Changes in soil organic matter after conversion from irrigated to dryland cropping systems

Global water resources are under increasing pressure, and some regions face the need to retire irrigation due to groundwater depletion or to meet governmental regulations. In arid and semiarid climates, irrigated lands tend to have more soil organic carbon (SOC) than non-irrigated croplands. However, little is known about how SOC might change following irrigation retirement. Our objective was to quantify changes in SOC and nitrogen stocks after irrigation retirement in semiarid agroecosystems of the High Plains. We sampled fields that stopped using irrigation and transitioned into either dryland crops or ungrazed perennial grasslands and compared SOC and nitrogen stocks in these fields with still irrigated and long-term dryland situations. Currently irrigated fields had more SOC (83.0 vs 73.3 Mg ha-1) and nitrogen (10.2 vs 9.4 Mg ha-1) at 0–80 cm depth than their dryland counterparts, confirming the reported positive impact of irrigation in semiarid agroecosystems. However, there was no legacy effect of irrigation on SOC and nitrogen topsoil stocks 7–10 years after transition to dryland crops as retired fields had lower stocks than still irrigated fields and did not differ from long-term dryland cropping zones of the same fields. This lack of a legacy effect was related to the preferential accumulation, during irrigation, of carbon and nitrogen in the less stable particulate organic matter pool at 0–10 cm soil depth. The transition from irrigated agriculture to perennial grasses resulted in intermediate SOC stocks, that did not differ from the still irrigated nor from the sites retired to dryland crops. Our results suggest that perennial systems are a viable option to mitigate, at least partially, the negative impact of irrigation retirement on SOC. However, the advantage of perennial grasses was explained by the accumulation of carbon and nitrogen in the particulate organic matter fraction, which poses some questions about the stability of SOC in these systems after changes in management.