in the 5-50 cm layer and diffusion rates in the 0-60 cm layer were positively and negatively correlated with soil moisture, respectively, but weakly influenced by N fertilizer treatment. Dry-wet alternation and low soil moisture limited the response of CH₄ concentration and diffusion to changes in mineral N. Additionally, weak correlations were found between surface CH₄ uptake rates and profile diffusion rates across all treatments. The reduced surface CH₄ uptake with ESN and SuperU was mainly due to increased NO₃^- concentrations. This study highlights the risk of reduced CH₄ uptake with ESN and SuperU and improves understanding of CH₄ concentration and diffusion in drip-mulch irrigation systems.