COS 125-3 - Denitrification response to warming in late summer and early spring at a Northeast temperate forest

Background/Question/Methods

Denitrification can be an important pathway for the loss of nitrogen (N) from ecosystems, but the effect of global warming on denitrification and gaseous losses of N₂O and N₂ from temperate forests is uncertain. Denitrification rates generally increase with increasing temperature, but the magnitude of that change is extremely variable and can be countered by drying and substrate limitations. Here, we tested whether warming increased denitrification fluxes in summer or early spring at a northeast temperate forest. We collected soil cores from sites along an elevation gradient spanning ~2°C at the Hubbard Brook Experiment Forest (HBEF) in NH during late summer 2020 and at snowmelt in April 2021. We imposed warming and other treatments on those cores in the lab and measured N₂O and N₂ fluxes at 20% oxygen using an intact core-based Nitrogen-Free Air Recirculation Method (NFARM). In late summer, we tested how warmer temperatures and drier soils can have opposing effects on denitrification rates. During snowmelt, we were interested in testing how high soil moisture and quickly warming temperatures can lead to pulses of N gases.

Results/Conclusions

Different environmental conditions during the two seasonal periods, late summer and early spring snowmelt affected response to our imposed warming treatments. In late summer, there was no response to either warming by ~2°C or wetting (~20%). At snowmelt, warming treatments in the lab suppressed N₂ fluxes relative to controls, while warming in the field produced marginally larger N₂O fluxes on a warmer date than a cooler date. There were no significant correlations between either N₂O or N₂ fluxes and field temperatures, experimental lab temperatures, or gravimetric water content. The only environmental factor that correlated significantly with either N gas flux was soil extractable nitrate concentration, which correlated positively with N₂O fluxes in August 2020. Though climate warming may not always directly affect denitrification rates at HBEF, the combination of effects on soil moisture and nitrate pools in the summer and rapid temperature changes in early spring may be important for future losses of N₂O and N₂.