Assessing the condition of forest soils in the Basque Country (Spain) with site-specific thresholds for soil health indicators

The assessment of soil health needs to consider the context of soil-forming factors and land use history when identifying reference soils, and set thresholds and management targets specific to different soil types. In the Basque Country (N Spain), rural landscapes and forests have been subjected to anthropogenic transformations and uses since Antiquity, with a profound expansion of intensive forestry plantations during the 20^th century. Hence, the establishment of reference soil health status is challenging but essential for guiding sustainable forest management. The aims of this study are: 1) to apply digital soil mapping for the delineation of soil monitoring units in the Basque Country, 2) set up thresholds and targets for soil indicators for managed forest soils, and 3) map the soil health condition of forest plantations. We established a framework for assessing the condition of forest plantations using three indicators suggested by the EU Soil Monitoring and Resilience Law proposal (soil organic carbon (SOC): clay, pH, and bulk density) using data from Basonet, the permanent network for forest monitoring in the Basque Country. The soil units were created applying unsupervised classification to a set of environmental covariates, proxies of the soil-forming factors (pedogenon mapping). Semi-natural native forests were used as reference for setting unit-specific thresholds for soil indicators (i.e., reference approach), and we tested the influence of the selected threshold on the soil health assessment. 61% of plots were in poor condition at the interval 0-20 cm and 90% at 20-40 cm for loss of SOC according to the EU level threshold of SOC:clay <1/13. The proportion of plantations in poor condition for loss of SOC with the reference approach ranged between 14-50% depending on the percentile used to set thresholds (5^th and 25^th percentiles). Forest plantations acidified the soil compared to semi-natural forests, with 15-60% of plantation plots with pH lower that the thresholds. All plantation plots were in good condition in terms of subsoil compaction with the EU criteria, but 9.6% of semi-natural plots suffered from compaction. We emphasize that the assessment of soil health needs to consider the context of soil-forming factors and inherent soil properties (e.g., mineralogy) when identifying thresholds for soil health indicators. Future work will continue the search for reference soils combining historical aerial photographs and long-term satellite imagery.