Carbon sequestration and soil aggregation in center-pivot irrigated and dryland cultivated farming systems

Although irrigation is considered a beneficiary management for increasing soil organic C (SOC) stocks in (semi)arid environments, our understanding of the impact of irrigation on soil organic matter (SOM) dynamics in the field remains limited. We investigated the effect of irrigation on soil C storage in relation to soil aggregation by measuring C stocks of bulk soil and different aggreagate fractions in the top 20-cm layer of center-pivot irrigated vs. dryland farming systems in semiarid southwestern Nebraska. The irrigated fields (IRR) showed increased C inputs and larger SOC stocks than the dryland cultivated fields (DRY). Fractionation of bulk soil samples into non-microaggregate-associated particulate organic matter (free POM) and microaggregate-associated POM, silt, and clay fractions indicated that the larger bulk SOC stock under IRR was explained solely by an increase in microaggregate-associated C storage. Wet sieving of bulk soil showed that microaggregation was remarkably low under DRY and did not increase under IRR, suggesting that the protection of microaggregates inside macroaggregates was no prerequisite for C sequestration under IRR. The results of this study confirm the potential of irrigation to increase soil C stocks through preferential sequestration of C inside microaggregates, but question our understanding of the mechanisms underlying this preferential sequestration. © Soil Science Society of America.