The impact of harvester ants on decomposition, N mineralization, litter quality, and the availability of N to plants in the Mojave Desert

In arid areas of North America, nests of the seed-harvesting ant Pogonomyrmex rugosus tend to be elevated in mineral nitrogen and other soil nutrients relative to other microhabitats. We investigated the roles of decomposition, N mineralization, and plant nutrient uptake in maintaining high standing stocks of nutrients in P. rugosus ant nests. Decomposition rates of standard cellulose substrates placed on the surface of ant nests and other desert microhabitats suggest that conditions found in ant nests and bare areas are conducive to higher rates of decomposition than conditions under shrubs. In laboratory incubations of moist soil, net N mineralization rates were significantly higher in soil from ant nests than from bare areas and under two of three plant species. Net N mineralization rates measured in situ were much lower than those measured in laboratory incubations, but ant nest soil still exhibited higher rates at one of two sites. Litter collected from ant mounds, composed chiefly of seed chaff, was similar in N content to litter collected from underneath the dominant plant species, but had a significantly higher mean δ15N. Using this distinctive isotope signature as a tracer, we found no evidence that large perennial shrubs tap ant nests as a source of N. An invasive, annual grass species was significantly enriched in 15N, had higher leaf %N, and produced more seeds when growing on the mound than when growing several meters away; however P. rugosus nest surfaces are typically free of such annuals. We conclude that both high rates of nutrient cycling relative to other Mojave Desert microhabitats and low N utilization by the surrounding vegetation contribute to high standing stocks of mineral N in P. rugosus nests.