VEgetation NUtrient-retention Service (VENUS) index: an indicator to assess favourable conditions for nutrient retention by vegetation.

Ecosystems role in preserving water resources is acknowledged by EU Water Framework Directive 2000/60/EC and by Groundwater Directive 2006/118/EC. In this view nutrient concentration represent a critical aspect with regard to water quality and eutrophication. Nitrate pollution is specifically addressed by the Nitrate Directive 91/676/EEC, which provides nitrate management rules for farming activities, defines nitrate vulnerable zones and gives indications on nitrate monitoring in water bodies.

To preserve water quality levels an important role is played by Riparian Buffer Zones (RBZ) through the provision of the Ecosystem Service (ES) nutrient retention, the uptake process operated by plant roots. Research has shown how ES valuation is a very effective approach to support land management process, as it allows a better understanding of the importance of the role of ecosystems in guaranteeing wellbeing conditions for human communities and for the environment.

In order to perform a complete assessment of ES, the use of maps only providing information on processes occurring at/above the surface (e.g land use maps, or ecosystem maps, or ecosystem function maps) does not allow a complete analysis of underground dynamics.

In the case of the valuation of the ES “nutrient retention” provided by RBZ it is necessary to include the hydrogeological model and its links with the riverine network. The simple presence/absence of vegetation, or even a detailed vegetation classification map, cannot provide a complete description of all conditions required to fully assess this ES, as information on groundwater flow, sediments and soils characteristics is needed.

For the Riparian Vegetation Management Plan of the Gesso Stura Riverine Park a specific index to study vegetation contribution to nutrient retention was introduced: the VEgetation NUtrient-retention Service (VENUS) Index. This index provides land managers a semi-quantitative indication on the spatial distribution of “best conditions” (suitability map) for nutrient retention by vegetation; it is based on a simple set of parameters representing the main factors controlling the interactions between vegetation roots and water flows (runoff and groundwater) from surrounding areas towards water bodies.

The application of the VENUS Index improved the definition of management measures for riparian vegetation and allowed reaching the following results: (1) identification of homogeneous sectors in terms of RBZ relevance for nutrient retention; (2) assessment of different RBZ management scenarios with regard to nutrient retention performance; (3) definition of specific management measures to preserve and/or foster the provision of nutrient retention.