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 delta D of four co-occurring species reveal contrasting patterns of plant resource acquisition in response to this variation. Mineralization potential and nitrogen concentration of surface soils associated with plant canopies were greater than inter-canopy locations, and values decreased 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 corresponding changes in plant delta(15)N such that plant delta(15)N values were negatively correlated with nitrogen concentration of surface soils. Plants occurring on soils with relatively high nitrogen concentrations had lower delta(15)N, and higher leaf nitrogen concentrations, than plants occurring on soils with relatively low nitrogen concentrations. Two 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 values 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 stable water and nitrogen source during drought. delta(15)N values of Chrysothamnus increased throughout the year, while xylem pressure potentials and stem-water delta D values remained constant. The contrasting patterns of resource acquisition have important implications for community stability following disturbance. Disturbance can cause a decrease in nitrogen concentration at the soil surface, and so plants that rely on surface water and nitrogen may be more susceptible than those that switch to more stable water and nitrogen sources at depth during drought.