Depth impacts on the aggregate-mediated mechanisms of root carbon stabilization in soil: Trade-off between MAOM and POM pathways

Agricultural practices that promote the formation of soil organic matter (SOM) are considered important climate change mitigation strategies by increasing resilience to climate shocks and promoting soil carbon sequestration. Efforts to increase root production and depth distribution through planting deep rooted crops and selective crop breeding have been identified as a promising strategy to achieve these goals. However, we lack a complete understanding of how the decomposition of roots in the deep soil (e.g., below 30 cm), contributes to SOM formation and stabilization. Here using unique soil-biomass microcosms in the field to trace 13C enriched root litter to a depth of 90 cm, we show that as decomposition dynamics change with depth, so do the SOM formation pathways. At our study site, root residues decomposed faster in the top 0-30 cm, achieving 97 % mass loss by 13 months of incubation compared to 77 % and 81 % in the 30-60 and 60-90 cm depths, respectively. Litter derived carbon (LDC) was preferentially recovered as stable mineral associated organic matter (MAOM), primarily within aggregates, with 67 % more in the 0-30 cm than in the 60-90 cm depth. At depth, root residues decomposed slower and accumulated as the less stable particulate organic matter (POM) within macroaggregates with 145 % more LDC recovered in light POM in the 60-90 cm depth than the 0-30 cm depth. We found that bulk SOM measurements were too coarse to elucidate the likely fate of newly incorporated litter in the soil, but our detailed fractionation demonstrated the relative contribution of new root inputs to functionally different SOM pools, MAOM and POM, and allowed us to interpret the role of microaggregates in these dynamics in new detail, particularly microaggregates within macroaggregates (i.e., occluded microaggregates). Our results highlight the importance of balancing the trade-off between MAOM and POM formation when considering strategies to enhance both carbon sequestration and soil health in agroecosystems. If POM is critical for aggregate formation and microaggregates play an important role in MAOM formation, efforts to increase soil carbon sequestration need to focus on both fractions and on supporting overall soil structure.