: The Sensitivity of Nutrient Availability to Changes in Atmospheric CO2 Influenced by Soil Type.

Numerous studies have indicated that increases in atmospheric CO₂ has the potential to decrease nitrogen availability through the process of progressive nitrogen limitation (PNL). The timing and magnitude of PNL in field experiments is varied due to numerous ecosystem processes. Here we examined the responsiveness of nutrient dynamics to changes in atmospheric CO₂ in prairie monoliths from three soil types. The three soil types span from high clay vertisol to sandy loam mollisol. We hypothesized that the high-clay vertisol would be the most responsive soil because it had the lowest soil water potential, which can influence nutrient mineralization. We used in situ ion exchange resins, gross flux measures, and enzyme activity assays to characterize soil nutrient dynamics. Our four years of data indicate that the vertisol monoliths show a decrease in mid-growing season N and P availability. In the fourth year of the experiment, the alfisol also showed a decrease in N availability mid-summer. Accompanying the decrease in N-availability is a decrease in the magnitude of the CO₂ effect on aboveground Net Primary Production in the vertisol monoliths. In the mollisol monoliths, there were no significant relationships between CO₂ and nutrient availability. Enzyme assays show that aminopeptidase activity in the vertisol correspond to N availability in the vertisol monoliths. These data indicate that soil type has a strong influence on the magnitude and timing of ecosystem responses to changes in atmospheric CO₂. Regional responsiveness to changes in atmospheric CO₂ may be heterogeneous depending on the diversity of soil types within an area.