WATER AND NITROGEN DYNAMICS IN AN AVID WOODLAND

Arid environments are characterized by spatial and temporal variation in water and nitrogen availability. Differences in delta(15)N and delt a D of four co-occurring species reveal contrasting patterns of plant resource acquisition in response to this variation. Mineralization pot ential and nitrogen concentration of surface soils associated with pla nt canopies were greater than inter-canopy locations, and values decre ased with increasing depth in both locations. Mineralization potential and nitrogen concentration were both negatively correlated with soil delta(15)N. The spatial variation in soil delta(15)N caused correspond ing changes in plant delta(15)N such that plant delta(15)N values were negatively correlated with nitrogen concentration of surface soils. P lants occurring on soils with relatively high nitrogen concentrations had lower delta(15)N, and higher leaf nitrogen concentrations, than pl ants occurring on soils with relatively low nitrogen concentrations. T wo general temporal patterns of water and nitrogen use were apparent. Three species (Juniperus, Pinus and Artemisia) relied on the episodic availability of water and nitrogen at the soil surface. delta(15)N val ues did not vary through the year, while xylem pressure potentials and stem-water delta D values fluctuated with changes in soil moisture at the soil surface. In contrast, Chrysothamnus switched to a more stabl e water and nitrogen source during drought. delta(15)N values of Chrys othamnus increased throughout the year, while xylem pressure potential s and stem-water delta D values remained constant. The contrasting pat terns of resource acquisition have important implications for communit y stability following disturbance. Disturbance can cause a decrease in nitrogen concentration at the soil surface, and so plants that rely o n surface water and nitrogen may be more susceptible than those that s witch to more stable water and nitrogen sources at depth during drough t.