Modeling soil aggregation at the early pedogenesis stage from the parent material of a mollisol under different agricultural practices

Summarization: Soil aggregation and development of soil structure play a crucial role in determining soil functions and ecosystem services in Earth's Critical Zone. Soil organic matter (SOM) is the main constituent that binds mineral particles together into larger-sized aggregates. Many theoretical concepts have been proposed to explain soil aggregation and SOM accrual processes, but quantification of the processes remains lacking. We observed changes in aggregate size mass distribution and aggregate carbon content in 2 discrete years from an 8-year field experiment, which was conducted to determine how to speed up soil development and restoration with different agricultural practices. The 2-year data showed that the proportion of macroaggregates (> 250 μm) increased with the decreasing proportions of microaggregates (53–250 μm) and soil texture units (< 53 μm) and increasing proportion of particulate organic matter occluded in the macroaggregates. This aggregation process was more noticeable in the field treatments with higher organic carbon input. By using these data, we successfully calibrated the recently developed CAST mathematical model, which assumes that primary macroaggregates are first formed around plant-derived organic matter and primary microaggregates are then formed within the macroaggregates as the occluded organic materials are decomposed. The secondary microaggregates and macroaggregates are assumed to form after the breakdown of primary macroaggregates. The calibration processes separated the plant growth season from the frozen winter season. Some calibrated parameters were the same for all of the field treatments, and these parameters included the first-order rate constants of the fragmentation and decomposition of plant litter within the macroaggregates, the decomposition rates of organic matter incorporated and protected in microaggregates, and organic carbon associated with the free soil texture units. Other calibrated parameter values were affected by the field treatments, and the parameters were the first-order decay rate constants of the plant-derived organic matter in nonaggregated soils and in the secondary macroaggregates, the least carbon sources for the formation of the secondary macroaggregates, and the correction factor to adjust carbon with the mass flow of the soil texture units. The differences in the calibrated parameter values suggested that the rates of the primary macroaggregation and the secondary microaggregation were likely controlled by the intrinsic properties of the parent material rather than by the land use and agricultural practices, whereas the rates of the secondary macroaggregation process were likely affected by land use through controlling plant litter quality and quantity and by agricultural practices, such as soil tillage, and to a less extent by fertilizers that control organic input and then carbon and mass content of the soil texture units and microaggregates. The model results were consistent with measured changes over time of the aggregate mass distribution by size class, the total organic matter content, and the SOM bound in aggregates. However, the CAST model did not reproduce particularly well the mass of the soil texture units within different aggregate size classes nor the relative contribution of carbon stocks from different carbon pools within aggregates. The gap between observations and model results points the way forward to develop further the detailed process descriptions in the CAST modeling approach, for example, to consider primary microaggregation directly from basic soil texture units. Nevertheless, this study demonstrates that the CAST model is very powerful to predict the dynamics of soil aggregation and concurrent SOM change in response to changes in land use and soil management.Appearing in: Quantifying and Managing Soil Functions in Earth's Critical Zone Combining Experimentation and Mathematical Modellin