In the present paper, we review studies of stable isotopes in soil carbonates and offer some preliminary data from our current studies of how biotic processes may affect the formation of caliche and its stable isotope content. Studies of the isotopic ratios in soil carbonates suggest that biota play a major role in the soil-forming processes of desert ecosystems. The isotopic composition of caliche forming in current conditions appears to be related to brad climatic patterns and to local conditions of vegetation. Many previous compilations of such data are uninterpretable because samples were collected from sites dominated by calcareous parent materials and from soil horizons that developed under unknown paleoclimatic conditions. Gardner (1984) reported that Pleistocene age caliches contain lower values for 18O than Holocene samples (cf. Cerling et al. 1077; Table 18.1). Such changes may suggest that the deposition occurred in a cooler climate, but changes in the evaporative removal of soil moisture may also be involved. Careful studies of the seasonal change in the 13C and 18O values of soil CO2 and soil H2O would help determine the mechanisms controlling the deposition of carbonate in desert soils.