Tractor-induced soil compaction drives nitrous oxide emissions

Machinery-induced soil compaction adversely affects soil physical properties and enhances nitrous oxide (N₂O) emissions. While machinery traffic creates substantial spatial heterogeneity in soil structure, its contribution to field-scale N 2 O remains insufficiently quantified. This study aimed to (i) quantify N 2 O emissions from wheel-compacted versus non-compacted soil, (ii) evaluate how repeated traffic changes the soil structure, and (iii) assess field-scale N 2 O emissions. An observational field experiment was conducted from August 2024 to April 2025 at the SmartField Super Site A, Denmark, on an Oxyaquic Argiudoll soil. Grass-clover was undersown in spring barley, and the field remained under grass-clover throughout the experiment without tillage. Ten automated flux chambers were installed on wheel-compacted and adjacent non-compacted areas. Further, 30 undisturbed soil cores from 0 to 5 cm depth were analyzed for soil physical properties, including micro-CT-derived pore structure. Machinery traffic significantly increase bulk density from 1.36 ± 0.02–1.53 ± 0.02 g cm⁻³ and reduce relative gas diffusivity from 0.066 ± 0.006–0.019 ± 0.003. The maximum N₂O flux was 1300 µg N₂O–N m⁻² h⁻¹ in the wheel-compacted area, approximately one order of magnitude higher than non-compacted area (133 µg N₂O–N m⁻² h⁻¹). Cumulative fluxes over the observation in the wheel-compacted area were approximately 4.8 times higher than in the non-compacted area. Accounting for wheel-compacted areas increased the field-scale estimate from 0.32 to 0.50 kg N₂O–N ha⁻¹. Excluding wheel-compacted areas, cumulative N₂O emissions would have been underestimated by approximately 36%. We postulate that field-scale N 2 O emissions are likely underestimated when machinery-induced soil compaction is disregarded.