Strontium isotopes and nutrient sourcing in a semi-arid woodland

Abstract

Sr isotopes are widely used as a tracer of Sr and Ca in surficial systems. Basalt flows ranging in age from 3ka (kiloyears ago) to>200ka from El Malpais National Monument (EMNM), New Mexico provide an ideal setting to examine strontium, and hence calcium cycling by plants in a semi-arid woodland. To gauge plant dependence on atmospheric dust versus local weathering products for strontium and calcium, we measured 87Sr/86Sr ratios in local bedrock and soils, and compared them to leaf/wood cellulose of four different conifers, a deciduous tree, three shrubs, an annual C4 grass, and a lichen. Sampling sites varied by parent material (limestone, sandstone, granite, and basalt) and age (Quaternary to Precambrian), providing a wide range in end-member 87Sr/86Sr ratios, whereas the target plant species varied in physiognomy, life history, and rooting depth. On non-basalt parent material, the contribution from dust changed with the supply of weatherable Sr-bearing minerals in local bedrock. Soils developed on Paleozoic limestone showed significant bedrock contributions. On basalts, the Sr budget of soils at EMNM is dominated by atmospheric dust on young, 3ka flows, incorporates a mixture of basalt-dust in 9ka flows, and is basalt-dominated in 120ka flows. This is unlike the pattern observed in tropical soils developed on basalt in Hawaii, where basalt weathering dominates the Sr inventory of the youngest soils and aerosols dominate in older, deeply weathered soils. This contrast is mainly due to different water/rock (W/R) ratios: bedrock subjected to high W/R over short periods is quickly (<10ka) depleted in Sr (and Ca), except for the ongoing replenishment from aerosols. In arid settings where W/R are lower, soil Sr is still abundantly available first from dust, and increasingly from bedrock even after 120 ka. For plants, 87Sr/86Sr variations within and across sites at EMNP showed that evergreen trees varied most in 87Sr/86Sr ratios, shrubs were least dependent on eolian input of Sr, and both foliage density and rooting depths influence soil Sr pools.