Effects of land-use history, fertilization, and precipitation on short-term N2O emissions from agricultural soils using open-path eddy flux N2O and static chamber methods.

Nitrous oxide (N2O) is an important greenhouse gas with an atmospheric lifetime of ~ 120 years and a global warming potential ~300 times that of CO2. Atmospheric N2O concentrations have increased from ~270 ppbv during pre-industrial times to ~330 ppbv today. Anthropic emissions are a major source of atmospheric N2O and about half of global anthropic emissions are from the agricultural sector. N2Oemissions from soils exhibit high spatial and temporal variability. Estimation of N2O emissions from agricultural soils is particularly challenging because N2O fluxes are affected by fertilizer type and application rates, land-use history and management, as well as soil biological activity. We studied ecosystem level N2O emissions from agricultural lands using a combination of static chamber methods and continuous N2O exchange measured by a quantum cascade laser-based, open-path analyzer coupled with an eddy-covariance system. We also compared N2O emissions between different static chamber methods, using both laboratory-based gas chromatography (GC) and an in situ quantum cascade (QC) laser for N2O analyses. Finally, we compared emissions estimated by the two static chamber methods to those estimated by eddy-covariance. We examined pre- and post- fertilization N2O fluxes from soils in two no-till continuous corn fields with distinct land-use histories: one field converted from permanent grassland (CRP-C) and the other from conventional corn-soybean rotation (AGR-C). Both fields were fertilized with ~160 kg urea-N ha-1. We compared N2O emissions from these fields to those from an unmanaged grassland (REF). In addition, we examined the potential effect of post-fertilization precipitation on N2O emissions by applying 50 mm of artificial rainfall to the static chambers at all three locations. Measurements of N2O emissions using both GC and QC laser methods with static chambers were in good agreement (R2 = 0.96). Even though average soil N2O fluxes before fertilization were low, they still exhibited high temporal and spatial variability. Fluxes from the CRP-C site were higher than fluxes from the AGR-C site, and fluxes from the REF site were lowest, ranging from 2 - 22, 1 - 3, and ~1 g N2O-N ha-1 day-1, respectively. Post-fertilization fluxes were minor as well due to very dry soil conditions in 2012. However, after applying artificial rain, soil N2O fluxes were distinctly higher in all systems, increasing to 106 - 208 g N2O-N ha-1 day-1 at the CRP-C site, to 36 g N2O-N ha-1 day-1 at Ag-C, and to 5 g N2O-N ha-1 day-1 at the REF site. Fluxes decreased to pre-rain levels 1-2 days after wetting. This single rain event resulted in total emissions of 5, 43, and 251 g N2O-N ha-1 from REF, Ag-C, and CRP-C systems, respectively. A comparison between static chambers and the open-path method at CRP-C system revealed similar diurnal trends in N2O fluxes and similar cumulative N2O-N emissions. Overall, we found a strong relationship between land-use history and soil N2O emissions: soils with higher organic carbon content (CRP-C) exhibited greater fluxes. In addition, we found that N2O emissions increased significantly after a post-fertilization rain event, accounting for a significant proportion of typical total annual emission from these no-till corn fields. We also present the first measurements of ecosystem level N2O fluxes using an open-path N2O analyzer and show the potential of this novel system to study ecosystem level N2O fluxes.