Earthworms drastically change fungal and bacterial communities during vermicomposting of sewage sludge

Wastewater treatment plants produce hundreds of million tons of sewage sludge every year all over the world. Vermicomposting is well established worldwide and has been successful at processing sewage sludge, which can contribute to alleviate the severe environmental problems caused by its disposal. Here, we utilized 16S and ITS rRNA high-throughput sequencing to characterize bacterial and fungal community composition and structure during the gut- and cast-associated processes (GAP and CAP, respectively) of vermicomposting of sewage sludge. Bacterial and fungal communities of earthworm casts were mainly composed of microbial taxa not found in the sewage sludge; thus most of the bacterial (96%) and fungal (91%) taxa in the sewage sludge were eliminated during vermicomposting, mainly through the GAP. Upon completion of GAP and during CAP, modified microbial communities undergo a succession process leading to more diverse microbiotas than those found in sewage sludge. Consequently, bacterial and fungal community composition changed significantly during vermicomposting. Vermicomposting of sewage resulted in a stable and rich microbial community with potential biostimulant properties that may aid plant growth. Our results support the use of vermicompost derived from sewage sludge for sustainable agricultural practices, if heavy metals or other pollutants are under legislation limits or adequately treated.

One-Year Monitoring of Daily Earthworm Cast Production: Surface Cast Contribution to Soil Fertility in a Subtropical Forest

The process of earthworm casting plays an important role in soil fertility. The contributions of earthworm casts to soil nutrients dynamics cannot be fully understood unless the temporal changes in earthworm cast production and the related nutrient resource pools are explored in the field. Here, we recorded the daily surface cast production of earthworms and monthly change patterns of carbon (C), nitrogen (N) and phosphorous (P) in casts in a subtropical plantation for one year and compared the nutrient concentrations and microbial community traits (structure and bacterial activity) in casts with those in bulk soils. Our objectives were to characterize the temporal patterns of earthworm cast production and the C and nutrients in casts and to assess the potential contribution of earthworm casts to soil fertility. The results showed that the annual surface cast production at the study site was 8.3 tons ha−1; monthly cast production was >80 g m−2 during April to November and <30 g m−2 during December to March. The annual outputs of total organic C (TOC), total N (TN) and total P (TP) in surface casts were 68.10, 4.84 and 0.38 g m−2, respectively; the annual outputs of available P (AP), NH4+-N and NO3−-N were 22.4, 147.0 and 61.3 mg m−2, respectively. The concentrations of TOC and TN in casts were higher during warm months, but NH4+-N was higher during cold months, while both the concentrations of TP and AP in casts were constant throughout the year. The nutrient concentrations were 1.6–4.3-fold greater, and the microbial biomass C and N contents were 7–8-fold greater in earthworm casts than those in bulk soils. However, the bacterial stress index (BSI) was significantly greater in casts than in the bulk soil, which implies that bacteria in casts were under stress. Accordingly, the ratio of fungal to bacterial biomass (F/B) was significantly higher in casts than in the bulk soil. Earthworm casting activity can process forest soil into a “new soil”, which forms large amounts of earthworm-induced hotspots of soil fertility. These resource pools in casts, with monthly fluctuated C and N and constant P, could be more readily accessed by fungi rather than bacteria. Earthworm casting activity potentially changes both the nutrients available and the interactions of plants and rhizosphere microbiota.