Carbon dioxide emissions from Chihuahuan desert soils

Soil carbonate carbon (CaCO3) is present in all dryland regions of the world. Globally amounts of carbonate are estimated to be 855 PgC. Only the global terrestrial content of carbon stored as soil organic matter is higher with 1550 PgC. Soil carbonate contains from 50.2 Pg to 65.7 Pg of C in drylands of the US. In southern New Mexico it is often exposed to the atmosphere by soil erosion. Theoretically, carbonate exposure at the land surface to slightly acidic rain (pH from 6 to 6.5) and microbiotic crusts might cause carbonate dissolution and contribute additional volumes of CO2 to the atmosphere, causing soil carbonate to become a source instead of a sink of atmospheric CO2.; The purpose of the study was to determine if exhumed petrocalcic horizons from Chihuahuan desert soils might contribute CO2 to the atmosphere. Two methods were proposed. (1) To compare rates of CO2 emissions from eroded soils that have exposed petrocalcic horizons (EPHS) with neighboring non-eroded soils (NEPS), and soils without petrocalcic horizons (SWoPH). (2) To determine the source of soil CO2 by comparing its isotopic composition (delta13C) with published delta13C data for soils, plants and atmosphere. Soil CO2 fluxes were measured using 0.4M NaOH and soda lime traps, and delta13 C signatures determined with a mass spectrometer.; From the comparison of CO2 emissions for NaOH and soda lime alkali traps almost all the sampling period average soil CO2 emissions from EPHS were similar (alpha = 0.05) to those observed in neighboring soils. With respect to the isotopic comparison, EPHS average delta13C signatures were similar to average delta13C signatures of neighboring soils close or into the C3 plants range. Therefore, any increase in EPHS CO2 emissions were not generally due to dissolution of exhumed CaCO3. Considering other alternatives like differences in soil CO2 emissions of small magnitude between EPHS and neighboring soils, or the mixing of different sources of CO2 in the upper soil profiles aided to understanding of CO2 field results. Collateral experiments are discussed. Based on both the magnitude (soil CO2 flux) and origin (carbon isotopic signatures) of soil CO2 emissions, it was concluded that EPHS soils are not emitting significantly more CO 2 to the atmosphere than NEPS or SWoPH soils in the Chihuahuan desert.