Tillage-induced effects on organic carbon in earthworm casts through changes in their physical and structural stability parameters

Earthworm casts have been recognized as contributing to long-term carbon protection, but few studies have focused on the mechanism by which this is achieved. Most of the literature documenting earthworm casts revolves around the difference between the earthworm cast and natural soil, whereas little attention has been given to deciphering the effects of different tillage systems on earthworm casts. Further, whether or not an earthworm cast, like soil, affects its own organic carbon content through physical structure stability is still somewhat vague, and very little is known. The objective for this study was to determine the effect of tillage on organic carbon and physical structure stability parameters in earthworm casts. Undisturbed soil columns were collected from a 17year tillage experiment including no tillage (NT), ridge tillage (RT) and mouldboard ploughing (MP) in Northeast China. Each column was seeded with ten earthworms (Eisenia foetida), incubated at 18 ◦C for 120 days and casts were collected and analysed. Both NT and RT led to significantly greater organic carbon (20.5% and 14.4%), total pore volume (54.6% and 75.6%), total pore area (45.4% and 60.4%), permeability (21.8% and 66.4%), fluid conductivity (26.2% and 31.1%), tensile strength (34.1% and 51.2%) and water resistance index (16.7% and 22.3%) in the casts compared to MP (P < 0.05), whereas water repellency showed a significant decrease (42.9% and 52.4%) (P < 0.05). Correlation analysis showed that total pore area, tensile strength and water repellency were the three most critical factors affecting organic carbon in earthworm casts (r = 0.943, 0.742, 0.762 and 0.715, 0.758, 0.743 for NT and RT, respectively). The results of tillage effects on organic carbon and physical structure stability parameters combined with Structural Equation Modelling analysis identified that NT and RT could increase organic carbon by increasing total pore area, tensile strength and water repellency in earthworm casts. These results demonstrate that tillage through physical structure stability (total pore area, tensile strength and water repellency) changes affect organic carbon in earthworm cast which are major new findings. This signifies similarity in the underlying processes by which tillage affects organic carbon and physical structure stability parameters in earthworm casts and the surrounding bulk soil. Moreover, the increased abundance of earthworms and greater organic carbon in earthworm casts under conservation tillage, suggest that earthworms enhance the conversion of residue into organic carbon and therefore enhance the long-term soil organic carbon sequestration.