Cover crop residue decomposition triggered soil oxygen depletion and promoted nitrous oxide emissions

Cover cropping is a promising strategy to improve soil health, but it may also trigger greenhouse gas emissions, especially nitrous oxide (N₂O). Beyond nitrogen (N) availability, cover crop residue decomposition may accelerate heterotrophic respiration to limit soil O₂ availability, hence promote N₂O emissions from denitrification under sub-optimal water-filled pore space (WFPS) conditions that are typically not conducive to large N₂O production. We conducted a 21-day incubation experiment to examine the effects of contrasting cover crop residue (grass vs legume) decomposition on soil O₂ and biogeochemical changes to influence N₂O and CO₂ emissions from ¹⁵N labeled fertilized soils under 50% and 80% WFPS levels. Irrespective of cover crop type, mixing cover crop residue with N fertilizer resulted in high cumulative N₂O emissions under both WFPS conditions. In the absence of cover crop residues, the N fertilizer effect of N₂O was only realized under 80% WFPS, whereas it was comparable to the control under 50% WFPS. The N₂O peaks under 50% WFPS coincided with soil O₂ depletion and concomitant high CO₂ emissions when cover crop residues were mixed with N fertilizer. While N fertilizer largely contributed to the total N₂O emissions from the cover crop treatments, soil organic matter and/or cover crop residue derived N₂O had a greater contribution under 50% than 80% WFPS. Our results underscore the importance of N₂O emissions from cover crop-based fertilized systems under relatively lower WFPS via a mechanism of respiration-induced anoxia and highlight potential risks of underestimating N₂O emissions under sole reliance on WFPS.