Quantification of micron-scale structural and hydraulic properties of long-term pig manure and lime amended red soil aggregates using the lattice Boltzmann method and pore network modelling

Lime and animal manure can have major impacts on soil physical properties, particularly in degraded and highly weathered soils that are naturally acidic. Here we evaluate how treatment of a regular till Acrisol in southeast China with different amounts of lime and/or pig manure, and planted with maize, affects pore scale properties down to micron size using synchrotron microtomography (SR–mCT). Soil macroaggregates (2 - 5 mm) from 4 treatments were measured: 1) Control, no manure amendment; 2) low manure (150 kg N ha^-1 y^-1); 3) high manure (600 kg N ha^-1 y^-1); and 4) high manure (600 kg N ha^-1 y^-1)+ lime (3000 kg Ca(OH)₂ ha^-1 every 3 years). Pore structure at a resolution of 3.7 µm was reconstructed in 3D and the Multi- Relaxation- Time (MRT) Scheme for Multi- Phase Lattice Boltzmann Method (LBM) was used to simulate water flow and retention. Topological analysis was performed based on the extracted pore network by using the maximal ball-based pore network extraction. A quasi-static pore network solver was applied to compute the capillary pressure based on the extracted pore networks. The application of a high amount of pig manure increased the fraction of macropores (>100 µm) to 38.61% compared to the controlled level (18.15%). A high amount of pig manure also decreased total porosity to 8.08% compared to 11.35% for the control, suggesting less micropores caused by high pig manure treatment. The application of high amount of pig manure and lime also caused more uniform water flow. Control samples had a velocity frequency at around e¹¹ of the normalized velocity (respect to the mean), while the samples from the other treatments had more evenly distributed peaks. Water flows most quickly due to least impediment by pores in the samples with high manure amendment. The slope between permeability and porosity increased from 8.10 Darcy (controlled) to 174.47 Darcy (high amount of manure treatment). The amendment of 600 kg N ha^-1 y^-1 pig manure increased water retention ability calculated by the simulations. For the capillary pressure > -50 kPa, control samples had the greatest water saturation level compared with the samples from the other treatments, while there were no significant differences of water saturation of samples from all the treatments for the capillary pressure < -1000 kPa . The simulated water retention results had the same trend with the measured results.