CHARACTERIZATION OF PHYSICAL AND HYDRAULIC PROPERTIES OF SOILS IN SEVILLETA WILDLIFE NATIONAL REFUGE: IMPLICATIONS FOR PEDOGENIC CARBONATE ACCUMULATIONS ON WATER MOVEMENT

Declining water tables and increased societal demands on water supply, pose an urgent need to understand the various controls on the water movement in thick vadose zones to infer the water balance of the region. Calcic horizons are ubiquitous features of the soils in semi-arid Southwest US. The accumulation of calcium carbonate over time produces systematic morphological and textural changes in the soils described as stages of calcic horizon development. These changes in soil properties influence the partitioning of precipitation into infiltration and runoff in semi-arid environments. Calcium carbonate arrives at the soil surface along with dust (primarily silt and clay) so there is also a systematic increase in the silt and clay content of semi-arid soil with time. The purpose of this study is to understand the effects of pedogenic carbonate accumulation on water movement within a soil profile.

The physical and hydraulic properties of two members of a soil chronosequence developed on the Sevilleta Long Term Ecological Research (LTER) site were studied. Soil textures, total carbonate content, and bulk densities were measured for each horizon. In situ measurements of soil hydraulic conductivity were taken at every soil horizon using a tension disk infiltrometer. Samples were taken from different stages of calcic horizon to determine their morphology and pore structure. The amount and morphology of the calcium carbonate, and thus the hydraulic properties, varied with depth for each of the soils. Field and meteorological data recorded at the LTER sites were used as input in a 1-D Hydrus model. Initial results from the model reiterate the fact that soil pCO2 and moisture content are the governing variables for calcite dissolution and precipitation within a soil profile. However the precipitation of pedogenic carbonate within a soil profile is not uniform and is controlled by the distribution of carbonate already present in the soil. Thus the rate of carbonate precipitation and its spatial distribution within a soil profile is a complex response to the changing soil permeability characteristics with depth.