An Innovative Framework Fosters Practical Application of Nematode-Based Indices in Soil Health Assessment

Nematodes are versatile bioindicators of the soil food web in both agricultural and natural ecosystems. Multiple nematode-based indices (NBIs), derived from morphological, life history and community traits, provide invaluable information on various aspects of soil health. However, a standardised approach is required to explicitly link NBIs to the soil health concept. Moreover, unifying all NBIs into a single quantitative index could offer a more comprehensive and straightforward bioindicator for soil health. The ecological foundations for individual NBIs have been well established, but a single standardised bioindicator for soil microfaunal communities including nematodes, remains absent. Here, we integrated existing knowledge on NBIs into an innovative framework for quantitatively assessing soil health and ecosystem functions. Moreover, we propose a new Nematode Soil Health (NSH) index which summarises all NBIs into a single quantitative bioindicator. The framework was tested with five case datasets covering different soil types, depths, land uses and seasonal variations. Results for Datasets 1 and 2 indicated no significant difference in NSH values among soil types (Ferrosol, Chromosol and Vertosol) but significantly greater NSH in topsoil compared to subsoil layers. Dataset 3 revealed that soil amendments with fauna significantly increased the NSH index compared to defaunated soils, supporting the role of soil faunal communities in maintaining soil health. The NSH index (in Dataset 4) was also significantly higher in perennial pastures than annual croplands and exhibited (in Dataset 5) seasonal variation, with higher values in spring compared to autumn. Although this framework requires further calibration, testing and standardisation on more nematode community datasets, it could be combined with quantitative estimations or graphical representations of NBIs to provide additional information relevant to soil health conditions. The NSH index has the potential to foster the practical application of NBIs in soil health assessment programs, enhancing their adoption by practitioners and farmers.