Impacts of size reduction and alkaline-soaking pretreatments on microbial community and organic matter decomposition during wheat straw composting

[Display omitted] •Alkaline soaking (Alk) rather than size reduction (M) altered bacterial communities.•Alk promoted lignocellulosic saccharification and humification.•The combined pretreatments M + Alk enhanced composting temperature by 14.4 °C.•M + Alk pretreatments decreased NO3-N and increased NH3N in the compost.•M + Alk pretreatments improved the GI of products by 49.70%–60.20%. This study assessed the impacts of size reduction and alkaline-soaking pretreatments on microbial community shifts and organic matter decomposition in wheat straw composting. Bacterial communities were altered by alkaline soaking rather than size reduction, while fungal communities were altered by both pretreatments. Alkaline-soaking pretreatment promoted lignocellulosic saccharification and humification. A combination of both pretreatments increased the proportion of the fungal genus Coprinopsis (39%) at the early stage and promoted the proliferation of Ornithincoccus (15%) at the late stage. This facilitated the mineralization of ammonium N from amino acids; decreased the total lipids, free fatty acids, and nitrate N contents; and greatly improved the germination index of the final composting product to a high level of 149% as tested with radish seeds. The findings demonstrate that the combined application of size reduction and alkaline-soaking pretreatments is an effective strategy for improving the product quality of wheat straw compost.