Emily Warner , Licida M Giuliani, Grant A Campbell, Pete Smith, Nathalie Seddon, Dan Seddon, Alison Smith 

1. As the development of nature-based solutions (NbS) increases globally, it is important to ensure that projects meet the objective of benefiting biodiversity, alongside tackling societal challenges. However, most NbS projects do not directly monitor ecological outcomes, and those that do often focus on a limited set of metrics. It is therefore challenging to assess whether projects fulfil the aim of benefiting biodiversity.
2. We aimed to identify the most informative and feasible ecological metrics, both above- and below-ground, for monitoring ecological outcomes of NbS. We conducted a structured non-systematic literature review to identify possible biodiversity and soil health metrics, then developed a scoring system to rank these based on their informativeness and feasibility for monitoring.
3. Metrics were categorised into compositional, structural, and functional aspects of biodiversity, and biological, physical, and chemical aspects of soil health. We grouped biodiversity and soil health metrics into Tier 1 (the most informative and feasible metrics), Tier 2 (informative metrics with some limitations in scope or feasibility), and Future metrics (highly informative metrics which are currently less feasible to monitor). Tier 1 metrics collectively address multiple aspects of biodiversity and soil health and are currently the highest priority for NbS projects to assess. For biodiversity nine Tier 1, six Tier 2, and 15 Future metrics were identified, and for soil health 11 Tier 1, six Tier 2, and five Future metrics.
4. We identified existing standardised methodologies for monitoring the proposed metrics, noting that for many metrics standardised methodologies are not available and threshold or reference values for each metric are missing.
5. Solution: Our study provides practitioners with a framework for selecting optimum metrics for assessing above- and below-ground ecological outcomes of NbS relevant to the place in which they are being implemented. A definition of each metric and standardised methodologies for collecting data are summarised, providing information to develop an ecological monitoring protocol for an NbS project. The information on each metric has been made freely available as a searchable database in an interactive online interface geared towards UK practitioners, but with wider applicability.