Optimising soil profiles to support calcareous grassland habitat creation

Calcareous grasslands are some of Europe’s most species-rich plant communities and important biodiversity sites. These habitats are, however, threatened; many in the UK were lost to changing land use during the 20^th century and pressure continues on the remaining (often scarce and fragmented) sites. Habitat restoration and rewilding are increasingly important both in the public consciousness and in governmental policy, and the ecological value and threatened nature of calcareous grasslands make them a prime target for restoration efforts. A growing number of projects are therefore working to restore or re-create chalk grassland ecosystems. An under-explored aspect of this, however, is how by-products from land development projects might be re-purposed to create the specialised soil environment needed to support calcareous grassland communities. This has the potential to combine sustainable re-use of construction materials with novel ways to create or restore calcareous grassland habitats and thus ensure infrastructure projects contribute to net biodiversity gain.

 

In this study we investigate how to optimise the design of soil profiles to support calcareous grassland ecosystems. The study site is located in within the Central 1 section of the HS2 (High Speed 2) Phase One rail development in the Colne Valley (England) being delivered by the Align joint venture. The aim is to create a large area of calcareous grassland as part of a broader (127 hectare) mosaic habitat creation including calcareous grassland, wood pasture and wetland in land that is currently used for construction but was previously arable land. The ‘Colne Valley Western Slopes’ will, when complete, be 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.

 

The properties of underlying soils are critical for the establishment, development and health of this internationally important chalk grassland ecosystem. Physical, chemical and biological properties such as soil structure, drainage and restricted nutrient availability are essential for supporting the diverse plant assemblages found in calcareous grasslands. In this project, we are testing through a combination of controlled environment studies and field trials four constructed soil profiles using different configurations of site-derived materials/construction by-products, using both controlled environment studies and field trials. The site-won 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) that contain highly variable percentages of CaCO₃. Here we present results from a large-scale mesocosm trial alongside initial field trial data to assess how these constructed soil profiles affect key factors for habitat creation, including soil hydrology, soil microbial dynamics, nutrient cycling, and vegetation establishment and diversity. The findings for this project will inform and effectively complete the earthworks design for the creation of 88 hectares of calcareous grassland in the Colne Valley, as well as providing insights for other chalk grassland restoration projects elsewhere.