Effects of fresh spent mushroom substrate of Pleurotus ostreatus on soil micromorphology in Brazil

• We clarified the micromorphology of a soil amended by fresh spent mushroom substrate.
• The amendment changed the topsoil and subsoil structures.
• Porosity and complexity increased in the topsoil and subsoil of the amended soil.
• Excrements were observed in the topsoil, and hyphae were present in the subsoil.
• Fresh spent mushroom substrate directly and indirectly affected soil formation.

Spent mushroom substrate (SMS) is a good organic material for soil amendment over a short period of time. Several studies have investigated the stability of soil aggregates in amended fields; however, few have attempted to clarify the formation of soil aggregates. In this study, we investigated soil aggregates in fields amended with fresh SMS (from Pleurotus ostreatus) (SMS field; SF) and those without amendment (control field; CF) in Brazil by soil morphological analysis. The results demonstrated that SF was dominated by the strong development of a granular microstructure in the A horizon (15–20 cm) and a spongy structure in the B horizon (45–50 cm and 70–75 cm). These horizons had a high porosity and a high fractal dimension. Therefore, these results suggest that the addition of fresh SMS changed the soil structure and the porosity in both topsoil and subsoil. In the A horizon of SF, the granules contained a large amount of SMS residues and excrement. In the B horizon of SF, we found a large amount of plectenchymae. These results suggest that soil structure formation was related to SMS and soil fauna in the A horizon and to fungi in the B horizon.