Effects of Organic Fertilizer Substitution for Chemical Fertilizer Nitrogen and Limited Irrigation on Soil Carbon Emissions in Spring Wheat Fields

The Hetao Irrigation District in Inner Mongolia is a major spring wheat production region in China. To synergize high wheat yield, water conservation, and carbon emission reduction in this region, a 2023 and 2024 field experiment was conducted. This study systematically analyzed the effects of organic fertilizer substitution for chemical nitrogen (T1:0%, T2:25%, T3:50%, T4:75%, T5:100%) on soil carbon emissions dynamics and carbon footprint of wheat fields, under two irrigation regimes: water-saving irrigation (twice at jointing and heading stages, 2W) and conventional irrigation (four times at tillering, jointing, heading, and grain-filling stages, 4W). The results showed that during the wheat-growing season, soil CO₂ emission rate exhibited a single-peak trend (peak at flowering stage), while cumulative soil CO₂ emission showed a "decrease-increase-decrease" pattern (peak at jointing to heading). At different growth stages, both CO₂ emission and its rate increased with higher organic fertilizer substitution ratios, and were higher under 4W than 2W. Irrigation and substitution treatments significantly affected the total carbon emissions, carbon sequestration, and carbon footprint: total emissions increased with substitution ratios, while sequestration and footprint first increased then decreased; all three indices were higher under 4W than 2W. Regression analysis revealed that maximum net carbon budget was achieved at 21.6-31.7% substitution (1402.3-1879.9 kg ha^-1) under 2W, and 31.0-33.8% substitution (2295.5-2822.0 kg ha^-1) under 4W. In conclusion, water-saving irrigation (900 m³ ha^-1 per application at jointing and heading stages) combined with an optimal organic-nitrogen ratio (1008.0 kg ha^-1 organic fertilizer, 193.1 kg ha^-1 chemical nitrogen) effectively coordinates water conservation and carbon emission reduction. This study provides a basis for synergizing these goals in Hetao's wheat production.