No-tillage with continuous maize cropping enhances soil aggregation and organic carbon storage in Northeast China

In Northeast China, conventional tillage practices involve removal of crop residue after harvest and prior to moldboard plowing; this has been shown to cause a decline of soil organic carbon (SOC) and degradation of soil structure. No-tillage has been suggested to be an effective way to increase SOC storage but its effectiveness in some soils and climates has been questioned. Different cropping systems also influence SOC storage. Hence, we established an experiment to evaluate how a combination of different tillage and cropping systems could improve soil aggregation and organic carbon storage. We included five treatments: a) NTMS: no tillage with maize (Zea mays L.)-soybean (Glycine max Merr.) (MS) rotation; b) MPMS: moldboard plowing with maize-soybean rotation; c) NTMM: no tillage with continuous maize (MM); d) MPMM: moldboard plowing with continuous maize; e) CTMM: conventional tillage with continuous maize (the conventional farming practice in Northeast China). All crop residues were returned to the soil except in the CTMM treatment. Returning residue to the soil significantly increased SOC storage in all tillage/cropping systems with NTMM having the highest increase in rate of SOC storage at 0.80 Mg C ha−1 yr−1 relative to the start of the experiment. The CTMM depleted SOC storage at rate of 0.52 Mg C ha−1 yr−1 relative to the start of the experiment. Soil under NTMS exhibited a significant SOC decline deep in the soil (5-30 cm) but overall SOC storage in 0-30 cm profile was equal to that under MPMS. The NTMM had the highest SOC storage and the highest proportion and associated organic carbon (OC) in occluded micro-aggregates (53-250 μm, inside of 250 μm aggregates) across all experimental treatments. The OC in occluded micro-aggregates was much higher than that associated with unprotected micro-aggregates (53-250 μm, outside of 250 μm aggregates). The effects of tillage on aggregate size and OC concentration occurred mainly in the surface layer (0-5 cm) whereas the effect of cropping system on aggregate size and OC concentration occurred at deeper depths. The MS cropping system increased the proportion of silt-clay (<53 μm, outside of 250 μm aggregates) over MM while occluded silt-clay (<53 μm, inside of 250 μm aggregates) in MM was greater than in MS in all layers. The NTMM treatments improved SOC storage and aggregation over the other treatments.