There remains considerable uncertainty about the net effects and underlying soil processes of irrigation management on soil C sequestration. A series of field comparisons of soil C stocks and aggregate size distribution between center-pivot irrigated cropland, dryland cropland and native grassland were performed in silt loam Argiustolls from two farms near Imperial, Nebraska. Conversion of grassland to dryland cropland resulted in a loss of soil organic C (SOC). SOC was higher under irrigated conditions compared to dryland, although the effect was greater where center-pivot irrigation was practiced longer. At one farm, irrigation even restored the SOC level of the cultivated soil close to native grassland levels in the 0 - 20 cm depth. Analysis of aggregate size distributions showed much higher proportions of soil weight in the macroaggregate size fraction in the grassland compared to the cultivated systems, but no significant difference in macroaggregation was observed between irrigated and dryland systems. This suggests that differences in the residue inputs between the irrigated and dryland systems are not a major control on macroaggregation, and that increased soil C sequestration under irrigation is not related to macroaggregate dynamics. We hypothesize that soil disturbance though tillage (similar in both irrigated and dryland but lacking in the grassland), rather than residue-C input, is the overriding control on macroaggregate dynamics in these systems. This study however demonstrated the potential of center-pivot irrigation management for increasing SOC levels through long-term stabilization of C by microaggregates: at those sites where center-pivot irrigation increased total SOC storage, 74 to 85 percent of the difference in total SOC was accounted for by the difference in C protected by microaggregates. This was mainly due to higher amounts of microaggregates as well as higher microaggregate-C concentrations.
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