Soil Organic Carbon Sequestration Under Pastures in Arid region

Two sites in Iraq were chosen to study the affect of annual pasture and perennial grasses (C4). The perennial grass pastures had SOC stocks, 1.6 (Baghdad) and 1.4 (Babylon) times that of the annual pastures. Soil Organic Carbon (SOC) pools were 1.90, 2.97 and 2.88% for annuals, perennials and tagasaste at Baghdad site. At Babylon the SOC pools were 2.7, 4.70, and 3.71% under annuals, perennials and tagasaste respectively. Estimated total C sequestration contribution to the resident soil organic C pool was 2.8 times greater for perennials and 2.7 times for tagasaste than annual pasture at the Baghdad deep sandy duplex site and 1.2 times greater for perennial pasture and 1.2 times greater for tagasaste than annual pasture at the Babylon deep-sand site. Both the sites were sampled to a depth of 1.6m. Perennial grasses in this region generally produced more above ground biomass than annual pastures. However, the differences in biomass input are unlikely to be large enough to explain the high rate of sequestration of these perennials. We hypothesise that the perennial grasses promote fungi such as mycorrhiza that convert a greater proportion of labile carbon to stable humic forms than under annual pastures.