Irrigated saline-alkaline croplands exhibit N2O emission factors substantially exceeding IPCC defaults due to enhanced asymmetric denitrification: Three-year field evidence for emission factor refinement

Saline-alkaline soils encompass approximately 1.1 billion hectares globally, yet no dedicated IPCC emission factor exists for these pedogenically distinct croplands. Because alkaline pH and the wet–dry cycling imposed by drip fertigation both bias N pathway products toward N 2 O rather than N 2 , applying generic defaults is expected to underestimate actual emissions in national inventories of nations with extensive salt-affected agricultural land. We measured N 2 O fluxes, soil organic carbon (SOC) stocks, mineral nitrogen dynamics, 15 N isotope source partitioning, and denitrification enzyme activities over three consecutive years (2021–2023) on a Haplic Solonchak (EC 4.2 dS m⁻ 1 , pH 8.6) in the Tarim Basin, China, across five nitrogen rates (0–300 kg N ha⁻ 1 ) under drip-fertigated silage maize. The slope-derived emission factor was 2.41 ± 0.18% (95% CI: 2.05–2.77%), and the per-treatment mean was 2.79 ± 0.45%, both substantially exceeding the IPCC Tier 1 default (1.0%). Enzyme assays revealed a 47% decline in the N 2 O reductase activity ratio across the N gradient alongside a 2.8-fold increase in potential denitrification rate, indicating an enhanced functional asymmetry between rapidly induced upstream denitrification reductases and the more slowly assembled terminal N 2 O reductase under repeated wet–dry cycling. 15 N site preference analysis corroborated denitrification-pathway dominance (estimated 68% of N 2 O). Despite complete aboveground biomass removal, silage maize maintained a net carbon sink (ΔSOC = 0.48 Mg C ha⁻ 1 yr⁻ 1 ), supported by substantial belowground carbon inputs (1.31–2.47 Mg C ha⁻ 1 yr⁻ 1 ). The net greenhouse gas balance, including estimated indirect emissions from NH 3 volatilization, transitioned from climate benefit to climate cost at approximately 195 kg N ha⁻ 1 — roughly 19% below the regional recommendation — and optimizing N to 180 kg ha⁻ 1 maintained 94% of maximum yield while delivering a net benefit of 230 kg CO 2 -eq ha⁻ 1 yr⁻ 1 . These site-level findings provide a candidate emission-factor value for irrigated saline-alkaline croplands (EC > 4 dS m⁻ 1 , pH > 8.0) in the range 2.5–3.0% to enter future multi-site syntheses, pending validation across the heterogeneous saline-alkaline envelope.