Land cultivation, drainage and irrigation not only arise movement and deposit of soil dissolved organic carbon (DOC) within the canals and ditches, but also lead to the difference of sediment DOC. Previous studies were mainly performed at plot-to-field scale, but little is known about the effect of intensive agricultural catchments on the distribution of sediment DOC beyond field edges. This study was conducted to examine the distribution characteristics of sediment DOC in multi-stage canal and ditch systems in the Sanjiang Plain. Sediment samples were collected from multi-stage canal and ditch systems, which have similar soils and land use patterns, and analyzed for DOC content. Those different units of canals and ditches varied from field-edge waterways to sizeable drainages. The proportion of sediment DOC increased gradually and the sediment DOC was transferred to the deeper sediment layers (40-60cm) with increased drainage area, such as from field canal, to lateral, to branch, to trunk, and to general main canal. Overall, the mean DOC content in sediment was the highest in general main canal (488.747mg/kg) and the lowest in the field canal (269.672mg/kg). The sediment DOC in 40-60cm layers was mainly accumulated in general main canal (1270.947mg/kg). The regions with the highest values for 20-40cm, 10-20cm and 0-10cm sediment layers were mainly in the trunk canal (326.147mg/kg), field or lateral canal(360.547mg/kg and 360.547mg/kg), and branch or trunk canal (425.747mg/kg and 450.947mg/kg), respectively. Those canals and ditches collected and stored the DOC mass flows from wetland and farm, and then transferred them into rivers and lakes.
Temporally repetitive drainage of canals and ditches transported DOC flows into canal and ditch systems by sporadic flows. The Land erosion controlled flows of DOC mass during storm events. Continuous flows of DOC interacted with multi-stage canal and ditch systems. The spatial distribution of sediment DOC appears, in certain extent, as horizontal distribution with hypsometric characteristics. Close relationships existed among DOC concentration, flux, period, frequency and duration and the distribution characteristics of DOC in sediment of multi-stage canal and ditch systems. In addition, the vegetative characteristics of the canals and ditches exerted an important influence on DOC distribution in the sediment of the multi-stage canal and ditch systems that had a response to processes of organic matter in the sediments such as decomposition, humification, and stabilization of organic matter.