Abstract
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.
Highlights
Earthworm casts are considered to be more fertile than bulk soil [1], the contributions of earthworm casting activity to soil fertility have not been fully understood
The casting process resulted in a greater soil microbial biomass C (MBC) or N (MBN) [16,17] and improved nutrient availability [18]
Recent research reported that the increase in soil NH4+ induced by the earthworm Pontoscolex corethrurus was efficiently utilized by the plants with greater colonization of arbuscular mycorrhizal fungi (AMF) [22], which suggested that nutrients derived from earthworm casts may only be accessible to certain groups of soil biota
Summary
Earthworm casts are considered to be more fertile than bulk soil [1], the contributions of earthworm casting activity to soil fertility have not been fully understood. The positive effects of earthworm casts on vegetation were attributed to high concentrations and availability of soil nutrients, improved physical structure of the soil, enzyme activities and an effect of microbial inoculation [2,3,4,5,6]. Many studies have focused on the effects of earthworm casts on the physical structure and nutrient cycling of the soil [8,9,10,11]. The casting process resulted in a greater soil microbial biomass C (MBC) or N (MBN) [16,17] and improved nutrient availability [18]. Recent research reported that the increase in soil NH4+ induced by the earthworm Pontoscolex corethrurus was efficiently utilized by the plants with greater colonization of arbuscular mycorrhizal fungi (AMF) [22], which suggested that nutrients derived from earthworm casts may only be accessible to certain groups of soil biota
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