The Stable Isotope Chemistry of a Native and Irrigated Typic Natrargid in the San Joaquin Valley of California

The stable C and O isotope chemistry of native and irrigated Typic Natrargids was examined in order to determine the relationship between the climate and vegetation and the δ 13 C and δ 18 O of the native pedogenic carbonates (CaCO 3 ) and the effect of irrigation on the CaCO 3 in the cultivated soils. The native soil had a sparse vegetative cover consisting of grasses and salt tolerant shrubs, especially Atriplex polycarpa (Torr.), S. Wats. The δ 13 C of the CaCO 3 reflected the mixture of C3 and C4 vegetation and the δ 18 O of the CaCO 3 suggested that it had formed from soil H 2 O that had undergone extensive amounts of evaporation. At the time of sampling, alfalfa ( Medicago sativa L.) was being grown on the irrigated soils. These soils had CO 2 partial pressures of up to 7 KPa, nearly 20 times that of the native soil. The δ 13 C of the CaCO 3 in the irrigated soils was not significantly different than that of the native soils which was due, in part, to the relatively small isotopic difference between the newly formed CaCO 3 in the irrigated soil and much of that which occurred naturally. However, the δ 18 O of CaCO 3 in the irrigated soils appeared to be shifted to more negative values than the δ 18 O of the native CaCO 3 . Based on the differences in the δ 18 O of the CaCO 3 in the irrigated and native soils, it was semiquantitatively estimated that between 10 to 40% of the carbonate in the irrigated soils was newly formed or had dissolved and reprecipitated as a result of irrigation, increased biological activity, and the application of chemical amendments.