Nitrogen Mineralization in Soils of a Chaparral Watershed in Arizona

This study was undertaken to extend our knowledge of nutrient availability in soils of the Arizona chaparral. Our objective was to determine the effect of shrub species and topographic aspect on N mineralization of soils from a watershed in central Arizona and to relate N mineralization to other soil properties. Soil for an incubation study was collected from the 0to 2and 2to 10-cm soil layers beneath the canopy of 32 randomly selected shrubs, eight each of birchleaf mountain mahogany (Cercocarpus betutoides Nutt. ex. Torr. & A. Gray) and shrub live oak (Quercus turbinetta Greene) from both north and south aspects. Neither cumulative N mineralization (Nm) nor potentially mineralizable N (N0) was influenced by shrub species or aspect. Aspect significantly influenced NH4 released in the 2to 10-cm layer, with greater amounts released in soils from north and south aspects. Both Nm and N0 were markedly higher in the 0to 2-cm soil layer than in the 2to 10-cm layer; NO, was the dominant form of mineralized N. Nitrogen-mineralized in both soil layers correlated highly with soil C, N, P, and N/P and C/P ratios. Phosphorus supply (P.v) differed markedly between the two soil layers (3.8 and 0.7% of total P, respectively) and was related to Nm. The association of Pav with N, however, leaves the real effect of P.v on Nm in doubt. /^HAPARRAL ECOSYSTEMS cover extensive areas of \^ the southwestern USA including 1.3 million ha in Arizona (Bolander, 1982). Knowledge of soil-plantnutrient relations for chaparral is fragmented and sketchy. Research has focused on California chaparral, much of it on N status of the soil-plant system, and including mechanisms of N loss and replenishment (DeBano and Conrad, 1978; DeBano et al., 1979; Dunn and Poth, 1979; Dunn et al., 1979; Schlesinger and Hasey, 1981). However, P also merits attention based on its occasional deficiency (Hellmers et al., 1955; McMasters et al, 1982), susceptibility to loss from chaparral systems, and the important role P plays in accumulation of N and organic C in soil-plant systems (Cole and Heil, 1981; Stevenson, 1986). In a companion study, we observed that availability of N in soils from a chaparral watershed in central Arizona was quite high, while availability of P was very low (Klemmedson and Wienhold, 1991). In that study, barley (Hordeum vulgare L.) plants grown in soils supplemented with P alone produced 70% of the yield of plants grown in soils with both N and P, while yield of barley grown in soils with N alone was low and similar to yield of plants grown in soil with no nutrients added. These results contrast with much of J.O. Klemmedson, School of Renewable Natural Resources, Univ. of Arizona, Tucson, AZ 85721; and B.J. Wienhold, USDA-ARS Hydrology Lab., Beltsville, MD 20705. This is a contribution of the Arizona Agric. Exp. Stn., Journal Article no. 7352. Research was funded by the Rocky Mountain Forest and Range Experiment Station, Cooperative Agreement 28-K4-321. Received 6 Aug. 1991. 'Corresponding author. Published in Soil Sci. Soc. Am. J. 56:1629-1634 (1992). the literature on California chaparral, which reports that soil fertility is quite low and that both N and P are usually deficient (Hellmers et al., 1955; Christensen and Muller, 1975; McMasters et al., 1982; Marion and Black, 1988). Several studies have been conducted to assess Nmineralization rates in soils from California chaparral, but little is known about this process in Arizona chaparral. The purpose of this study was to extend our knowledge of N availability in soils of Arizona chaparral with a study of N mineralization and its controlling factors. Because biota and aspect commonly have marked effects on N status of soil-plant systems (Aandahl, 1949; Klemmedson, 1964; Jenny, 1980; Klemmedson and Tiedemann, 1986; Marion and Black, 1988), this study was designed to test the null hypothesis that shrub species and topographic aspect have no effect on N mineralization in Arizona chaparral soils. In addition, N-mineralization rates were related to other soil characteristics of the study area. MATERIALS AND METHODS Study Area The study area was a 55-ha drainage of the Battle Flat Watershed in the Bradshaw Mountains of central Arizona (34°19'N, 112°22'W). The topography is rough and highly dissected. The prevailing aspect of the watershed is southeast; slope gradient ranges from 15 to 60%. Elevation is = 1700 to 1770 m above sea level. Geologic parent material is massive bedded crystalline tuff with recent gravels along stream beds (Anderson and Blacet, 1972). The Moano very rocky loam soils common to the study site are classified as loamy, mixed, nonacid, mesic Lithic Torriorthents. The dense shrubby vegetation averages =75 to 80% crown cover. Canopy height varies from 1 to 3 m; shrub height and cover are greatest on northerly aspects. Dominant shrubs are manzanita (Arctostaphylos pungens HBK), shrub live oak, and birchleaf mountain mahogany. Subdominant shrubs include emory oak (Quercus Emoryi Torr.), alligator juniper (Juniperus Deppeana Steud.), desert ceanothus (Ceanothus Greggii A. Gray), apache plume (Fallugia paradoxa D. Don), and yerba santa (Eriodictyon angustifolium Nutt.). The herbaceous understory is sparse. Mean annual precipitation of 480 mm is about equally divided between cyclonic winter and convective summer storms. About 15 to 20% of total precipitation is snow. Mean daily temperature is 15°C; the annual maximum range is from 29 to 39°C.