Tue, Aug 16, 2022: 4:30 PM-4:45 PM
515B
Background/Question/MethodsThe 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/ConclusionsHigh-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.
Results/ConclusionsHigh-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.