BIOLOGICAL AND CLIMATE CONTROLS ON CALCIUM CARBONATE PRECIPITATION IN A SMALL FIRST ORDER DRAINAGE BASIN, SEVILLETA LONG TERM ECOLOGICAL RESEARCH SITE, NEW MEXICO. USA

The precipitation of calcium carbonate is a diagnostic feature of soils developing in semi-arid environments. Empirical data show a relationship between the depth to calcium carbonate precipitation and average annual rainfall. Other studies have also shown that micro- biological organisms play a role in determining the location of calcium carbonate precipitation within a soil profile, principally through their influence on PCO2 which controls the concentration of HCO- ions in the soil. On the Sevilleta LTER a small first order drainage basin displays a dramatic change in vegetation with an arid Chihuahuan Desert grassland (creosote/black gramma grassland ) on the south facing slope and a mesic Great Plains grassland (Juniper/black gramma grassland) on the north facing slope. Conventional wisdom attributes these changes to differences in solar radiation experienced by the different slope orientations resulting in variation in soil moisture regime. An understanding of soil forming processes would suggest that if there are significantly different climates on the opposing hillslopes then eventually different soils will also form.. Significant variation in the amount and depth of calcium carbonate was found in the soils throughout the drainage basin. On the shady north facing slope, soils have higher moisture retention and a more active vegetation community resulting in higher PCO2 and ultimately higher levels of CaCO3 than the soils on the drier south facing slope. At the head of the basin however, a sequence of soils downslope shows a progressive increase in depth to CaCO3 to the point where the lowest soil in the sequence has no calcium carbonate at all. Thus regional precipitation is a poor estimate of pedogenic calcium carbonate characteristics in sloping terrain. The pattern of calcium carbonate precipitation in the drainage basin shows complex relationships to soil moisture regime. Greater moisture availability on shady north facing slopes causes increased biological activity, higher PCO2 and more soil carbonate. In the convergent head slope of the drainage basin the moisture flux enhanced by topography causes complete leaching of pedogenic CaCO3 from the soil profile.