Development of harvester ant colonies alters soil chemistry

Nests of the harvester ant Pogonomyrmex barbatus typically contain higher concentrations of organic matter, nitrogen and phosphorus than surrounding soils. The difference between nest soils and surrounding soils is due, at least in part, to ant foraging behavior. Ants retrieve seeds from the environment and concentrate seed nutrients in the vicinity of the nest. But elevated nutrient concentrations in nests may also reflect initial conditions, if nest-founding queens are more likely to choose or survive in soils with high organic matter and nutrient content. By measuring the soil nutrient content and surface area of P. barbatus nests ranging from 1 to 20 years of age over two sampling periods, we (a) investigated the relationship between nest soil characteristics and colony age, and (b) tested the hypothesis that nest soils differ from background soils when nests are established. Nest surface area increased with colony age until age 5–10 years and leveled off thereafter. Relative to surrounding soils, concentrations of total nitrogen and orthophosphate increased, and pH decreased, with increasing colony age. The difference between nest soils and surrounding soils in total nitrogen, nitrate, and ammonium concentrations also increased over a 9-month interval between sampling bouts. Extrapolations from regressions of soil chemical variables against colony age provided no evidence that nest founding and early colony survival is more likely to occur in high-nutrient soils.