Effects of manure on topsoil and subsoil organic carbon depend on irrigation regimes in a 9-year wheat-maize rotation

•Interaction between irrigation and fertilization on SOC and components was found.•The SOC was decreased by CF while was increased by CFM following reduced irrigation.•The CFM increased SOC predominately via increasing more stable carbon pool.•The SOC changes under CF and CFM were related to the varied enzyme activities.•The SOC origin shifted from wheat-C to maize-C following increased soil depth. On the North China Plain, low soil organic carbon (SOC) content and a consistently dropping underground water level are seriously threatening food production security. To date, the interactive effect of fertilization types and irrigation regimes on surface and subsurface SOC contents has rarely been quantified. Here, we conducted a field study of three irrigation regimes with two fertilization types between 2009 and 2018. The three irrigation regimes included presowing irrigation (W0); presowing and jointing irrigation (W1); and presowing, jointing and anthesis irrigation (W2). The two fertilization types were mineral fertilizer only (CF) and mineral fertilizer plus composted chicken manure (CFM). Soil samples were collected from a depth of 0−100 cm in 20-cm increments to measure SOC and its composition, including labile pool I (LPI) (polysaccharides), labile pool Ⅱ (LPⅡ) (cellulose), and recalcitrant pool (RCP). Four SOC-related enzymes were determined, i.e., α-glucosidase, β-glucosidase, peroxidase and polyphenol oxidase. Stable carbon isotopic (13C) analysis was determined to distinguish the contributions of wheat- and maize-derived SOC in a wheat-maize rotation. The results showed that the 0−20-cm SOC content was highest in the W0 + CFM treatment. In the 0−20-cm layer, SOC decreased from W2 to W1 or W0 under CF, which is attributed to a reduction in the labile pool and maize-derived C and higher polyphenol oxidase. Conversely, under CFM, the 0−20-cm SOC content increased following decreased irrigation from W2 to W1 and from W1 to W0, mainly due to the increased LPⅡ and RCP. Compared with CF at W0 and W1, the 0−20-cm SOC content increased by 30.5 % and 21.8 % under CFM, respectively. Relative to the 0−20-cm soil layer, the SOC stock was larger in the 20−100-cm layer, which of the treatments had a variation manner similar to the 0−20-cm soil layer. Our results suggest that reduced irrigation regimes have significantly reduced the SOC stock and altered its composition in the 0−100-cm soil layer under CF and that CFM can increase the SOC stock in this layer when