During the first two years of the study, Hungate et al. (1999) reported that nitrogen fixation by the legume, Galactia elliotia., was stimulated by growth in elevated CO2 immediately following the burn. Continuation of this study has now shown that this stimulation was short lived and that during the past three years, nitrogen fixation by this legume has gone from stimulation to inhibition, owing to the combined effects of increased shade and nutrient limitation (Hungate et al. 2004). This finding contrasts with the widely held expectation that elevated CO2 will stimulate biological nitrogen fixation.
Nitrogen uptake by the dominant oaks also initially increased in response to elevated CO2, but the stimulation has declined since 1999, a pattern mirrored but even more pronounced in the uptake of tracer 15N, applied in 1998. Greater accumulation of N in biomass and in the surface organic layer of soil may be causing progressive N limitation in this site, possibly contributing to the low growth response to elevated CO2 observed in 2001. This may have been caused by progressive N limitation, although the subsequent stimulation of carbon assimilation in 2002 runs counter to this explanation. Johnson et al.(2003) reported that elevated CO2 alters the stoichiometry of concentration of all elements although not uniformly.