COS 105-5 - Controls on N₂O and N₂ losses from a northern hardwood forest

Background/Question/Methods

The availability of nitrogen limits terrestrial carbon sequestration, yet ecosystem losses of N gases to denitrification are poorly quantified in both field observations and model simulations. Denitrification is the microbial conversion of nitrate to N gases (NO, N₂O, and N₂) under low-oxygen conditions, a process that removes inorganic N from ecosystems and produces a potent greenhouse gas (N₂O). However, measurements of denitrification rates are hindered by technical challenges, particularly for N₂, and by tremendous spatiotemporal variation of substrate (nitrate, organic C) availability and environmental conditions (oxygen, temperature, moisture). We employed field measurements (soil N₂O and CO₂ fluxes), high-frequency field sensors (O₂, temperature, moisture) at several soil depths, lab measurements of N₂ and N₂O fluxes from intact soil cores (N-FARM, or Nitrogen-Free Air Recirculation Method), field (soil moisture) and lab (nitrate, organic C) manipulation experiments, as well as model analyses (Community Land Model-CN5.0) to characterize denitrification-driven N gas losses from the Hubbard Brook Experimental Forest (HBEF) and their environmental controls. We expected that denitrification rates would be greatest in soils with larger nitrate supplies, and would increase after natural and experimental increases in soil moisture that suppressed O₂ concentrations and enhanced substrate availability in both surface and deeper soils

Results/Conclusions

High-frequency O₂ sensors revealed that Hubbard Brook soils, traditionally considered “well-drained,” regularly incur low-O₂ conditions after rainfall and snowmelt events to O₂ levels sufficient to promote denitrification, particularly in deeper mineral soils. N-FARM measurements showed that these soils produce both N₂O and especially N₂, particularly in mineral soils where low-O₂ conditions were more common. N-FARM- and collar-based measurements showed greatest N₂O fluxes from soils with more extractable nitrate supplied internally or delivered from upslope, with more N₂O in the field after rainy than dry days, but little response to experimentally increased soil moisture or temperature. Lab-based substrate amendments to soils from several depths found no response of potential denitrification rates to additions of nitrate or labile C alone, but that rates instead corresponded with initial concentrations of soil extractable nitrate, perhaps due to the relative presence of microbial populations to support these processes. Together results show that environmental conditions sufficient for denitrification occur frequently within Hubbard Brook soils, particularly at depth, and that N gas losses are often constrained by nitrate supply. These results, and associated model analyses with CLM-CN5.0, highlight the importance of nitrate supply and coupled nitrification-denitrification processes in regulating N gas losses from temperate forests.