The Effects of Elevated Co₂ on Nutrient Distribution in a Fire‐adapted Scrub Oak Forest

Elevated carbon dioxide (CO 2 ) caused greater accumulation of carbon (C) and nutrients in both vegetation and O horizons over a 5‐yr sampling period in a scrub oak ecosystem in Florida. Elevated CO 2 had no effect on any measured soil property except extractable phosphorus (P), which was lower with elevated CO 2 after five years. Anion and cation exchange membranes showed lower available nitrogen (N) and zinc (Zn) with elevated CO 2 . Soils in both elevated and ambient CO 2 showed decreases in total C, N, sulfur (S), and cation exchange capacity, and increases in base saturation, exchangeable Ca 2+ , and Mg 2+ over the 5‐yr sampling period. We hypothesize that these soil changes were a delayed response to prescribed fire, which was applied to the site just before the initiation of the experiment. In the ambient CO 2 treatment, the increases in vegetation and O horizon C, N, and S were offset by the losses of soil total C, N, and S, resulting in no statistically significant net changes in ecosystem C, N, or S over time. In the elevated CO 2 treatment, the increases in vegetation and O horizon C content outweighed the losses in soil C, resulting in a statistically significant net increase in ecosystem C content. Nitrogen and S contents showed no statistically significant change over time in the elevated CO 2 treatment, however. Comparisons of vegetation contents and soil pools of potassium (K), calcium (Ca), and magnesium (Mg) suggest that a substantial proportion of these nutrients were taken up from either groundwater or deep soil horizons. This study demonstrates that changes in ecosystem C sequestration due elevated CO 2 or any other factor cannot be accurately assessed in the absence of data on changes in soils.