Diversity and abundance of soil fauna as influenced by long-term fertilization in cropland of purple soil, China

• Organic–inorganic compound fertilizers were beneficial for soil fauna diversity.
• Soil fauna diversity, abundance and functional groups are key biological parameters.
• Some indices of soil fauna community can indicate certain features of soil fertility.
• These indices may not be used to reflect integrated assessment for soil fertility.

The relationship between soil fauna groups and different fertilization regimes has been of continuous concern. Little attention has been paid to the response of soil fauna to changes in soil fertility. Thus, the aim of this study was to investigate the response of soil fauna communities to fertilizer management practices, to explore the inter-relationships between soil fauna communities and fertilization regimes so as to identify soil fauna species or groups that are sensitive to changes in soil fertility. This long-term fertilization experiment was conducted with a no fertilizer control and six fertilization regimes: CK (control, no fertilizer), N (synthetic N fertilizer), NPK (synthetic fertilizer: nitrogen, phosphorus and potassium), OM (pig manure), OMNPK (pig manure plus nitrogen, phosphorus and potassium), RSD (crop residues returned) and RSDNPK (crop residues returned with nitrogen, phosphorus and potassium). The application of organic fertilizers (i.e., pig manure or crop residues) promoted the diversity of the soil fauna communities due to the abundant organic matter the fertilizers supplied for the communities’ survival and development. Furthermore, organic–inorganic compound fertilizers (i.e., treatment of OMNPK or RSDNPK) were beneficial for richness and diversity of soil fauna communities due to their abundance of organic matter and supply of nutrients. Indices of soil fauna communities (i.e., TI, the DG (density-group) index, the number of individuals of Nemata, Lumbricida, Collembola and Oribatida) showed the same trend with an increase in the soil organic matter. Therefore, indices of soil fauna communities can be applied to indicate certain features of soil fertility, such as soil organic matter content, but they cannot be used to reflect integrated assessment for soil fertility.