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Abstract Soil can act as a source or sink for methane (CH 4 ), a potent greenhouse gas and key component of atmospheric carbon cycling. We monitored CH 4 fluxes over 4 years in a semiarid, sprinkler‐irrigated crop rotation [spring wheat ( Triticum aestivum L.), potato ( Solanum tuberosum L.), barley ( Hordeum vulgare L.), and sugar beet ( Beta vulgaris L.)] in southern Idaho to evaluate the effects of dairy manure and synthetic fertilizer. Treatments included an unfertilized control, synthetic fertilizer, and three manure rates (18, 36, and 52 Mg ha − 1 ). Fluxes were measured using static closed chambers. Across all treatments and years, soils acted as a net CH 4 sink, where negative flux values indicate net uptake. Fluxes ranged from −1.5 ± 1.0 g CH 4 ha − 1 day − 1 (control) to −1.3 ± 1.0 g CH 4 ha − 1 day − 1 (high manure). Manure weakened uptake in proportion to rate, while synthetic fertilizer and low manure maintained control‐like fluxes. Uptake peaked in October (2.3 ± 0.6 g CH 4 ha − 1 day − 1 ) and December (2.1 ± 0.7 g CH 4 ha − 1 day − 1 ), was lowest in June (0.9 ± 0.6 g CH 4 ha − 1 day − 1 ), and was moderate during freeze–thaw transitions. Uptake was highest in 2014 (2.0 ± 0.7 g CH 4 ha − 1 day − 1 ) during a potato year with extensive tillage and lowest in 2016 (0.8 ± 0.3 g CH 4 ha − 1 day − 1 ) when monitoring ended earlier. Cumulative uptake was weakest in high manure (1.72 kg CH 4 ha − 1 ) and strongest in low manure (2.04 kg CH 4 ha − 1 ) and control (2.01 kg CH 4 ha − 1 ). Overall, organic amendments reduced uptake but did not shift soils to a net source.