It is important to understand soil organic carbon (SOC) sequestration and its relationship with crop productivity. Based on two long-term experiments under irrigated vs rain-fed conditions conducted in the China Loess Plateau, we evaluated SOC sequestration efficiency under diverse fertilization regimes and quantified the relationship between crop yield and SOC. The experiments consisted of a winter wheat-summer maize system under irrigated conditions with nine treatments-nitrogen (N), N and phosphorus (P), N and potassium (K), P and K (PK), combined NPK, crop straw (S) in combination with NPK (SNPK) and dairy manure (M) integrated with NPK (M1NPK and M2NPK)-and a winter wheat-summer fallow system under rain-fed conditions with seven treatments-CK, N, NK, PK, NP, NPK, and MNPK. After 25 years, the SOC storage was generally higher in the irrigated plots than in the rain-fed plots by 6% on average. The carbon sequestration efficiency was higher under the rain-fed (28%) than under the irrigated (19%) condition. The relationship between relative yield and SOC content suggested that the threshold SOC contents for obtaining the highest crop yield were 10.0 and 8.8 g kg −1 for the winter wheat-summer maize and winter wheat-summer fallow cropping systems, respectively. This difference suggests that a higher SOC level is required to support high crop productivity in the irrigated system than in the rain-fed system.

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