NUTRIENT SOURCING OF NINE PLANT SPECIES IN THE SOUTHWESTERN U.S. USING STRONTIUM ISOTOPES

The nutrient enrichment anomalies surrounding trees and shrubs in the Southwestern U.S. has long been a source of contention. Patches of increased nutrient concentrations at the base of trees may be due to the lateral import of nutrients by tree roots with a resulting depletion in peripheral areas or due to foliar trapping of nutrient-rich dust followed by incorporation of the throughfall into the underlying soil. If the former were true, the nutrient source for all plants at a particular site would be similar, and so would the 87Sr/86Sr ratio of all plant species at a given site. If life history and growth form influences the efficiency to trap atmospheric dust, however, nutrient sources and radiogenic isotopes of cellulose could vary among species. Plant reliance on dust inputs can vary depending on surface-to-volume ratio of the foliage, the plant's degree of dependency on soil waters, and the depth of the rooting zone. We suspect that the grasses will be the most dependent on silicate mineral weathering for their nutrient supply, and therefore exhibit 87Sr/86Sr ratios much more similar to that of bedrock. Trees with dense foliage, on the other hand, should be much more efficient at trapping eolian dust, and therefore should exhibit 87Sr/86Sr ratios closer to that of atmospheric dust ratios.

We measured 87Sr/86Sr ratios in cellulose and bedrock to gauge the dependence of trees, grasses, and shrubs on atmospheric dust as a nutrient source for 8 prevalent species of trees, shrubs, and an annual grass (Pinus ponderosa, Pinus edulis, Juniperus monosperma, Juniperus scopulorum, Chrysothamus nauseosus, Fallugia paradoxa, Rhus trilobata, and Bouteloua gracilis) at El Malpais National Monument, New Mexico. Sampling sites varied by bedrock type (limestone, sandstone, granite, cinder and basalt) and by age (Quaternary to Precambrian). Local dust 87Sr/86Sr ratios at El Malpais and surrounding areas are ted by marine carbonates with minor additions from local bedrock. In our study, we found local dust ranges (.7059-.7083) within tree species with minor basalt contributions at sites with basaltic bedrock. Basalt in the area is known to efficiently trap dust within crevices. Such crevices may allow for more chemical contribution by local bedrock. The resulting data demonstrate the ability of Sr isotopes to decipher nutrient source variability within the study site.