Aspect and Species Influences on Nitrogen and Phosphorus Availability in Arizona Chaparral Soils

Biota and topography are among the most important factors affecting nutrient status of wildland soils. Knowledge of these relations has a fundamental bearing on management of chaparral ecosystems. This study was conducted to determine the effect of shrub species and topographic aspect on availability of soil N and P in Arizona chaparral soils. Soil was collected under the canopy of 32 randomly selected shrubs, eight each of birchleaf mountain mahogany (Cercocarpus betuloides Nutt.) and shrub live oak (Quercus turbine/la Greene) from both north and south aspects. A pot culture technique, using barley (Hordeum vulgare L. var. gustoe) and mountain mahogany (C. montanus Torr.) as test plants, was used to estimate availability of soil N and P. Both test plants showed that shrub species and aspect influenced nutrient availability. Availability of P was very low, while that of N was quite high. Lower availability of P in soils from southerly aspects was associated with low amounts of total soil P, probably the result of long-standing differential erosion between north and south aspects, aggravated by fire. High N availability was probably associated with inputs of N via symbiosis between actinomycetes and mountain mahogany. Higher fertility of soils from oak systems is associated with lower lignin content of litter from these shrubs. /^HAPARRAL ecosystems cover extensive areas of the \^ U.S. Southwest including 1.3 million ha in Arizona (Bolander, 1982). Resource values are not high, but these ecosystems are important because of watershed and wildlife values, and their susceptibility to damage from disturbance. Knowledge of soil-plantnutrient relations for chaparral is fragmented and sketchy. Research has been directed chiefly at the California chaparral, much of it focused on N status of the soil-plant system, including mechanisms of loss and replenishment (DeBano and Conrad, 1978; DeBano et al., 1979; Dunn and Poth, 1979; Dunn et al., 1979; Schlesinger and Hasey, 1981; Marion and Black, 1988). However, P also merits attention based on its occasional deficiency (Hellmers et al., 1955; McMaster et al., 1982), susceptibility to loss in chaparral soils (DeBano and Conrad, 1978), and because of the important role P plays in accumulation of N and organic C in soil-plant systems (Stevenson, 1986). Although we have some insight into how nutrient supply of chaparral soils is influenced by shrub species (Hellmers et al., 1955; Vlamis et al., 1971; Delwiche et al., 1965) and fire (Christensen and Muller, 1975; Marion and Black, 1988), and can speculate about the effects of other state factors such as parent material and topography based on information from other systems (Aandahl, 1949; Klemmedson, 1964; KlemmedJ.O. Klemmedson, School of Renewable Natural Resources, Univ. of Arizona, Tucson, AZ 85721; and B.J. Wienhold, USDA-ARS Hydrology Lab., Beltsville, MD 20705. Contribution of the Arizona Agric. Exp. Stn., Journal Article no. 7246. Research was funded by the Rocky Mountain Forest and Range Exp. Stn. Cooperative Agreement 28-K4-321. Received 5 July 1990. 'Corresponding author. Published in Soil Sci. Soc. Am. J. 55:1735-1740 (1991). son and Jenny, 1966), specific information on factors that influenced nutrient availability would improve our ability to make management decisions concerning productivity of chaparral ecosystems. In a companion study (Klemmedson and Wienhold, 1985, unpublished data)* we observed a significant influence of biota (shrub species) and topography (aspect) on accumulation of N and P in a small watershed of the Arizona chaparral. The purpose of the present study was to go one step further and test the null hypothesis that shrub species and topographic aspect have no effect on availability of N and P in Arizona chaparral soils.