Plant response to variations in nitrogen availability in a desert shrubland community

Spatial variations in nitrogen availability were studied in a desert community codominated by <i>Larrea tridentata</i> (DC.) Cov. and <i>Prosopis glandulosa</i> Torr. Measurements of natural 15N values in tissues suggested that <i>Prosopis</i> obtains approximately half of its nitrogen through direct symbiotic fixation. Soils were collected under 1) <i>Prosopis</i> shrubs, 2) <i>Larrea</i> shrubs < 2 m from <i>Prosopis</i> (LP), and 3) <i>Larrea</i>< 2 m from other <i>Larrea</i> but > 5 m from the nearest <i>Prosopis</i> (LL). <i>Prosopis</i> soils showed significantly higher rates of nitrogen mineralization than LL soils in both A and B horizons. Rates of mineralization in LP soils were significantly higher than rates in LL soils only in the B horizon and were not significantly different from rates in <i>Prosopis</i> soils. Leaf nitrogen concentrations were significantly higher in LP shrubs (2.06%) than in LL shrubs (1.78%), although 15N values did not differ between the two shrub types. Nitrogen concentrations in <i>Perezia nana</i> Gray, a perennial herb, were greater in plants under <i>Prosopis</i> shrubs (2.09%) than under LP shrubs (1.93%) or LL shrubs (1.67%). Despite apparent differences in nitrogen availability, biomass of <i>Larrea</i> and density of <i>Perezia</i> did not differ significantly among these sites.