Heavy precipitation triggers pulse N2O emission with high N2O/N2 ratios from drained and/or climatic drought-stressed peatlands

Background/Question/Methods

Peatlands, covering 3% of land area, not only store one-third of all terrestrial soil carbon but also contain vast amounts of organic nitrogen, amounting to approximately 12-21% global soil N (12-20 Pg N). Generally, peatlands are important sinks of nitrogen with low mineralization rates because of waterlogged conditions. However, this huge pool of organic nitrogen is threatened by more frequent extreme hydrologic events–prolonged droughts and heavy storms–associated with climate change. Some studies showed that nitrogen mineralization rate (NMR) can be up to 500-600 kg ha^-1 yr^-1 in drained histosols and low pH significantly increases N₂O emission. Therefore, a huge N₂O pulse might be emitted following storms in drained and/or drought-stressed peatlands. Most studies of drought/drainage effects on peatlands focus on carbon processes; evidence of the potential impact of storms/rewetting on episodic N₂O emission in drained and drought-stressed peatlands is very poor. In this study, we collected intact peat monoliths from natural, drained and hydrologically restored pocosin peatlands in the southeastern U.S and conducted a microcosm experiment simulating 15-month drought and 3-month rewetting to investigate whether: 1) droughts increase nitrogen mineralization, 2) storms pump out mineralized nitrogen as N₂O and 3) past hydrologic management affects these processes.

Results/Conclusions

During the drought we found significant temporal NMR variations and strong hydrologic management effects. The highest NMR occurred in the third month; after that it dropped. Monoliths from the drained site released nitrogen at a rate of 490±110 kg ha^-1 yr^-1, which was about 5 times higher than the restored site. Nitrate from the drained site was about 16 fold higher than the restored site.

After the drought and before the rewetting, N₂O emissions from all monoliths were similarly low (5.5–7.8 µg m^-2 h^-1). After the rewetting, the emissions increased rapidly to 2355.6±487.8, 1310.1±237.9, and 1252.1±625.3 µg m^-2 h^-1 within ten days in the monoliths from drained, natural and restored sites, respectively. With the decreasing NO₃^- concentrations, N₂O emissions quickly reduced after one month. During the rewetting, monoliths from the drained site emitted 194.3±51.6 kg N₂O km^-2, which was 3 and 4 times higher than natural and restored sites, respectively. The first-month emission contributed 80-87% of total 3-month emission, and emitted N₂O covers 38.3-66.5% of total nitrogen loss.

Our results indicate that drought-rewetting events trigger pulse N₂O emission with a high N₂O/N₂ ratio from peatlands and that hydrologic history has substantial effects on the magnitude of this process.