Wetting and drying cycles improving aggregation and associated C stabilization differently after straw or biochar incorporated into a Vertisol

Wetting and drying (WD) cycle drives soil structure dynamics more intensively in swelling soils than in non-swelling soils, and its feedback on soil organic C stabilization. Such relationship may be further dependent on the quality of organic amendment. Our objectives were to evaluate the role of WD cycle on soil aggregation after organic incorporation and to determine its physical protection on C stabilization in aggregates. Two organic inputs (maize straw and straw-derived biochar) were incorporated into a Vertisol subjected to two WD intensities (constant soil moisture and WD cycles). After 56 day incubation, CO2, microbial biomass C (MBC), aggregate stability (MWD), hot water extractable C (HWC), and delta(13)c were measured. Our results showed that straw amendment significantly enhanced soil respiration, MBC, and hence improved aggregation (P < 0.05), as compared to the control treatment but biochar did not. The WD cycle improved soil aggregation process (MWD) but decreased the MBC relative to the constant moisture condition in organic amended soils or not (P < 0.05). Natural delta C-13 indicates that straw derived C joined soil aggregation process as a binding agent greater than biochar derived C did (P < 0.05). The WD cycle further enhanced the new C stabilization in large macro aggregates in straw amended soils. However, this improvement was observed in microaggregates and silt + clay fraction in biochar amended soils. Thus, our results demonstrate that the WD) cycle can improve aggregation and associated C stabilization after organic amendment in the Vertisol, but this process is organic type dependent.