Total soil carbon and water quality: an implication for carbon sequestration

Carbon sequestration in soil has been suggested as a means of reducing the rate of increase of atmospheric C[O.sub.2]. Most soil science research has been on soil organic carbon (SOC) sequestration but in arid and semiarid climates, soil inorganic carbon (SIC) may offer another option for C sequestration. A field study was conducted in Bakersfield, CA, to determine if irrigation water quality (fresh water [FW] vs. treated effluent [TE]) affected the distribution and amount of SIC and SOC in the upper 4 m of soil and parent material compared to a nonirrigated (NI) field. Significant carbonate depletions were found in the upper 2 m in both irrigated fields compared with the NI. Differences in carbonate content between irrigated fields were also related to soil texture. Total carbonate and clay-size carbonate were more abundant at the sites irrigated with TE than at the sites irrigated with FW, indicating that the TE had inhibited carbonate dissolution. Based on stable isotope analyses ([sup.13]C and [sup.18]O) and radiocarbon dating, we estimated that irrigation for >75 yr sequestered about 7.15 kg [m.sup.-2] [(4 m).sup.-1] of SIC under FW and between 0.9 and 2.4 kg [m.sup.-2] [(4 m).sup.-1] under TE, if carbonate dissolution is C sequestration. Adding C loss due to SOC decomposition to the SIC sequestration, the fields may be a source for 8.8 and 17.4 to 15.9 kg [m.sup.-2] [(4 m).sup.-1] of C under FW and TE, respectively. This study provides some of the first evidence of how water quality affects the C budget in an arid region. Abbreviations: FW, freshwater; NI, nonirrigated; PSD, particle size distribution; SIC, soil inorganic carbon; SOC, soil organic carbon; TE, treated effluent.