Paper
Earthworm communities influence soil carbon and nitrogen circulation by altering the diversity and composition of microbial communities, which improves soil fertility. Studying the soil nutrient composition and bacterial communities change in response to earthworm community natural invasion may be key to exploring earthworm ecological functions and accurately assessing C and N mineralization in artificial forests and croplands. In this study, we examined the communities of five earthworm species in ecosystems characterized by six different land-use types, such as buttonwood forest, walnut forest, apple orchard, kiwi orchard, ryegrass land, and corn field. The Metaphire baojiensis (d) and Amynthas carnosus planus were dominant earthworm species. Among different land-use types, earthworm densities ranged from 2 to 27 ind·m−2 in summer and 15 to 40 ind·m−2 in spring. However, surface vermicompost weight in summer (296.7 to 766.0 g·m−2) was greater than in spring. There was a positive correlation between the weight of the vermicompost and earthworm numbers in the same season. Soil carbon (C) and total nitrogen (N) of vermicompost ranged from 5.12 to 20.93 g·kg−1 and from 0.52 to 1.35 g·kg−1, respectively. Compared with soil, the contents of vermicompost C and N increased 2.0 to 4.3 times and 1.6 to 7.7 times, respectively. The average C/N of vermicompost (9.5~23.5) was higher than in the soil (7.3~19.8). Due to the higher abundances of C and N in the soil of corn fields and kiwi orchards, which cultivate higher abundances of earthworms and more vermicompost, the C and N and C/N of vermicompost is higher than in the soil. C and N were accumulated by earthworms’ excreting and feeding activity instead of vegetation in vermicompost. Earthworm community structure plays key roles in decreasing bacterial diversity and adding Proteobacteria, Actinobacteria, Acidobacteria, Bacteroidetes, and Chloroflex in vermicompost, resulting in enriching soil C and N content and increasing C/N in vermicompost. Therefore, the evaluation of different vegetation ecosystems in soil C and N pool accumulation and mineralization should be given more attention regarding the function of earthworm communities in the future.