Carbon isotope discrimination and foliar nutrient status of Larrea tridentata (creosote bush) in contrasting Mojave Desert soils

We investigated the relationships between foliar stable carbon isotope discrimination (Delta), % foliar N, and predawn water potentials (psi(pd)) and midday stomatal conductance (g(s)) of Larrea tridentata across five Mojave Desert soils with different age-specific surface and sub-surface horizon development and soil hydrologies. We wished to elucidate how this long-lived evergreen shrub optimizes leaf-level physiological performance across soils with physicochemical characteristics that affect the distribution of limiting water and nitrogen resources. We found that in young, coarse alluvial soils that permit water infiltration to deeper soil horizons, % foliar N was highest and Delta, g(s) and psi(pd) were lowest, while %N was lowest and Delta, g(s) and psi(pd) were highest in fine sandy soils; Larrea growing in older soils with well-developed surface and sub-surface horizons exhibited intermediate values for these parameters. Delta showed negative linear relationships with % N (R-2=0.54) and a positive relationship with psi(pd) (R-2=0.14). Multiple regression analyses showed a strong degree of multicolinearity of g(s) and Delta with psi(pd) and N, suggesting that soil-mediated distribution of co-limiting water and nitrogen resources was the primary determinant of stomatal behavior, which is the primary limitation to productivity in this shrub. These findings show that subtle changes in the soil medium plays a strong role in the spatial and temporal distribution and utilization of limiting water and nitrogen resources by this long-lived desert evergreen, and that this role can be detected through carbon isotope ratios.