After eleven years of exposure to elevated atmospheric CO₂ (ambient + 350 µmol CO₂ mol^-1), a scrub-oak forest has consistently maintained greater net primary productivity (NPP) compared to ambient treatments, despite inherent severe N limitation at the site.  We hypothesized that forest soils have alleviated potential N constraints by liberating more N in soil organic matter and through increased exploitation of deep sources of inorganic nitrogen.  We employed a suite of assays performed in the sixth and eleventh year of a long-term atmospheric CO₂ enrichment experiment, designed to address N dynamics in the entire soil profile, which included: extractable inorganic N, microbial respiration, and potential C and N mineralization.

Results/Conclusions

In the eleventh year, we found significantly greater microbial respiration (F = 10.03, P ≤ 0.01) under elevated CO₂, with the strongest differences at 0-100 cm.  However, field extractable N concentrations were less (F = 3.12, P = 0.096) at all depths to 190 cm under elevated CO₂ with the strongest differences at 10-130 cm.  Conversely, net N mineralization, although not significant considering the entire profile (F = 0.58, P = 0.460), tended to greater under elevated CO₂ at all depths to 190 cm and was significantly greater at 30-130 cm.  In contrast, in the sixth year, differences in field extractable N and net N mineralization were constrained to the top 30 cm, and there were no differences in microbial respiration.  Similar net N mineralization rates, but lower extractable inorganic N concentrations in elevated CO₂ treatments suggests more intensive utilization of inorganic N in almost the entire soil profile.  Enhanced microbial activity may promote a sustained supply of inorganic N in elevated CO₂ treatments.  Also, since the sixth year of this long-term study, exploitation of deep inorganic N appears to be increasingly important in avoiding increased plant N limitation, maintaining greater NNP under elevated CO₂.