Short-term patterns in water and nitrogen acquisition by two desert shrubsfollowing a simulated summer rain

A field experiment was conducted at the Jornada Long-Term Ecological Resear ch (LTER) site in the Chihuahuan Desert of New Mexico to compare the rapidi ty with which the shrubs Larrea tridentata and Prosopis glandulosa utilized water, CO2 and nitrogen (N) following a simulated summer rainfall event. S elected plants growing in a roughly 50-m(2) area were assigned to treatment and control groups. Treatment plants received the equivalent of 3 cm of ra in, while no supplemental water was added to the control plants. Xylem wate r potential (Psi(x)) and net assimilation rate (A(net)) were evaluated one day before and one and three days after watering. To monitor short-term N u ptake, soils around each plant were labeled with eight equally distant patc hes of enriched N-15 before watering. Each tracer patch contained 20 ml of 20 mM (NH4NO3)-N-15-N-15 (99 atom%) solution applied to the soil at 20 cm f rom the center of the plant at soil depths of 10 and 20 cm. Nitrogen uptake , measured as leaf delta(15)N, was evaluated at smaller time intervals and for a longer period than those used for Psi(x) and A(net). Both A(net) and Psi(x) exhibited a significant recovery in watered vs. control Larrea plant s within 3 days after the imposition of treatment, but no such recovery was observed in Prosopis in that period. Larrea also exhibited a greater capac ity for N uptake following the rain. Leaf delta(15)N was five-fold greater in watered compared to unwatered Larrea plants within 2 days after watering , while foliar delta(15)N was not significantly different between the water ed and unwatered Prosopis plants during the same period. Lack of a signific ant change in root (NO3-)-N-15 uptake kinetics of Larrea, even three days a fter watering, indicated that the response of Larrea to a wetting pulse may have been due to a greater capacity to produce new roots. The differential ability of these potential competitors in rapidly acquiring pulses of impr oved soil resources following individual summer rainfall events may have si gnificant implications for the dynamic nature of resource use in desert eco systems.