Large-scale mesocosm trials to optimise soil profiles for calcareous grassland habitat creation

Calcareous grasslands are important biodiversity sites and among Europe’s most floristically rich habitats. These habitats are, however, threatened; many of the UK’s calcareous grasslands were lost to changing land use during the 20th century and pressure on the surviving (often fragmented) sites persists. Due to their ecological value and threatened status there is significant interest in restoring and creating new areas of calcareous grassland, and an increasing number of projects are working to restore or re-create these internationally important ecosystems.

A major calcareous grassland creation project in the Colne Valley in the UK is being undertaken as part of the Central 1 section of the HS2 (High Speed 2) Phase One rail development, delivered by the Align joint venture. This will form a large area (90 hectare) of calcareous grassland as part of a larger (127 hectare) mosaic habitat including wood pasture and wetlands on former low-grade arable land subsequently used for construction. This represents the largest single area of habitat creation along the HS2 route and will significantly contribute to the project’s commitment to deliver ‘No Net Loss’ in biodiversity. To create soil profiles suitable for supporting species-rich calcareous grassland by-products from the HS2 development will be used; this will combine sustainable re-use of construction materials with the development of novel ways to create or restore chalk grassland habitats. These materials include 2.6 M m³ of excavated chalk from 16 km of tunnel construction, crushed limestone and concrete from decommissioned compounds/haul roads, and subsoils (stripped during site clearance).

However, how to best create soil profiles to support calcareous grassland habitat creation, including the potential for re-use of construction by-products, is not well understood. Success depends on establishing the specialised soil physical, chemical and biological environment required to support the diverse calcareous grasslands plant communities, including suitable soil structure, infiltration capacity and nutrient levels. We have therefore tested constructed soil profiles using different configurations of site-derived materials / construction by-products, using numerous soil and plant metrics through a combination of controlled environment and field trials to assess their ability to support calcareous grassland creation.

Here we present results from the main large-scale, controlled-environment trial at Cranfield University, in which we tested four soil profile configurations and the effect of upper soil layer depth in large (1 m³) soil mesocosms. The development of calcareous grassland on these profiles was closely monitored over a six-month period, including above- and below-ground imaging to monitor sward and root development, alongside close monitoring of soil hydrology, microbial dynamics, nutrient cycling, and vegetation establishment and diversity. This included a simulated drought period to assess how soil profile configurations affected the developing grassland vegetation’s resistance to water stress. Our results provide a uniquely high-resolution examination of how constructed soil profiles can be used to support calcareous grassland establishment and how profile design affects the ability of restored grassland to withstand environmental stress. This will allow improvements in circular re-use of infrastructure construction by-products in habitat restoration and development of novel strategies for the (re-)creation of biodiverse habitats.