Soil can act as a source or sink for methane (CH4), a potent greenhouse gas and key component of atmospheric carbon cycling. We monitored CH4 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 CH4 sink, where negative flux values indicate net uptake. Fluxes ranged from -1.5 +/- 1.0 g CH4 ha- 1 day- 1 (control) to -1.3 +/- 1.0 g CH4 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 CH4 ha- 1 day- 1) and December (2.1 +/- 0.7 g CH4 ha- 1 day- 1), was lowest in June (0.9 +/- 0.6 g CH4 ha- 1 day- 1), and was moderate during freeze-thaw transitions. Uptake was highest in 2014 (2.0 +/- 0.7 g CH4 ha- 1 day- 1) during a potato year with extensive tillage and lowest in 2016 (0.8 +/- 0.3 g CH4 ha- 1 day- 1) when monitoring ended earlier. Cumulative uptake was weakest in high manure (1.72 kg CH4 ha- 1) and strongest in low manure (2.04 kg CH4 ha- 1) and control (2.01 kg CH4 ha- 1). Overall, organic amendments reduced uptake but did not shift soils to a net source.