The long-term effect of sugarcane crop residues and fertilization on soil physical properties may be mediated by microbial communities

Soil physical structure, a key indicator of soil health and biomass production potential, can be altered by agricultural practices. In South Africa, the intensive and long-term sugarcane (Saccharum officinarum L.) monoculture is known to degrade soil characteristics. The objective of this study was to evaluate the effect of crop residue management practices (mulching, burning with residues scattered or removed) and mineral fertilization (with or without) on soil physical structure and to investigate possible link with soil microbial communities. To this aim, we analyzed soil aggregation and shrinkage curves (SSCs) in a long-term sugarcane trial established in 1939. The SSC provides descriptive structural soil data by differentiating and characterizing two pore systems (plasma and structural pores). We also quantified soil microbial communities’ abundance by qPCR as well as exopolysaccharides (EPS). Residue management and fertilization practices were found to have significant effects both on physical and microbial properties. Partial redundancy analysis showed that residue management practices had a slightly higher effect (19% of total variance) on hydrostructural variables compared with fertilization (12%). Total soil shrinkage, specific volume, and swelling capacity of the plasma were higher in mulched and/or unfertilized plots, indicating that soil was less compact, and shrinkage was more intense, including at the plasma level. The stronger structural dynamics and aggregate stability of the soil were explained by the behavior of the primary aggregates (peds), which were more porous and reactive during the drying process. In addition, swelling capacity of the plasma and the mean weight diameter of aggregates were both correlated to the amount of microbial EPS and the fungal abundance. This study highlights the importance of mulching and limited fertilization to maintain soil structure over the long term through the action of microbial communities.