The effect of the New Mexican locust (Robinia neomexicana Gray) on N, P, S, and Ca, Mg, K and organic C in the soil-litter system of a ponderosa pine (Pinus ponderosa Douglas, ex P. Lawson & Lawson) forest in central Arizona was studied on sites with either basalt or limestone parent material. Samples of the Oi and Oe + Oa horizons (forest floor) and 0- to 5- and 5- to 15-cm soil layers from 10 pairs (with and without locust) of randomly located pine stands were sampled and analyzed for total N, P, and S, total (forest floor) and exchangeable (mineral soil) Ca, Mg, and K, and organic C (Corg). The 10 stand pairs represented five sites with basalt and five with limestone parent material. There were no significant parent material × locust interactions for the nutrients considered. Locust presence increased nutrient concentrations of N and Ca most strongly (in both Oe + Oa and soil layers) and Corg, P, S, and K concentrations to a lesser extent. Concentrations of N, P, and K were significantly higher in mineral soils derived from basalt. Increased amounts of N in forest floor and mineral soil, and increased amount of P, Ca, Mg, and K in the forest floor were associated with locust presence. Amounts of Corg, N, P, and exchangeable K were greater in soil components of stands with basalt parent material. Forest floor mass was greatest for basalt sites with locust; this and nutrient composition differences between locust leaves and pine needles suggest greater nutrient differences between parent materials may accrue with age of locust stands.
Research supported by the McIntire-Stennis Cooperative Forestry Research Program in cooperation with the Arizona Agric. Exp. Stn.