Plant stoichiometric response to landscape-scale variation in bedrock geochemistry: Interactions of P, Ca and Fe in a semi-arid environment.
Neff, Jason1, Belnap, Jayne2, Sanford, Robert 3, Miller, Mark2, Reynolds, Richard4, 1 University of Colorado, Boulder, CO, USA2 US Geological Survey, Moab, UT, USA3 Denver University, Denver, CO, USA4 US Geological Survey, Denver, CO, USA
ABSTRACT- The semi-arid landscape of southeastern Utah is characterized by striking geologic formations and spatially diverse vegetation communities. At a landscape-scale, there are broad differences in the geochemistry of bedrock units that are reflected in patterns in foliar stoichiometry. However, different plant species have varying capacity to respond to nutrient availability, and these underlying physiological may help explain patterns in species composition across desert landscapes. We carried out a study of bedrock, soil, and plant chemistry in four distinct geologic settings that differ in P content as well as Ca and Fe concentrations (both of which can affect P availability in soils). Bedrock P content ranges from 0.01% for Navajo and Cedar Mesa Sandstones to 0.04-0.05% in Organ Rock and Moenkopi/Chinle substrates. The lowest foliar C:P ratios in our study (217) occurred in soils derived from Navajo sandstone where low Ca and Fe content limits formation of secondary P minerals and resulting reductions in bioavailable P. In contrast, the highest average foliar C:P ratio of 300 occurred in the Cedar Mesa Sandstone, which has identically low bedrock P (0.01%) but much higher Ca content than the Navajo bedrock (7% vs. 1%, respectively). Plants on soils derived from bedrock with abundant Fe oxides (Organ Rock or Chinle) have foliar C:P ratios between 250 and 270. Collectively these results illustrate the importance of Ca in restricting the availability of P to vegetation in these semi-arid landscapes. For specific plant species, two of the most widely distributed plant species blackbrush (Coleogyne ramosissima) and juniper (Juniperus osteosperma) also have the highest foliar C:P ratios (350-400), whereas more unevenly distributed native bunchgrass species (Stipa comata, Stipa hymenoides, and Hilaria jamesii) and a native shrub (Atriplex canescens) have more narrow C:P ratios that vary between 200 and 250. This difference suggests that flexibility in plant P use could be one mechanism influencing vegetation composition in SE Utah.
Key words: biogeochemistry, desert, Phosphorus, stoichiometry
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