*Principal Investigator Quantitative and Qualitative Assessment of Soil Organic Carbon in Native and Cropland Soils in

Irrigation in semi-arid regions can alter soil carbon (C) sequestration processes compared to those of native soils. Towards understanding the effect of these altered processes, we studied the soil C dynamics of two soils from major irrigated agriculture regions of California (the San Joaquin Valley and the Imperial Valley). Soils were sampled from selected native and cultivated fields to provide a time spanning almost a century of irrigated cultivation. Field soil samples were analyzed for total soil C and soil inorganic C as carbonate (SIC). Soil organic carbon (SOC) was then calculated from the measured data. Results showed that the SOC stock increased above that stored in the native soil after five decades of irrigated cultivation. SIC inventory showed opposing trends within the top meter of the two studied soils. In the San Joaquin Valley, SIC decreased over a 55-year period, while in the Imperial Valley SIC increased after 85 years of irrigated agriculture and appears to represent a significant form of sequestered soil C. Our results indicate that long-term irrigated cultivation can significantly increase SOC due primarily to increased SOC added to below the 10-cm soil depth, while significant increases in SIC may be partially due to the conversion of increased soil CO2 to carbonates under a regime of Colorado River irrigation water. Thus, when considering carbon sequestration in irrigated agriculture in semi-arid regions, it is important to determine both SOC and SIC contents