No-tillage, cover crops, and N fertilization play important roles in conserving or increasing soil organic carbon (SOC). However, the effects of their interaction are less well known, particularly in Asian countries. We examined the effects of three tillage management systems, moldboard plow/rotary harrow (MP), rotary cultivator (RC), and no-tillage (NT); three winter cover crop types (FL: fallow, RY: rye, and HV hairy vetch); and two nitrogen fertilization rates (0 and 100 kg N ha(-1) for upland rice and 0 and 20 kg N ha(-1) for soybean production) on changes in SOC. Vertical distributions at 0-2.5, 2.5-7.5, 7.5-15, and 15-30 cm depths of soil carbon content and bulk density were measured each year. From 2003 to 2011, NT and RC management increased SOC by 10.2 and 9.0 Mg ha(-1), whereas SOC under the MP system increased only by 6.4 Mg ha(-1). Cover crop species also significantly increased SOC in the same period by 13.4 and 8.6 Mg ha(-1) for rye and hairy vetch, respectively, although SOC with fallow increased only by 5.4 Mg ha(-1). Continuous soil management for 9 years enhanced SOC accumulation. Summer crop species between upland rice and soybean strongly affected SOC; the SOC increases were 0.29 Mg ha(-1) year(-1) for the upland rice rotation and 1.84 Mg ha(-1) year for the soybean rotation. However, N fertilization levels did not significantly affect SOC. These results suggest that the NT system and rye cover crop enhance carbon sequestration in Kanto, Japan, but that their contributions differ depending on the combination of main and cover crops. (C) 2014 Elsevier B.V. All rights reserved.