Contrasting Net Ecosystem Carbon Dynamics for Managed Cool-Season Forages.

Grasslands have a role as carbon sinks in mitigation of global warming.  Information exists on the sink-strength of  rangelands, but little is available on management impacts or processes affecting C-sequestration for tame cool-season grasslands in North America.  In previous research (State College, PA) we showed that net ecosystem C exchange (total NEE) (eddy covariance system) in a cool-season grass ecosystem was due to high CO₂ uptake (daytime NEE) during May and June. Even though day time CO₂ (daytime NEE) uptake occurred from April to November it was overcome by respiratory processes (indicated by night time NEE) after initial growth.   Our objective was to follow day time, night time and total NEE accumulated monthly during the growing and dormant seasons for a meadow bromegrass sward in a contrasting short season cool environment at Lacombe, AB.  A Bowen ratio energy balance system (BREB) was used to determine comparable CO₂ flux parameters over a fertilized sward from 2003 until 2009. In AB positive day time NEE occurred from mid April to October and positive total NEE in May, June, August and September (mean 141 d yr^-1).  Night time NEE, peaked in August (-3.25 g C m^2-d^-1) and decreased from August to September (-2.45 g C m^2-d^-1), but daytime NEE remained constant (3.45 g C m^2-d^-1). A Q₁₀ value of 2.2 for AB compared to 1.8 for PA described the exponential relationship of night time NEE (ecosystem respiration) with temperature. Soil organic-C concentration (0-5 cm), which can affect respiration rate, was higher for AB than PA (50 vs. 36 mg C g^-1 soil). Late summer daytime NEE was large enough to offset high night time NEE in AB. Night time air temperature may have enhanced respiration in summer more in PA than AB.   PA average minimum air temperature for the June to Sept. period was 14.5 ^o C compared to 7.5 ^o C for AB.