The effects of elevated CO2 on nutrient distribution in a fire‐adapted scrub oak forest

Elevated carbon dioxide (CO₂) 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 CO₂ had no effect on any measured soil property except extractable phosphorus (P), which was lower with elevated CO₂ after five years. Anion and cation exchange membranes showed lower available nitrogen (N) and zinc (Zn) with elevated CO₂. Soils in both elevated and ambient CO₂ showed decreases in total C, N, sulfur (S), and cation exchange capacity, and increases in base saturation, exchangeable Ca²⁺, and Mg²⁺ 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 CO₂ 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 CO₂ 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 CO₂ 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 CO₂ or any other factor cannot be accurately assessed in the absence of data on changes in soils.