Enhancing Inorganic Carbon Sequestration by Irrigation Management in California

We conducted a field study to determine if the distribution and amount of soil inorganic carbon was changed by the application of treated effluent for irrigation compared to fresh-water irrigated soils. The main site for the study was in a wastewater utilization area near Bakersfield, Calif., that had received varying amounts of treated effluent for 70 years. At the time, the sampled cotton was growing at the site. A nearby field, irrigated with fresh water, also was sampled for comparison. Significant depletions of carbonate were found in the upper 2m of all irrigated fields, compared to non-irrigated fields. Carbonate did not accumulate in soils with textures as coarse as loamy sand. Differences in carbonate content among fields irrigated with treated effluent and fresh water were a function of both soil texture and water quality. Soils with carbonate contained both natural pedogenic carbonate and carbonate inherited from the parent material. It was not possible to isolate and measure the young pedogenic carbonate that might have resulted from effluent utilization. Total carbonate content and clay-size carbonate was more abundant at sites irrigated with treated effluent than at sites irrigated with fresh water, indicating that effluent had increased carbonate precipitation. Stable isotope analyses of carbon and oxygen indicated that a significant amount of the carbonate found below 2.5 meters depth in the treated-effluent irrigated field was pedogenic. The pedogenic carbonate was a mixture of young and old carbon according to 14C dating of the carbonate. More of the carbonate was “young ” carbonate at shallow depths of the soil compared to deeper in the soil profile. This is additional evidence that treated-effluent irrigation contributes to carbonate precipitation. Treated-effluent irrigation can increase soil inorganic carbon sequestration in arid and semiarid region soils and it should be used as another method for balancing the increase in CO2 in the atmosphere from anthropogenic activity