Diminishing effects of ant mounds on soil heterogeneity across a chronosequence of prairie restoration sites

It is well recognized that mound-building ants affect soil and vegetation heterogeneity, yet little is known about the extent to which such changes are propagated through time. We compared soil properties from ant mounds (built by Formica montana and Acanthamyops claviger) and from adjacent prairie soils at three tallgrass prairie restoration sites in the Midwestern United States, ranging in age from 8 to 26 years post-restoration of native species on former row-crop agriculture plots. We were particularly interested in N-dynamics on and off the mounds. All of the soil variables measured showed the greatest difference between mound and prairie soils at the 8-year old site. At that site, we found that ant mounds had significantly higher concentrations of total N, dissolved organic N, and NH4+ compared to prairie soil. Differences in concentrations of these nitrogen pools between mound and prairie soils were, however, lost with increasing time since restoration. The lack of N-enrichment at the older sites suggests that nutrient enrichment of ant mounds may be a transient phenomenon in certain ecosystems. To our knowledge, this is the first report that illustrates a critical, but a temporally dynamic role of ant mounds in creating resource heterogeneity in a restoration project.