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Title: Growth and foliar d15N of a Mojave desert shrub in relation to soil hydrological dynamics

item Hamerlynck, Erik

Submitted to: Journal of Arid Environments
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2010
Publication Date: 9/2/2010
Citation: Hamerlynck, E.P., Mcauliffe, J.R. 2010. Growth and foliar d15N of a Mojave desert shrub in relation to soil hydrological dynamics. Journal of Arid Environments. 74:1569-1571.

Interpretive Summary: Desert shrubs are generally thought to act as “islands of fertility”, serving as focal points for the accumulation of plant litter, dry deposition, and animal activity. However, it is not clear how soil physical features that strongly determine soil hydrology affect the ability of long-lived desert shrubs to act as fertile islands. In this study, we showed that younger soils with little or no soil horizon development supported dramatically larger creosotebush (Larrea tridentata) and white bursage (Ambrosia dumosa) plants, and had leaf stable nitrogen isotope ratios (del15N) highly enriched in the heavier 15N isotope, suggesting these large plants utilized soil N from deeper in the soil profile, where it had likely undergone repeated microbial transformation, and were better traps for dry deposited nitrogen sources compared to plants growing in soils with strong surface and below-surface horizons that limited infiltration and percolation deeper into the soil profile. Overall, this study shows the over-riding importance of soil hydrology – which is strongly determined by geological processes – has on plant ecological processes in desert systems.

Technical Abstract: Foliar 15N ratios (del15N), % N, and canopy volumes were measured in the two Mojave Desert dominant shrubs, the evergreen Larrea tridentata and drought deciduous Ambrosia dumosa growing across a geomorphically determined soil mosaic. Across three soils with increasingly strong age-dependent surface and sub-surface horizons, del15N decreased with soil age and plant size, and were 1.6-2.0 ‰ higher in Ambrosia compared to Larrea, suggesting Ambrosia consistently uses water and N-sources from shallower soil depths. Larrea del15N were markedly higher in poorly developed recently-derived Holocene-aged alluvial deposit soils and in sand dune soils (6.18 and 7.48 ‰, respectively), and were concurrent with a respective 4 to 30 fold increase in plant canopy volumes compared to Larrea growing in older soils with stronger surface and sub-surface soil horizons that limit infiltration and percolation, where del15N ranged from 0.37 – 4.17 ‰, and canopy volumes from 0.46 to 0.57 m3. These findings suggest Larrea growing in more strongly developed soils increasingly relied on N directly from precipitation. Overall, this study shows that a strong geomorphic soils context can clarify the importance of seasonal soil hydrology in determining the effectiveness of desert plants to accumulate and incorporate organic material, sustain microbial activity, and act as fertile islands in these water-limited systems.