D. P. Fernandez

Controls of Bedrock Geochemistry on Soil and Plant Nutrients in Southeastern Utah

The cold deserts of the Colorado Plateau contain numerous geologically and geochemically distinct sedimentary bedrock types. In the area near Canyonlands National Park in Southeastern Utah, geochemical variation in geologic substrates is related to the depositional environment with higher concentrations of Fe, Al, P, K, and Mg in sediments deposited in alluvial or marine environments and lower concentrations in bedrock derived from eolian sand dunes. Availability of soil nutrients to vegetation is also controlled by the formation of secondary minerals, particularly for P and Ca availability, which, in some geologic settings, appears closely related to variation of CaCO3 and Ca-phosphates in soils. However, the results of this study also indicate that P content is related to bedrock and soil Fe and Al content suggesting that the deposition history of the bedrock and the presence of P-bearing Fe and Al minerals, is important to contemporary P cycling in this region. The relation between bedrock type and exchangeable Mg and K is less clear-cut, despite large variation in bedrock concentrations of these elements. We examined soil nutrient concentrations and foliar nutrient concentration of grasses, shrubs, conifers, and forbs in four geochemically distinct field sites. All four of the functional plant groups had similar proportional responses to variation in soil nutrient availability despite large absolute differences in foliar nutrient concentrations and stoichiometry across species. Foliar P concentration (normalized to N) in particular showed relatively small variation across different geochemical settings despite large variation in soil P availability in these study sites. The limited foliar variation in bedrock-derived nutrients suggests that the dominant plant species in this dryland setting have a remarkably strong capacity to maintain foliar chemistry ratios despite large underlying differences in soil nutrient availability.

Soil carbon storage responses to expanding pinyon-juniper populations in southern Utah

Over the past several decades, the expansion and thickening of woodlands in the western United States has caused a range of ecological changes. Woody expansion often leads to increases in soil organic matter (SOM) pools with implications for both biogeochemical cycling and ecological responses to management strategies aimed at restoration of rangeland ecosystems. Here we directly measure C and N stocks and use simple non-steady-state models to quantify the dynamics of soil C accumulation under and around trees of varied ages in southern Utah woodlands. In the two pinyon-juniper forests of Grand Staircase Escalante National Monument studied here, we found approximately 3 kg C/m2 and approximately 0.12 kg N/m2 larger C and N stocks in soils under pinyon canopies compared to interspace sites. These apparent increases in soil C and N stocks under woody plant species were dominated by elevated SOM in the surface 10 cm of soil, particularly within non-mineral-associated organic fractions. The most significant accumulation of C was in the >850 microm fraction, which had an estimated C residence time of <20 yr. Rates of carbon accumulation following pinyon-juniper expansion appear to be dominated by changes in this fast-cycling surface soil fraction. In contrast, we found that after separating >850 microm organic matter from the remaining light fraction (LF), C had residence times of approximately 400 yr and mineral-associated (MA) soil C had residence times of approximately 600 yr. As a result, we calculate that input rates to the LF and MA pools to be 10 +/- 1 and 0.68 +/- 0.15 g m(-2) yr(-1) (mean +/- SE), respectively. These findings suggest that one consequence of management activities aimed at the reduction of pinyon-juniper biomass may be a relatively rapid loss of soil C and N pools associated with the >850 microm fraction. The temporal dynamics of the <850 microm pools suggest that carbon and nitrogen continue to accumulate in these fractions, albeit at very slow rates, and suggest that multidecadal storage of C following tree recruitment is limited to relatively small, subsurface fractions of the total soil C pool.