Inorganic carbon can play an important role in sequestering carbon that would otherwise enter the atmosphere and contribute to global warming. Our initial results show that inorganic carbon, as calcium carbonate, is precipitated under treated effluent irrigation in arid region soils. We found a large accumulation of carbonate in a field irrigated with treated effluent for more than 70-years near Bakersfield CA. Radiocarbon dating of 7030± 120 years, suggests that this carbonate is a mixture of old and new carbonates. We also found that the samples from the effluent-irrigated field contained significantly more clay-size carbonate than samples from the fresh-water irrigated ones. This is additional indirect evidence for the enhancement of carbonate precipitation in the effluent-irrigated field. The depth in which the majority of the carbonate is found in this field (2-4 m) is well below the zone of most active root growth. Less than 1 km away, a similar field irrigated with fresh water, accumulated a much smaller amount of carbonate and that accumulation took place at a shallower depth than in the effluent irrigated field (Table 1). The above observations may indicate that in addition to root respiration (providing CO₂ as the source of carbon), other processes have an important role in carbonate precipitation in effluent-irrigated fields. We propose a conceptual model that illustrates the different scenarios, depths and intensities, of the processes that control carbon sequestering in soils of arid and semi-arid regions.

Table 1: Mean carbonate content of similar fields at Bakersfield, CA under fresh water and long-term effluent irrigation

Type of irrigation water	Carbonate content†
	0-2 m			2-4m
	# of profiles	Mg/ha/depth		# of profiles	Mg/ha/depth
Effluent	7	92.6 b		4	1172 a
Fresh	6	68.2 b		3	8.7 b

^† Values followed by the same letter do not differ significantly (0.05 probability level)