GROWTH AND NITROGEN UPTAKE BY AGROPYRON DESERTORUM AND PSEUDOROEGNERIA-SPICATA WHEN EXPOSED TO NITRATE PULSES OF DIFFERENT DURATION

Plant growth and nitrate uptake were measured for two Great Basin pere nnial grasses, Agropyron desertorum and Pseudoroegneria spicata, in sa nd-filled pots in either monoculture or mixed culture (2 plants/pot). All plants were supplied with the same initial amount of nitrate but d elivered in five different pulse durations ranging from 0.5 to 72 h. T he pulse duration was controlled by flushing the pots with water at di fferent times after applying the nitrate pulse. The same concentration of nitrate was used in all pulse treatments. Increasing the pulse dur ation led to significantly increased plant biomass production for both species in both mono-and mixed cultures, and to reduced root/shoot bi omass ratio. Biomass and root/shoot ratio were greater for Agropyron i n mixed culture than in monoculture. To assess root nitrate uptake cap acity, a 30-min tracer pulse was applied to all plants. Plants that ha d been exposed to longer pulses acquired significantly more nitrate th an those that had been exposed to shorter pulses for both total plant acquisition and acquisition per unit root length. With greater root/sh oot ratio than Pseudoroegneria, total nitrate acquisition by Agropyron was significantly greater at most pulse durations. Root nitrate uptak e per unit mass was also greater for Agropyron than for Pseudoroegneri a, indicating that Agropyron is more responsive to nitrate pulsing.