R. Howard Skinner, Pasture Systems and Watershed Management Research Unit, USDA-ARS, University Park, PA
Increasing nitrogen fertilization of a mature cool-season pasture increased annual photosynthetic C uptake (GPP) and forage yield but also increased ecosystem respiration (Re), such that net ecosystem exchange (NEE) and soil C sequestration were not affected by the increased fertility. A nine-year study monitoring carbon dioxide flux at two pasture sites examined in detail the relationship between photosynthesis and respiration under a range of environmental conditions. As expected, forage yield was highly correlated with GPP (P = 0.0006). However, yield was also positively correlated with Re ((P = 0.0004). No significant relationship existed between GPP or Re and NEE. Regressing Re against GPP yielded an equation with y-intercept near zero and slope of -0.96 (r2 = 0.87, P < 0.0001) (according to meteorological sign convention, GPP is negative and Re is positive). In all cases, only a single equation was necessary to describe both pastures. Although the results suggest that GPP should exceed Re as productivity increased, the difference was so small that NEE would only be a slight sink of about -80 g CO2 m-2 yr-1 for the most productive year of the study.