Nitrification and denitrification as sources of nitrous oxide in coastal plain wetlands under contrasting land uses

Background/Question/Methods

Coastal plain wetlands are experiencing more variable hydrologic conditions and accelerated land use change.  Whether through wetland restoration or sea-level rise, microbial nitrification and denitrification in these soils are strongly influenced by altered hydrology and nitrogen availability. While these microbial processes can improve water quality, they also can be major sources of the greenhouse gas nitrous oxide (N₂O).  Because N₂O potentially represents a positive feedback to climate change and an unintended consequence of wetland restoration, estimating and understanding N₂O fluxes in these wetlands is important for global change science and for restoration planning.  Our objective is to examine the mechanisms of N₂O production in a range of coastal wetlands in North Carolina.  We sampled four adjacent sites with contrasting land uses:  two soil types within a 400ha wetland (restored in 2007; Timberlake Farms), a forested wetland, and a drained agricultural field in Tyrrell County, NC.  We estimated N₂O fluxes using static chambers and performed soil assays for nitrification and denitrification potential.  To determine nitrification and denitrification rates, and their relative importance to N₂O emissions under wet and dry conditions, we added ¹⁵NH₄-N and ¹⁵NO₃-N to separate sets of intact cores and measured headspace ¹⁵N₂O and ¹⁵NH₄-N and ¹⁵NO₃-N in soil extracts.  

Results/Conclusions

Field measurements of N₂O flux were highest in the restored wetland loam (RL: 95.5±39.0µg/m²/h; annual mean ± SE) and in the forested wetland muck (FM: 82.7±26.3µg/m²/h), while N₂O fluxes were near zero or negative in the agricultural field loam (AL: -7.1±17.0µg/m²/h) and in the restored wetland muck (RM: 17.6±15.3µg/m²/h).   Nitrification potential was highest in RL soils, while denitrification potential was highest in FM soils.  The ¹⁵N experiment showed significantly higher N₂O emissions from all sites under wet versus dry conditions, with FM soils higher overall.  We found different responses to ¹⁵NH₄-N and ¹⁵NO₃-N tracers by site.  In AL and RL soils, nitrification-derived N₂O was greater than or equal to denitrification-derived N₂O under both wet and dry conditions.  In FM and RM soils, denitrification was the dominant N₂O source.  These patterns are consistent with studies that have shown nitrification to produce more N₂O as oxygen availability decreases and denitrification yielding more N₂O when soils are not fully saturated.  Our results suggest that the more organic wetland soils are currently larger sources of N₂O owing to denitrification.  However, as sea levels rise and biogeochemical development proceeds in restored wetlands, N₂O fluxes from more mineral wetland soils may surpass those from organic soils.