Organic mulching alters the composition, but not the diversity, of rhizosphere bacterial and fungal communities

Abstract

Organic mulching is an effective forest management technique that provides carbon and nutrient sources to soil ecosystems, thereby improving the soil environment and promoting plant growth. Although the importance of rhizosphere microbiomes in plant and soil ecosystem functions has been widely recognised, the effect of organic mulching on rhizosphere microorganisms and the underlying mechanisms are unclear. We performed a field experiment in a 15-year-old Ligustrum lucidum forest of urban green space. The diversity and composition of the rhizosphere bacterial and fungal communities following organic mulching were assessed by combining 16S ribosomal RNA and internal transcribed spacer amplicon sequencing. The correlations between microbial diversity, composition, and fine-root traits, as well as rhizosphere soil properties, were also analysed. The results showed that organic mulching did not significantly affect the diversity of the rhizosphere bacterial or fungal communities on the whole, but it increased the bacterial diversity after 6 months, with a 20-cm-thick mulch layer showing a greater effect than 5- or 10-cm layers. Organic mulching significantly altered the rhizosphere bacterial and fungal community composition; after 6 months of mulching, the community compositions were significantly associated with fine-root traits (specific root length, nitrogen, and phosphorus concentration) and enzyme (urease and dehydrogenase) activity. Moreover, alterations in the bacterial and fungal communities occurred at the order level within each mulching stage. Bacterial diversity is affected by fungal diversity and rhizosphere soil properties (water content and organic carbon) in time-dependent manners. Hence, organic mulching appears to directly affect the fungal composition while indirectly affecting the bacterial composition via influencing rhizosphere soil properties (dissolved organic carbon and peroxidase activity). Our study suggests that organic mulching affects the rhizosphere bacterial and fungal community composition through different pathways; however, the underlying mechanisms, including the effects of time and soil layers, require further exploration combined with multi-index measurements and long-term dynamic monitoring.