Carbon sequestration and dynamics of two irrigated agricultural soils in California

Irrigation in semiarid regions can alter soil C sequestration processes compared with those of native soils. To better understand the effect of these altered processes, we studied the C sequestration and dynamics of two soils from major irrigated agricultural regions of California (the San Joaquin Valley and the Imperial Valley). Soils were sampled from selected native and cultivated fields to represent a span of almost a century of irrigated farming. Field soil samples were analyzed for total soil C and soil inorganic C as carbonate (SIC). Soil organic C (SOC) was then calculated from the measured data. Results showed that SOC stock increased above that stored in the native soil after five decades of irrigated farming. The SIC stock showed opposing trends within the top meter of the two studied soils: a decrease was measured after 55 yr in the San Joaquin Valley soil, while SIC in the Imperial Valley soil increased after 85 yr of irrigated agriculture and appears to represent a significant form of sequestered soil C. Our results indicate that long-term irrigated farming can significantly increase SOC due primarily to SOC added below the 10-cm soil depth, while significant increases in SIC may be partially due to the conversion of increased soil C[O.sub.2] to carbonates under a regime of Colorado River irrigation water. Thus, when considering C sequestration in irrigated agriculture in semiarid regions, it is important to determine levels of both SOC and SIC. Abbreviations: IMP, Imperial Valley; OM, organic matter; SIC, soil inorganic carbon; SJV, San Joaquin Valley; SOC, soil organic carbon; SOM, soil organic matter.