Dynamics of soil organic carbon in an innovative irrigated permanent bed system on sloping land in southern Spain

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Irrigated spring cropping systems on sloping soils require conservation agriculture (CA) to minimise soil erosion. In southern Spain, management of residues in the commonly-practiced maize–cotton rotation is a major hurdle against adoption of CA. Recently, a minimal-cultivation, irrigated permanent bed system has been developed in which, two weeks before sowing, maize or cotton residues are relocated from the seed rows on the top of beds into the furrows. This allows solar heating of the beds which accelerates seedling emergence. This paper presents the effects that the introduction of this system had on soil properties along a variably-sloping landscape in which six homogeneous zones were identified. It also discusses the system's potential for carbon sequestration and shows that a similar quantity of carbon is stored after 4 years under irrigated permanent beds to that after 11–21 years in rainfed no-tillage systems. The relocation of residues resulted on average in lower soil organic carbon concentration (SOCc) in the beds than in the furrows (1.40 and 1.51% respectively in the top 5 cm of soil). Under this system SOCc also varied among the landscape zones particularly in the top layers. SOCc was highest in the most eroded shoulder-backslope zone (Z2) though soil organic carbon storage (SOCs) there was the lowest because of the zone's high >2-mm soil fraction. The SOCs (0–30 cm) global average during the study was 35.2 Mg C ha −1 and ranged from 22.4 Mg C ha −1 in the shoulder Z2 zone to 41.4 Mg C ha −1 in the Z5 footslope zone. When compared to conventionally-managed fields nearby, following this management system appeared to improve soil characteristics significantly and protect against soil erosion.