Comparison of Simultaneous Soil Profile N2O Concentrations and Surface N2O Flux Measurements Overwinter and At Spring Thaw In An Agricultural Soil

In cold climates, after spring-thaw, large emissions of N₂O (often termed “bursts”) have been found to occur from agricultural soils. Of particular importance is determining whether nitrous oxide is produced during freezing and released at thaw, or if it is produced at the onset of thawing. The objectives of this research are to estimate soil N₂O storage and compare changes in soil N₂O storage to the accumulated surface N₂O flux from field-scale measurements. A field experiment was carried out in Ontario from October 2010 to May 2011, where the experimental area consisted of a 150 by 100 m plot (conventional till with residue returned). Surface N₂O fluxes were measured through the use of a trace gas analyzer employing the micrometeorological flux-gradient technique, and were calculated as half-hourly averages. Soil profile measurements were taken in four replicates at depths of 5, 10 and 15cm, consisting of: soil water content (water content reflectometers), soil temperature (thermocouples) and soil surface temperature (infared thermometers, surface only). Soil gas samples were taken manually at each replicate (using soil gas probes at depths of 5, 10, 15, 30 and 60 cm) weekly overwinter with increasing frequency approaching spring-thaw, and were analyzed for gaseous N₂O concentration. Using the water content reflectometer data and moisture content from soil samples the air filled porosity of the soil was estimated, and then soil gaseous N₂O storage was estimated using the soil N₂O concentrations. Using Henry's law for closed systems soil aqueous N₂O storage was estimated, giving total soil N₂O storage, which was found to reach a maximum of 25 mg N₂O m^-2. Upon spring thaw in March, soil N₂O storage decreased rapidly, and two events of heightened flux occurred. During the first event N₂O fluxes at the surface accounted for only 30% of the change in storage, however during the second event a minor change in storage occurred and surface fluxes were 5 times greater than the change in storage. These results suggest the importance of the contribution of the physical release of N₂O to heightened surface flux emissions at the onset of spring thaw. They also suggest the influence of other sinks to soil N₂O storage as surface fluxes did not account for the decrease in storage during the first event. Finally, findings display the decreasing contribution of physical release in time, as emissions during the second event were likely the result of “de novo” production from microbial activity.