The potential of conservation practices to maintain soil organic C (SOC) and reduce climate change impacts on yields is unknown. This study aimed to validate the DayCent model with observed yield data and the CQESTR model with measured SOC in two agroecosystems, and predict the best practices to sustain SOC under projected climate change. Data were from a conventional tillage (CT) wheat (Triticum aestivum L.)-fallow rotation without N or with 135 kg N ha(-1) fertilizer, and no-till (NT) experiments, NTA (NTA0W-F and NTA135W-F) and NTB with a wheat-pea (Pisum sativum L.) cover crop (NTB0W-P and NTB135W-P) in Oregon; and crop rotations with CT and NT silage or grain corn (Zea mays L.)-soybean [Glycine max (L.) Merr.]-alfalfa (Medicago sativa L.) with or without manure or stover removal or a cover crop in Pennsylvania. Measured and CQESTR-simulated SOC were significantly (p .0001) correlated (r = .90). In Oregon, predicted SOC to 60 cm increased under NTA135W-F and NTB135W-P at 0.05 and 0.08 Mg ha(-1) yr(-1) under projected climate change, respectively. The NT management under dryland production provided a limited SOC increase. In Pennsylvania, predicted SOC to 1 m decreased at 0.07 Mg ha(-1) yr(-1) in corn-soybean under CT or NT with stover removal, but increased by 0.71 Mg ha(-1) yr(-1) in dairy forage NT and manure added under climate change. The responses of SOC to climate change were affected by management, biomass type, edaphic properties, local climate, and agroecosystem.