Dynamics of profiles of soil greenhouse gases in a topographically variable landscape in western Canada

Greenhouse gas emission studies from temperate soil have received much attention in recent years, particularly freeze-thaw emission and their association with soil conditions. Field and laboratory experiments were conducted in the undulating landscape elements (Upper, Middle, Lower and Riparian) at the Manitoba Zero-Tillage Research Association's farm, to determine the association between seasonal and landscape variation in soil greenhouse gas concentration profiles, greenhouse gas surface emissions and soil conditions. Profile greenhouse gas concentrations, surface emission and soil conditions were monitored from August to November 2005 and then from March to August 2006. Highest nitrous oxide (N2O) profile concentrations (287.3 muL L-1 at 15 cm depth) and highest N2O surface emission (0.1 mug N m -2 s-1) was recorded in the Lower landscape element. Soil methane (CH4) concentrations ranged between 0.5 and 2,587 muL L-1 and were highest during freeze-thaw period for the Riparian element at 15 cm depth. The CH4 emission was highest in the Riparian (1.2 mug C m-2 s-1) followed by the Lower element (1.2 mug C m-2 s-1). The CH4 concentration was variable in all sections where the SO4 -2 levels were high. Carbon dioxide (CO2) concentrations increased with depth in all the landscape elements and were highest in the Riparian element (225,000 muL L-1 ) at 65 cm depth and unaffected by freeze-thaw but elevated during the cropped period. The CO2 emission was highest in the Riparian element and increased during the cropped period. Oxygen concentrations were highest in the Upper and Middle elements and lowest at the 65 cm depth in the Riparian element. Significant correlations were found between profile greenhouse gas concentrations and surface emission, soil moisture and temperature at all depths in the four landscape elements. The estimated N2O and CH4 profile emission values derived from greenhouse gas profile concentrations were closer to measured chamber emission values after normalization with CO2 surface emission in all elements and periods except the freeze-thaw period. A laboratory investigation with intact frozen and unfrozen soil cores obtained from the study site revealed that N2O emissions from unfrozen deeper soils were negligible and the frozen surface (0-5 cm) and shallow depth soils (10-15 and 30-35 cm) recorded highest N2O emissions during thaw events. In summary, the Lower landscape element had greater field N2O emissions, profile concentrations and intact soil core freeze-thaw emissions. Further, the results of this study provide more evidence that frozen surface soil and not unfrozen deeper soil is the source of field emissions of N2O during spring thaw.