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Depth Distribution and Seasonal Populations of Mesquite-Nodulating Rhizobia in Warm Desert Ecosystems

Dep. of Soil and Environmental Sciences and Dry Lands Res. Inst., Univ. of California, Riverside, CA 92521

Ross A. Virginia^*

Biology Dep. and Systems Ecology Res. Group, San Diego State Univ., San Diego, CA 92182

Wesley M. Jarrell

Dep. of Environmental Science and Engineering, Oregon Graduate Ctr. 19600 N.W. Von Neumann Dr., Beaverton, OR 97006

*Corresponding author.

ABSTRACT

Deeply rooted woody legumes are common in desert ecosystems yet little is known about the distribution of their rhizobial symbionts in relation to their roots and soil properties of the systems. The distribution of mesquite (Prosopis glandulosa Torr.)-nodulating rhizobia was investigated to depths of 13 m in warm desert ecosystems. Soils were collected under mesquite from sand dune and playa ecosystems in the California Sonoran Desert, and from sand dune, playa, arroyo, and grassland ecosystems in the Chihuahuan Desert of southern New Mexico. A Larrea tridentata (DC.) Coville (creosote bush) ecosystem in New Mexico, containing no mesquite, was sampled as a reference. Three intact soil cores from each ecosystem were removed during the winter, spring, and fall from the New Mexico sites, and the winter from the California sites. Significant rhizobial population densities were measured in soil from 1- to 4-m depths at the sand dune, arroyo, and playa ecosystems. At the playa ecosystem in New Mexico population densities > 10⁵ cells kg^–1 were measured in soils from 8-m depth, and root nodules containing rhizobia were recovered in soil samples from 3-, 4-, and 7-m depths. Multipleregression analysis of rhizobial concentration against soil NH₄-N, PO₄-P, electrical conductivity, and gravimetric water content indicated that no single soil factor was related significantly to rhizobial concentration across the ecosystems. Rhizobial densities varied with season in the dune and arroyo ecosystems. In these desert ecosystems significant rhizobial populations, root nodules, and presumably symbiotic N₂ fixation occur at soil depths rarely studied.

This research was supported by NSF Ecosystem Study Grant BSR-8506807 and the NSF Jornada Long-Term Ecological Research Program.

Received for publication February 1, 1988.

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