Carbon dioxide cycling through the snowpack, implications of change Gareth Crosby

CO 2 up north Carbon dioxide (CO 2 ) –largest component of carbon cycling between the biosphere and the atmosphere. –approximately 45% of total greenhouse forcing –industrial revolution = more than a 30% increase in atmospheric concentration Approximately 40% of the world’s soil carbon is stored in high-latitude ecosystems.

Seasonality CO 2 exchange during the growing season only represents 4 to 5 months of the year at most. growing season CO 2 data alone have been found to underestimate the actual magnitude of CO 2 flux from northern soils Cold season CO 2 emissions through the snowpack can contribute as much as 60%-81% of annual release.

Soil thermal dynamics influence the exchange of CO 2 between terrestrial ecosystems and the atmosphere. Field-based studies indicate both the importance of winter decomposition and freeze-thaw dynamics in the annual carbon budget in northern ecosystems.

Snow Characteristics: Insulation covers 44 to 53% of the northern latitudes during most of the year Insulation = structure + depth of snowpack Deeper = greater insulation Less packed = greater insulation

Snow Characteristics: CO 2 release Snowpack properties –depth, density, and layering dictate the rate and amount of CO 2 evolution from the soil. Release = structure and depth layering = compactness, ice lenses and crusts trapping the gas below lenses makes calculating fluxes difficult. In a homogeneous snowpack, gas transport by diffusion through the snow profile is linear.

Snow Characteristics: CO 2 release Intense wind and high temperature gradients can cause mass transport of gas by convection Windpumping –three types of windpumping: barometric pumping turbulent pumping topographic pumping.

Soil structure and composition also influence rate and distribution of CO 2 production and movement in the soil layer. Porous soils = more CO 2 to move upwards –frozen soils will become better traps for CO 2 produced in lower soil layers soils with high levels of organic matter will produce more CO 2 than soils depleted in organic matter at a given temperature.

Models Most models use monthly air temperature in the simulation of of seasonal dynamics of net primary production and decomposition. But many of these models have been predicting substantially different results

Model inputs Hydrological Dynamics Biogeochemical Dynamics Soil Thermal Dynamics Vegetation Characteristics Snow Depth Snow Properties Soil Temperatures Freeze-thaw Dynamics Version 5.0 of the Terrestrial Ecosystem Model (TEM) Zhuang et al., 2004) Based on WBM Based on TEM Based on a STM

Freeze-thaw In the temperate soils are prone to freezing. microbes are killed by freezing snow cover regulates temperatures that enable microbes that survived the freezing in lower soil layers to multiply use the dead microbes in the upper layers to produce a pulse of CO 2 under the snow though early season development of snowpack allows production of CO 2 to continue for most of the winter, sites that undergo hard freezes and then are covered with a consistent snowpack and allowed to thaw produced the highest fluxes.

Implications Lower average snowpack will increase the rate and depth of freezing in northern soils with longer lasting effects than simple wintertime gas fluxes. Frozen soils will decrease the production and movement of CO 2 up from the soil thus possibly becoming less of a source and more of a sink for CO 2 during the winter months.

Further Implications At the same time the freezing and melting of soils in the spring could produce a larger spring pulse of CO 2 emission as microbes that lasted the winter in deeper soils began to use the dead microbes as substrate for decomposition. Depending on the degree to which each of these processes affect the production or trapping of CO 2 northern ecosystems could become less of a source in the winter and spring or less of a source in the winter but more of a source in the spring