Long-term crop rotation and different tillage practices alter soil organic matter composition and degradation

Conservation agriculture such as conservation tillage and crop rotation can alter soil organic matter (OM) storage and cycling. However, little is known about how these methods alter the molecular-level soil OM composition and turnover. We collected soil samples (0-10 cm) from a 37-year experiment with contrasting diversity crop rotations under conventional tillage and conservation tillage. Soil organic carbon concentrations, molecular-level OM composition and microbial biomass and community structure were measured. Soil organic carbon concentrations were similar between the two tillage practices, likely due to the balance between carbon loss via degradation and carbon gains through incorporation of crop residues in soil. We observed significant changes in soil OM composition between conventional tillage and conservation tillage despite similar soil organic carbon concentrations. Under long-term conservation tillage practice, lower suberin-derived compounds and higher lignin-derived compounds were observed. The degradation of cutin-, lignin-derived compounds and soil OM were lower under conservation tillage among most rotations. The changes in lignin-derived compounds and degradation were microbially driven and likely controlled by substrate utilization. In contrast, the differences in suberin-derived compounds were likely associated with crop residue-induced OM inputs and/or the differential introduction of OM (e.g. root materials) to the upper layer of soil due to conventional tillage vs. conservation tillage managements. These findings reveal that microbial degradation, OM inputs from crop residues and management-induced OM distribution in soil layers may alter agricultural soil OM composition. Additionally, we found that the differences in some specific groups of compounds between conventional tillage and conservation tillage were lower in crop rotations with higher diversity, highlighting the benefits of the interaction between diverse rotations and conservation tillage.