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 (NO). Beyond nitrogen (N) availability, cover crop residue decomposition may accelerate heterotrophic respiration to limit soil O availability, hence promote NO emissions from denitrification under sub-optimal water-filled pore space (WFPS) conditions that are typically not conducive to large NO production. We conducted a 21-day incubation experiment to examine the effects of contrasting cover crop residue (grass vs legume) decomposition on soil O and biogeochemical changes to influence NO and CO emissions from N 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 NO emissions under both WFPS conditions. In the absence of cover crop residues, the N fertilizer effect of NO was only realized under 80% WFPS, whereas it was comparable to the control under 50% WFPS. The NO peaks under 50% WFPS coincided with soil O depletion and concomitant high CO emissions when cover crop residues were mixed with N fertilizer. While N fertilizer largely contributed to the total NO emissions from the cover crop treatments, soil organic matter and/or cover crop residue derived NO had a greater contribution under 50% than 80% WFPS. Our results underscore the importance of NO emissions from cover crop-based fertilized systems under relatively lower WFPS via a mechanism of respiration-induced anoxia and highlight potential risks of underestimating NO emissions under sole reliance on WFPS.