Mechanisms of N₂O production following chloropicrin fumigation

Soil fumigation has recently been shown to affect the greenhouse gas balance by increasing emissions of nitrous oxide (N₂O) following chloropicrin (CP) application. However, the exact mechanisms of this increase were not investigated. The purpose of this study was to elucidate potential mechanisms of CP-induced N₂O production through laboratory incubations using chemical inhibitors (acetylene, antibacterial, antifungal, and oxygen), isotopically labeled ¹⁵N-CP, and pH modifications of a forest nursery soil. Results showed that N₂O production increased by 12.6 times following CP fumigation. Microbial activity contributed 82% to the CP-induced N₂O production, with the remaining 18% from abiotic processes as determined by incubation with sterilized soil. Inhibitor studies suggested that 20% of the N₂O production was from bacteria and 70% from fungi. There were no significant differences in N₂O production following CP fumigation under various levels of acetylene (0, 10, and 10 kPa), suggesting that traditional nitrification and denitification reactions did not significantly contribute to N₂O production following CP fumigation. ¹⁵N labeled studies indicated that 12% of fumigant source N was incorporated into the produced N₂O. No enrichment in N₂ was observed, indicating that N₂O was one of the terminal biotic mineralization products of CP. Production of N₂O is aerobic and production rates increased with increasing oxygen concentrations. Our data strongly suggested that fungal mediated denitrification reactions under aerobic conditions were the primary mechanism for CP-induced N₂O production.