INFLUENCES OF SLOPE ASPECT, DUST AND PLANT COMMUNITY COMPOSITION ON THE STABLE ISOTOPE COMPOSITION OF PEDOGENIC CARBONATE IN SOILS ON A BASALTIC VOLCANO, CENTRAL NEW MEXICO

The semi-arid climate of central New Mexico supports several major biomes, two of which are locally extensive: the Great Basin Shrubland, dominated by C₄ grasses and C₄ and C₃ shrubs, and the Chihuahuan Desert Shrubland, often dominated by creosotebush, a C₃ shrub. The δ¹³C of pedogenic carbonate (PC) in modern soils associated with these two communities in gravelly alluvium in this region differ, partly due to contrasts in δ¹³C of soil-respired CO₂ resulting from C₄ and C₃ photosynthetic pathways (Breecker et al., 2009). In this region, creosotebush preferentially recruits basaltic landforms. This study was designed to determine the degree to which the stable isotopic composition of PC (matrix & recent discontinuous clast coatings) in dust-dominated soils on a basaltic landform reflects spatial and temporal patterns of creosotebush colonization. A Pliocene basaltic volcano, Black Butte, provides a highly suitable study site because plant community distribution on it reflects strongly aspect-related, topoclimatic variation. Creosotebush forms nearly monotypic communities on the warmer, xeric slopes with southern aspects; a grass-shrub community dominates the cooler, more mesic slopes. A mixed (approximately 50 : 50 creosotebush : grass/shrub) community is present in the ecotonal, transitional slopes areas. The δ¹³C and δ¹⁸O values of dust from eolian sediment from natural dust traps in the summit area and the isotopic composition of selected, unambiguously PC clast coatings show that soil matrix sample values are primarily pedogenic. The ^δ13C values of PC associated with grass/shrub communities range from 0.1‰ to -2.5‰, values that resemble those documented by Breecker for this biome. PC δ¹³C values from the creosotebush community are consistently far lighter (-7.0‰ to -11.3‰), consistent with the predicted isotopic compositions and substantially lighter than those documented by Breecker in a creosotebush community elsewhere in this region. The PC values in the mixed community (0.4‰ to -1.8‰) are virtually indistinguishable from the grass/shrub soil values, despite the significant creosotebush presence. These values suggest relatively advance of creosote into this ecotonal area and/or substantial differences in magnitude of soil respiration associated with these different communities.