Bio-converted organic wastes shape microbiota in maize rhizosphere: Localization and identification in enzyme hotspots

Organic fertilizers increase soil fertility, microbial diversity and heterogeneity of microbial activity hotspots. Black soldier fly (Hermetia illucens L.) frass (BSFF) derived from organic waste composts has been extensively marketed as organic fertilizer, but its effects on rhizosphere microbiota remain unknown. We compared the effects of BSFF from three organic wastes (straw, manure, and kitchen waste) on maize growth in acidic soils. Sufficient P, an optimal soil pH, and a satisfactory soil C/N ratio, as provided by the straw-derived frass, increased the maize growth more than the other frass types. Maize growth increased due to nutrient mobilization by active rhizosphere microbiota, which was localized using in situ soil zymography and identified through DNA amplicon sequencing and quantitative PCR. Specifically, the area of hotspots of acid phosphatase activity along the maize roots increased for 2.8 times after straw-derived frass application compared to unamended soils. Microbial diversity raised within these enzyme activity hotspots and was accompanied by the increased abundances of plant growth-promoting microbial taxa like Luteibacter, Chrysosporium, and Cladorrhinum. Microorganisms formed efficient and mutualistic networks within the rhizosphere hotspots induced by straw-derived frass to accelerate organic P mineralization, as inferred by random forest analysis and the quantitative of phoC and phoD genes. Concluding, the straw-derived frass benefits maize growth on acidic soils through abiotic (soil physico-chemical properties) and biotic (active rhizosphere microbiota) stimuli.