Does deep soil N availability sustain long‐term ecosystem responses to elevated CO 2 ?

A scrub‐oak woodland has maintained higher aboveground biomass accumulation after 11 years of atmospheric CO 2 enrichment (ambient +350 μmol CO 2  mol −1 ), despite the expectation of strong nitrogen (N) limitation at the site. We hypothesized that changes in plant available N and exploitation of deep sources of inorganic N in soils have sustained greater growth at elevated CO 2 . We employed a suite of assays performed in the sixth and 11th year of a CO 2 enrichment experiment designed to assess soil N dynamics and N availability in the entire soil profile. In the 11th year, we found no differences in gross N flux, but significantly greater microbial respiration ( P≤ 0.01) at elevated CO 2 . Elevated CO 2 lowered extractable inorganic N concentrations ( P =0.096) considering the whole soil profile (0–190 cm). Conversely, potential net N mineralization, although not significant in considering the entire profile ( P =0.460), tended to be greater at elevated CO 2 . Ion‐exchange resins placed in the soil profile for approximately 1 year revealed that potential N availability at the water table was almost 3 × greater than found elsewhere in the profile, and we found direct evidence using a 15 N tracer study that plants took up N from the water table. Increased microbial respiration and shorter mean residence times of inorganic N at shallower depths suggests that enhanced SOM decomposition may promote a sustained supply of inorganic N at elevated CO 2 . Deep soil N availability at the water table is considerable, and provides a readily available source of N for plant uptake. Increased plant growth at elevated CO 2 in this ecosystem may be sustained through greater inorganic N supply from shallow soils and N uptake from deep soil.