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 0- to 2- and 2- to 10-cm soil layers beneath the canopy of 32 randomly selected shrubs, eight each of birchleaf mountain mahogany (Cercocarpus betuloides Nutt. ex. Torr. & A. Gray) and shrub live oak (Quercus turbinella Greene) from both north and south aspects. Neither cumulative N mineralization (N(m)) nor potentially mineralizable N (N(o)) was influenced by shrub species or aspect. Aspect significantly influenced NH4 released in the 2- to 10-cm layer, with greater amounts released in soils from north and south aspects. Both N(m) and N(o) were markedly higher in the 0- to 2-cm soil layer than in the 2- to 10-cm layer, NO3 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(av)) differed markedly between the two soil layers (3.8 and 0.7% of total P, respectively) and was related to N(m). Tle association of P(av), with N, however, leaves the real effect of P(av) on N(m) in doubt.