Aggregate pore and shape properties were more strongly correlated to soil organic carbon in large aggregates: Evidence from a long-term management-induced soil carbon gradient

The interplay between soil structure and soil organic carbon (SOC) is complex and affects key soil functions. There is limited knowledge on how this relationship changes with the size of the structural unit studied. The objective of this study was to quantify the pore and shape characteristics of soil aggregates of varying sizes, and their relationships with SOC under different soil management regimes. Soils were sampled in March 2015 from the Highfield Ley-Arable Long-Term Experiment at Rothamsted Research. This experiment includes bare fallow, continuous arable rotation, ley-arable rotation, and grass treatments. A total of 24 aggregates from each treatment and size class (2–4, 4–8, and 8–16 mm) were subjected to X-ray micro-CT scanning at 40 μm voxel resolution. Results showed that permanent grass not only increased SOC accumulation, but also promoted pore connectivity of soil aggregates compared to bare fallow, regardless of aggregate size. Additionally, the pore and shape characteristics of larger aggregates (4–8 and 8–16 mm) were more sensitive to soil management compared to smaller aggregates (2–4 mm). The relationships between SOC and aggregate structural characteristics were strong for both 8–16 and 4–8 mm aggregates but weak for 2–4 mm aggregates. Furthermore, the responses of pore connectivity and sphericity to SOC increased with aggregate size. The results suggest that organic matter input plays an essential role in shaping aggregate structural characteristics and aggregate rearrangement (especially in larger aggregates).