Geochemical Constraint of Weathering and Soil Carbon Dynamics Across a Semiarid Environmental Gradient.

Arid and semiarid lands cover roughly 36% of global land area, yet the specific mechanisms controlling weathering and soil carbon dynamics in these systems remain poorly understood.  We address this knowledge gap in a regionally important set of semiarid ecosystems in southern Arizona, termed the Sonoran Desert Environmental Gradient (SDEG).  Specifically, we coupled geochemical and climatic data to provide constraint of soil organic and inorganic carbon cycle processes. The SDEG spans a steep elevation and environmental gradient on granitic parent materials: mean annual air temperature decreases (20-10 ºC) and mean annual precipitation increases (30-85 cm) with elevation, with concomitant changes in vegetation from mixed desert-scrub (<1200m) to grass and oak woodlands (1200-1700 m) to pinyon-juniper woodland (1700-2000 m) and ponderosa pine and fir forest (>2000 m).  We sampled soil and regolith material from each of vegetation community across the SDEG and established a long-term soil-moisture and temperature monitoring network.  Pedon physicochemical data were coupled with radiocarbon analyses of soil carbon, and saprolite cosmogenic nuclide (¹⁰Be) content to quantify rates of organic and inorganic carbon cycling across the SDEG.  One of the dominant pedogenic processes is the rapid cycling of soil organic matter, with radiocarbon data indicating soil organic matter dominated by carbon with a mean residence time of less than 15 years.  Application of a steady-state geochemical mass balance-denudation rate model indicates the mass flux and rate of weathering rates vary by an order of magnitude across the SDEG, e.g., Si mass flux ranged from from 4-20 g Si per square meter per year.  Both organic carbon and weathering rates scale directly with precipitation and soil-moisture content.  The combined data demonstrate significant variation in the relative importance of organic and inorganic carbon cycle processes across the SDEG that may be linked quantitatively to climate and soil-water dynamics.